Effects of positive end-expiratory pressure on respiratory function and hemodynamics in patients with acute respiratory failure with and without intra-abdominal hypertension: a pilot study

Phenotypes of esophageal pressure response to the change of positive end-expiratory pressure in patients with moderate acute respiratory distress syndrome

Background

Esophageal pressure (Pes) has been used as a surrogate of pleural pressure (Ppl) to titrate positive end-expiratory pressure (PEEP) in acute respiratory distress syndrome (ARDS) patients. The relationship between Pes and PEEP remains undetermined.

Methods

A gastric tube with a balloon catheter was inserted to monitor Pes in moderate to severe ARDS patients who underwent invasive mechanical ventilation. To assess the end-expiratory Pes response (ΔPes) to PEEP changes (ΔPEEP), the PEEP level was decreased and increased subsequently (with an average change of 3 cmH2O). The patients underwent the following two series of PEEP adjustment: (I) from PEEP-3 cmH2O to PEEPbaseline; and (II) from PEEPbaseline to PEEP+3 cmH2O. The patients were classified as "PEEP-dependent type" if they had ΔPes ≥30% ΔPEEP and were otherwise classified as "PEEP-independent type" (ΔPes <30% ΔPEEP in any series).

Results

In total, 54 series of PEEP adjustments were performed in 18 ARDS patients. Of these patients, 12 were classified as PEEP-dependent type, and six were classified as PEEP-independent type. During the PEEP adjustment, end-expiratory Pes changed significantly in the PEEP-dependent patients, who had a Pes of 10.8 (7.9, 12.3), 12.5 (10.5, 14.9), and 14.5 (13.1, 18.3) cmH2O at PEEP-3 cmH2O, PEEPbaseline, and PEEP+3 cmH2O, respectively (median and quartiles; P<0.0001), while end-expiratory transpulmonary pressure (PL) was maintained at an optimal range [-0.1 (-0.7, 0.4), 0.1 (-0.6, 0.5), and 0.3 (-0.3, 0.7) cmH2O, respectively]. In the PEEP-independent patients, the Pes remained unchanged, with a Pes of 15.4 (11.4, 17.8), 15.5 (11.6, 17.8), and 15.4 (11.7, 18.30) cmH2O at each of the three PEEP levels, respectively. Meanwhile, end-expiratory PL significantly improved [from -5.5 (-8.5, -3.4) at PEEP-3 cmH2O to -2.5 (-5.0, -1.6) at PEEPbaseline to -0.5 (-1.8, 0.3) at PEEP+3 cmH2O; P<0.01].

Conclusions

Two types of Pes phenotypes were identified according to the ΔPes to ΔPEEP. The underlying mechanisms and implications for clinical practice require further exploration.

A structured narrative review of clinical and experimental studies of the use of different positive end-expiratory pressure levels during thoracic surgery

OBJECTIVES

This study aimed to present a review on the general effects of different positive end-expiratory pressure (PEEP) levels during thoracic surgery by qualitatively categorizing the effects into detrimental, beneficial, and inconclusive.

DATA SOURCE

Literature search of Pubmed, CNKI, and Wanfang was made to find relative articles about PEEP levels during thoracic surgery. We used the following keywords as one-lung ventilation, PEEP, and thoracic surgery.

RESULTS

We divide the non-individualized PEEP value into five grades, that is, less than 5, 5, 5-10, 10, and more than 10 cmH₂ O, among which 5 cmH₂ O is the most commonly used in clinic at present to maintain alveolar dilatation and reduce the shunt fraction and the occurrence of atelectasis, whereas individualized PEEP, adjusted by test titration or imaging method to adapt to patients' personal characteristics, can effectively ameliorate intraoperative oxygenation and obtain optimal pulmonary compliance and better indexes relating to respiratory mechanics.

CONCLUSIONS

Available data suggest that PEEP might play an important role in one-lung ventilation, the understanding of which will help in exploring a simple and economical method to set the appropriate PEEP level.

Computed tomographic assessment of lung aeration at different positive end-expiratory pressures in a porcine model of intra-abdominal hypertension and lung injury

Background

Intra-abdominal hypertension (IAH) is common in critically ill patients and is associated with increased morbidity and mortality. High positive end-expiratory pressures (PEEP) can reverse lung volume and oxygenation decline caused by IAH, but its impact on alveolar overdistension is less clear. We aimed to find a PEEP range that would be high enough to reduce atelectasis, while low enough to minimize alveolar overdistention in the presence of IAH and lung injury.

Methods

Five anesthetized pigs received standardized anesthesia and mechanical ventilation. Peritoneal insufflation of air was used to generate intra-abdominal pressure of 27 cmH₂O. Lung injury was created by intravenous oleic acid. PEEP levels of 5, 12, 17, 22, and 27 cmH₂O were applied. We performed computed tomography and measured arterial oxygen levels, respiratory mechanics, and cardiac output 5 min after each new PEEP level. The proportion of overdistended, normally aerated, poorly aerated, and non-aerated atelectatic lung tissue was calculated based on Hounsfield units.

Results

PEEP decreased the proportion of poorly aerated and atelectatic lung, while increasing normally aerated lung. Overdistension increased with each incremental increase in applied PEEP. “Best PEEP” (respiratory mechanics or oxygenation) was higher than the “optimal CT inflation PEEP range” (difference between lower inflection points of atelectatic and overdistended lung) in healthy and injured lungs.

Conclusions

Our findings in a large animal model suggest that titrating a PEEP to respiratory mechanics or oxygenation in the presence of IAH is associated with increased alveolar overdistension.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40635-021-00416-5.

The Correlation between Head of Bed Angle and Intra-Abdominal Pressure of Intubated Patients; a Pre-Post Clinical Trial

INTRODUCTION

The recommended position for measuring Intra-Abdominal Pressure (IAP) is the supine position. However, patients put in this position are prone to Ventilator-associated pneumonia. This study was done to evaluate the relationship between bed head angle and IAP measurements of intubated patients in the intensive care unit.

METHODS

In this clinical trial, seventy-six critically ill patients under mechanical ventilation were enrolled. IAP measurement was performed every 8 hours for 24 hours using the KORN method in three different degrees of the head of bed (HOB) elevation (0 ° , 15 ° , and 30 ° ). Bland-Altman analysis was performed to identify the bias and limits of agreement among the three HOBs. According to World Society of the Abdominal Compartment Syndrome (WSACS), we can consider two IAP techniques equivalent if a bias of <1 mmHg and limits of agreement of - 4 to +4 were found between them. Data were analyzed using SPSS statistical software (v. 19), and the significance level was considered as 0.05.

RESULTS

The prevalence of intra-abdominal hypertension was 18.42%. Mean ± standard deviation (SD) of IAP were 8.44 ± 4.02 mmHg for HOB angle 0°, 9.58 ± 4.52 for HOB angle 15 ° , and 11.10 ± 4.73 for HOB angle 30^o (p = 0.0001). The IAP measurement bias between HOB angle 0°and HOB angle 15° was 1.13 mmHg. This bias was 2.66 mmHg between HOB angle 0° and HOB angle 30°.

CONCLUSION

Elevation of HOB angle from 0 to 30 degree significantly increases IAP. It seems that the measurement of IAP at HOB angle 15° was more reliable than 30°.

The Nature of Recruitment and De-Recruitment and Its Implications for Management of ARDS

Recruitment maneuvers in ARDS are used to improve oxygenation and lung mechanics by applying high airway pressures to reopen collapsed or obstructed peripheral airways and alveoli. In the early 1990s, recruitment maneuvers became a central feature of a variant form of lung-protective ventilation known as open-lung ventilation. This strategy is based on the belief that repetitive opening and closing of distal airspaces induces shear injury and therefore contributes both to ventilator-induced lung injury and ARDS-associated mortality. However, the largest multi-center randomized controlled trial of open-lung ventilation in moderate to severe ARDS reported that recruitment maneuver plateau pressures of 50-60 cm H₂O were associated with significantly higher mortality compared to traditional lung-protective ventilation. Despite being based on well conducted preclinical and clinical recruitment maneuver studies, the higher mortality associated with the open-lung ventilation strategy requires re-examining the assumptions and conclusions drawn from those previous studies. This narrative review examines the evidence used to design recruitment maneuver strategies. We also review the radiologic, rheologic, and histopathologic evidence regarding the nature of lung injury and the phenomena of recruitment and de-recruitment as it informs our perceptions of recruitment potential in ARDS. Major lung-protective ventilation clinical trial data and other clinical data are also examined to assess the practical necessity of recruitment maneuvers in ARDS and whether a subset of cases might benefit from pursuing recruitment maneuver therapy. Finally, a less a radical approach to recruitment maneuvers is offered that might achieve the goals of recruitment maneuvers with less risk of harm.

Impact of Different Positive End-Expiratory Pressures on Lung Mechanics in the Setting of Moderately Elevated Intra-Abdominal Pressure and Acute Lung Injury in a Porcine Model

The effects of a moderately elevated intra-abdominal pressure (IAP) on lung mechanics in acute respiratory distress syndrome (ARDS) have still not been fully analyzed. Moreover, the optimal positive end-expiratory pressure (PEEP) in elevated IAP and ARDS is unclear. In this paper, 18 pigs under general anesthesia received a double hit lung injury. After saline lung lavage and 2 h of injurious mechanical ventilation to induce an acute lung injury (ALI), an intra-abdominal balloon was filled until an IAP of 10 mmHg was generated. Animals were randomly assigned to one of three groups (group A = PEEP 5, B = PEEP 10 and C = PEEP 15 cmH₂O) and ventilated for 6 h. We measured end-expiratory lung volume (EELV) per kg bodyweight, driving pressure (ΔP), transpulmonary pressure (ΔP_L), static lung compliance (C_stat), oxygenation (P/F ratio) and cardiac index (CI). In group A, we found increases in ΔP (22 ± 1 vs. 28 ± 2 cmH₂O; p = 0.006) and ΔP_L (16 ± 1 vs. 22 ± 2 cmH₂O; p = 0.007), with no change in EELV/kg (15 ± 1 vs. 14 ± 1 mL/kg) when comparing hours 0 and 6. In group B, there was no change in ΔP (26 ± 2 vs. 25 ± 2 cmH₂O), ΔP_L (19 ± 2 vs. 18 ± 2 cmH₂O), C_stat (21 ± 3 vs. 21 ± 2 cmH₂O/mL) or EELV/kg (12 ± 2 vs. 13 ± 3 mL/kg). ΔP and ΔP_L were significantly lower after 6 h when comparing between group C and A (21 ± 1 vs. 28 ± 2 cmH₂O; p = 0.020) and (14 ± 1 vs. 22 ± 2 cmH₂O; p = 0.013)). The EELV/kg increased over time in group C (13 ± 1 vs. 19 ± 2 mL/kg; p = 0.034). The P/F ratio increased in all groups over time. CI decreased in groups B and C. The global lung injury score did not significantly differ between groups (A: 0.25 ± 0.05, B: 0.21 ± 0.02, C: 0.22 ± 0.03). In this model of ALI, elevated IAP, ΔP and ΔP_L increased further over time in the group with a PEEP of 5 cmH₂O applied over 6 h. This was not the case in the groups with a PEEP of 10 and 15 cmH₂O. Although ΔP and ΔP_L were significantly lower after 6 hours in group C compared to group A, we could not show significant differences in histological lung injury score.

Prospective Observational Study to Evaluate the Effect of Different Levels of Positive End-Expiratory Pressure on Lung Mechanics in Patients with and without Acute Respiratory Distress Syndrome

Background: The optimal level of positive end-expiratory pressure is still under debate. There are scare data examining the association of PEEP with transpulmonary pressure (TPP), end-expiratory lung volume (EELV) and intraabdominal pressure in ventilated patients with and without ARDS. Methods: We analyzed lung mechanics in 3 patient groups: group A, patients with ARDS; group B, obese patients (body mass index (BMI) > 30 kg/m²) and group C, a control group. Three levels of PEEP (5, 10, 15 cm H₂O) were used to investigate the consequences for lung mechanics. Results: Fifty patients were included, 22 in group A, 18 in group B (BMI 38 ± 2 kg/m²) and 10 in group C. At baseline, oxygenation showed no differences between the groups. Driving pressure (ΔP) and transpulmonary pressure (ΔP_L) was higher in group B than in groups A and C at a PEEP of 5 cm H₂O (ΔP A: 15 ± 1, B: 18 ± 1, C: 14 ± 1 cm H₂O; ΔP_L A: 10 ± 1, B: 13 ± 1, C: 9 ± 0 cm H₂O). Peak inspiratory pressure (P_insp) rose in all groups as PEEP increased, but the resulting driving pressure and transpulmonary pressure were reduced, whereas EELV increased. Conclusion: Measuring EELV or TPP allows a personalized approach to lung-protective ventilation.

Effect of moderate elevated intra-abdominal pressure on lung mechanics and histological lung injury at different positive end-expiratory pressures

INTRODUCTION

Intra-abdominal hypertension (IAH) is a well-known phenomenon in critically ill patients. Effects of a moderately elevated intra-abdominal pressure (IAP) on lung mechanics are still not fully analyzed. Moreover, the optimal positive end-expiratory pressure (PEEP) in elevated IAP is unclear.

METHODS

We investigated changes in lung mechanics and transformation in histological lung patterns using three different PEEP levels in eighteen deeply anesthetized pigs with an IAP of 10 mmHg. After establishing the intra-abdominal pressure, we randomized the animals into 3 groups. Each of n = 6 (Group A = PEEP 5, B = PEEP 10 and C = PEEP 15 cmH2O). End-expiratory lung volume (EELV/kg body weight (bw)), pulmonary compliance (Cstat), driving pressure (ΔP) and transpulmonary pressure (ΔPL) were measured for 6 hours. Additionally, the histological lung injury score was calculated.

RESULTS

Comparing hours 0 and 6 in group A, there was a decrease of EELV/kg (27±2 vs. 16±1 ml/kg; p<0.05) and of Cstat (42±2 vs. 27±1 ml/cmH2O; p<0.05) and an increase of ΔP (11±0 vs. 17±1 cmH2O; p<0.05) and ΔPL (6±0 vs. 10±1 cmH2O; p<0.05). In group B, there was no significant change in EELV/kg (27±3 vs. 24±3 ml/kg), but a decrease in Cstat (42±3 vs. 32±1 ml/cmH20; p<0.05) and an increase in ΔP (11±1 vs. 15±1 cmH2O; p<0.05) and ΔPL (5±1 vs. 7±0 cmH2O; p<0.05). In group C, there were no significant changes in EELV/kg (27±2 vs. 29±3 ml/kg), ΔP (10±1 vs. 12±1 cmH2O) and ΔPL (5±1 vs. 7±1 cmH2O), but a significant decrease of Cstat (43±1 vs. 37±1 ml/cmH2O; p<0.05). Histological lung injury score was lowest in group B.

CONCLUSIONS

A moderate elevated IAP of 10 mmHg leads to relevant changes in lung mechanics during mechanical ventilation. In our study, a PEEP of 10 cmH2O was associated with a lower lung injury score and was able to overcome the IAP induced alterations of EELV.

Transpulmonary thermodilution detects rapid and reversible increases in lung water induced by positive end-expiratory pressure in acute respiratory distress syndrome

PURPOSE

It has been suggested that, by recruiting lung regions and enlarging the distribution volume of the cold indicator, increasing the positive end-expiratory pressure (PEEP) may lead to an artefactual overestimation of extravascular lung water (EVLW) by transpulmonary thermodilution (TPTD).

METHODS

In 60 ARDS patients, we measured EVLW (PiCCO2 device) at a PEEP level set to reach a plateau pressure of 30 cmH₂O (HighPEEP_start) and 15 and 45 min after decreasing PEEP to 5 cmH₂O (LowPEEP_15' and LowPEEP_45', respectively). Then, we increased PEEP back to the baseline level (HighPEEP_end). Between HighPEEP_start and LowPEEP_15', we estimated the degree of lung derecruitment either by measuring changes in the compliance of the respiratory system (Crs) in the whole population, or by measuring the lung derecruited volume in 30 patients. We defined patients with a large derecruitment from the other ones as patients in whom the Crs changes and the measured derecruited volume were larger than the median of these variables observed in the whole population.

RESULTS

Reducing PEEP from HighPEEP_start (14 ± 2 cmH₂O) to LowPEEP_15' significantly decreased EVLW from 20 ± 4 to 18 ± 4 mL/kg, central venous pressure (CVP) from 15 ± 4 to 12 ± 4 mmHg, the arterial oxygen tension over inspired oxygen fraction (PaO₂/FiO₂) ratio from 184 ± 76 to 150 ± 69 mmHg and lung volume by 144 [68-420] mL. The EVLW decrease was similar in "large derecruiters" and the other patients. When PEEP was re-increased to HighPEEP_end, CVP, PaO₂/FiO₂ and EVLW significantly re-increased. At linear mixed effect model, EVLW changes were significantly determined only by changes in PEEP and CVP (p < 0.001 and p = 0.03, respectively, n = 60). When the same analysis was performed by estimating recruitment according to lung volume changes (n = 30), CVP remained significantly associated to the changes in EVLW (p < 0.001).

CONCLUSIONS

In ARDS patients, changing the PEEP level induced parallel, small and reversible changes in EVLW. These changes were not due to an artefact of the TPTD technique and were likely due to the PEEP-induced changes in CVP, which is the backward pressure of the lung lymphatic drainage. Trial registration ID RCB: 2015-A01654-45. Registered 23 October 2015.

Respiratory Mechanics, Lung Recruitability, and Gas Exchange in Pulmonary and Extrapulmonary Acute Respiratory Distress Syndrome

OBJECTIVES

Acute respiratory distress syndrome is a clinical syndrome characterized by a refractory hypoxemia due to an inflammatory and high permeability pulmonary edema secondary to direct or indirect lung insult (pulmonary and extrapulmonary form). Aim of this study was to evaluate in a large database of acute respiratory distress syndrome patients, the pulmonary versus extrapulmonary form in terms of respiratory mechanics, lung recruitment, gas exchange, and positive end-expiratory pressure response.

DESIGN

A secondary analysis of previously published data.

PATIENTS

One-hundred eighty-one sedated and paralyzed acute respiratory distress syndrome patients (age 60 yr [46-72 yr], body mass index 25 kg/m [22-28 kg/m], and PaO2/FIO2 184 ± 66).

INTERVENTIONS

Lung CT scan performed at 5 and 45 cm H2O. Two levels of positive end-expiratory pressure (5 and 15 cm H2O) were randomly applied.

MEASUREMENTS AND MAIN RESULTS

Ninety-seven and 84 patients had a pulmonary and extrapulmonary acute respiratory distress syndrome. The median time from intensive care admission to the CT scan and respiratory mechanics analysis was 4 days (interquartile range, 2-6). At both positive end-expiratory pressure levels, pulmonary acute respiratory distress syndrome presented a significantly lower PaO2/FIO2 and higher physiologic dead space compared with extrapulmonary acute respiratory distress syndrome. The lung and chest wall elastance were similar between groups. The intra-abdominal pressure was significantly higher in extrapulmonary compared with pulmonary acute respiratory distress syndrome (10 mm Hg [7-12 mm Hg] vs 7 mm Hg [5-8 mm Hg]). The lung weight and lung recruitability were significantly higher in pulmonary acute respiratory distress syndrome (1,534 g [1,286-1,835 g] vs 1,342 g [1,090-1,507 g] and 16% [9-25%] vs 9% [5-14%]).

CONCLUSIONS

In the early stage, pulmonary acute respiratory distress syndrome is characterized by a greater impairment of gas exchange and higher lung recruitability. The recognition of the origin of acute respiratory distress syndrome is important for a more customized ventilatory management.

Ventilation in patients with intra-abdominal hypertension: what every critical care physician needs to know

The incidence of intra-abdominal hypertension (IAH) is high and still underappreciated by critical care physicians throughout the world. One in four to one in three patients will have IAH on admission, while one out of two will develop IAH within the first week of Intensive Care Unit stay. IAH is associated with high morbidity and mortality. Although considerable progress has been made over the past decades, some important questions remain regarding the optimal ventilation management in patients with IAH. An important first step is to measure intra-abdominal pressure (IAP). If IAH (IAP > 12 mmHg) is present, medical therapies should be initiated to reduce IAP as small reductions in intra-abdominal volume can significantly reduce IAP and airway pressures. Protective lung ventilation with low tidal volumes in patients with respiratory failure and IAH is important. Abdominal-thoracic pressure transmission is around 50%. In patients with IAH, higher positive end-expiratory pressure (PEEP) levels are often required to avoid alveolar collapse but the optimal PEEP in these patients is still unknown. During recruitment manoeuvres, higher opening pressures may be required while closely monitoring oxygenation and the haemodynamic response. During lung-protective ventilation, whilst keeping driving pressures within safe limits, higher plateau pressures than normally considered might be acceptable. Monitoring of the respiratory function and adapting the ventilatory settings during anaesthesia and critical care are of great importance. This review will focus on how to deal with the respiratory derangements in critically ill patients with IAH.

Daily intra-abdominal pressure, Sequential Organ Failure Score and fluid balance predict duration of mechanical ventilation

BACKGROUND

Elevated intra-abdominal pressure (IAP) is a common occurrence in mechanically ventilated patients in the intensive care unit (ICU). This study was undertaken to determine the relationship between IAP, pulmonary compliance and the duration of mechanical ventilation.

METHODS

A prospective study of 220 consecutively enrolled mechanically ventilated patients admitted to a mixed surgical-medical ICU in a tertiary referral hospital. The IAP was measured at least twice daily, benchmarked against consensus guidelines. Dynamic pulmonary compliance was calculated together with admission Acute Physiology and Chronic Health Evaluation (APACHE III) score and daily Sequential Organ Failure Assessment (SOFA) score.

RESULTS

No relationship between highest IAP for the day and pulmonary compliance (P = 0.61) was found. For each 5 mm Hg increase in IAP, the risk of remaining intubated increased 19% (HR = 1.19, 95% CI: 0.98-1.44); for each standard deviation increase in SOFA score (3.7 points), the risk of remaining intubated increased by 14% (HR = 1.14, 95% CI: 0.98-1.33); and for each 1 L increase in fluid balance, the risk of remaining intubated increased by 11% (HR = 1.11, 95% CI: 1.04-1.19). A nomogram was developed to predict the probability of extubation based on daily highest IAP for the day, SOFA score and fluid balance.

CONCLUSION

IAPs did not correlate with pulmonary compliance in critically ill patients. Increased IAP was associated with a longer duration of mechanical ventilation. A nomogram integrating daily IAP, SOFA score and fluid balance may be used to predict the duration of mechanical ventilation.

Positive end-expiratory pressure titrated according to respiratory system mechanics or to ARDSNetwork table did not guarantee positive end-expiratory transpulmonary pressure in acute respiratory distress syndrome

PURPOSE

Pulmonary recruitment and positive end-expiratory pressure (PEEP) titrated according to minimal static elastance of the respiratory system (PEEP_Estat,RS) compared to PEEP set according to the ARDSNetwork table (PEEP_ARDSNetwork) as a strategy to prevent ventilator-associated lung injury (VALI) in patients with acute respiratory distress syndrome (ARDS) increases mortality. Alternatively, avoiding negative end-expiratory transpulmonary pressure has been discussed as superior PEEP titration strategy. Therefore, we tested whether PEEP_Estat,RS or PEEP_ARDSNetwork prevent negative end-expiratory transpulmonary pressure in ARDS patients.

MATERIAL AND METHODS

Thirteen patients with moderate to severe ARDS were studied at PEEP_ARDSNetwork versus PEEP_Estat,RS. Patients were then grouped post hoc according to the end-expiratory transpulmonary pressure (positive or negative).

RESULTS

7 out of 13 patients showed negative end-expiratory transpulmonary pressures (Ptp-) with both strategies (PEEP_ARDSNetwork: - 5.4 ± 3.5 vs. 2.2 ± 3.7 cm H₂O, p = .005; PEEP_Estat,RS: - 3.6 ± 1.5 vs. 3.5 ± 3.3 cm H₂O, p < .001). Ptp- was associated with higher intra-abdominal pressure and lower end-expiratory lung volume with both PEEP strategies.

CONCLUSIONS

In patients with moderate-to-severe ARDS, PEEP titrated according to the minimal static elastance of the respiratory system or according to the ARDSNetwork table did not prevent negative end-expiratory transpulmonary pressure.

The prophylactic effect of different levels of positive endexpiratory pressure on the incidence rate of atelectasis after cardiac surgery: A Randomized Controlled Trial

Background: The use of positive end-expiratory pressure (PEEP) can have an important role as one of the ways to prevent and treat atelectasis, but it seems that there is still no consensus about its beneficial level. The aim of this study was to determine the effect of different levels of PEEP on the incidence of atelectasis after heart surgery.

Methods: This is a double-blind randomized controlled trial that was adopted from a research project recorded in the Iranian Registry of Clinical Trials. This paper is the result of a research project undertaken at Fatemeh Zahra Hospital (Mazandaran Heart Center) in 2015. 180 patients underwent open heart surgery were selected and were divided randomly into three groups: control, PEEP=8, and PEEP=10 (60 in each group). The patients in the two PEEP8 and PEEP10 intervention groups separately received 8 cm H2O and 10 cm H2O PEEP, respectively, 30 minutes after admission to the ICU for 4 hours and then received 5 cm H2O PEEP until extubation. Atelectasis was examined two hours after the extubation and on the third day after surgery.

Results: The incidence rates of atelectasis two hours after extubation on the first day of surgery were 22 (36.7%), 20 (33.3%) and 10 (16.7%) patients in the control, PEEP8 and PEEP10 groups, respectively. The differences were statistically significant among the three groups (p=0.035). The incidence rates of atelectasis on the third day after surgery were 39 (65%), 36 (60%) and 21 (35%) patients in the control, PEEP8 and PEEP10 groups, respectively. The differences were also statistically significant among the three groups (p=0.003).

Conclusion: The use of 10 cm H2O PEEP can lead to a reduction in the incidence of atelectasis, intubation time at the ICU and length of ICU and hospital stay. Given that this level of PEEP is effective, this method is recommended to be used in postoperative care of patients.

Positive end-expiratory pressure adjusted for intra-abdominal pressure - A pilot study

PURPOSE

Intra-abdominal hypertension (IAH) is associated with impaired respiratory function. Animal data suggest that positive end-expiratory pressure (PEEP) levels adjusted to intra-abdominal pressure (IAP) levels may counteract IAH-induced respiratory dysfunction. In this pilot study, our aim was to assess whether PEEP adjusted for IAP can be applied safely in patients with IAH.

MATERIALS AND METHODS

We included patients on mechanical ventilation and with IAH. Patients were excluded with severe cardiovascular dysfunction or severe hypoxemia or if the patient was in imminent danger of dying. Following a recruitment manoeuvre, the following PEEP levels were randomly applied: PEEP of 5cmH₂O (baseline), PEEP=50% of IAP, and PEEP=100% of IAP. After a 30min equilibration period we measured arterial blood gases and cardio-respiratory parameters.

RESULTS

Fifteen patients were enrolled. Six (41%) patients did not tolerate PEEP=100% IAP due to hypoxemia, hypotension or endotracheal cuff leak. PaO₂/FiO₂ ratios were 234 (68), 271 (99), and 329 (107) respectively. The differences were significant (p=0.009) only between baseline and PEEP=100% IAP.

CONCLUSIONS

PEEP=100% of IAP was not well-tolerated and only marginally improved oxygenation in ventilated patients with IAH.

Prevalence and predictors of intra-abdominal hypertension and compartment syndrome in surgical patients in critical care units at Kenyatta National Hospital

BACKGROUND

Intra-abdominal hypertension (IAH) affects almost every organ sytem.If it is not detected early and corrected, mortality would be high. The prevalence of IAH and abdominal compartment syndrome (ACS) at Kenyatta National Hospital (KNH) critical care units is not known. The aim of this sudy was to determine the prevalence and factors associated with development of IAH/ACS among critically ill surgical patients.

METHODS

This was a cross sectional descriptive study involving surgical patients in critical care units at KNH, carried out from March 2015 to October 2015. One hundred and thirteen critically ill and ventilated patients 13 years or older were recruited into the study. Krohn's intravesical method was used to measure intra- abdominal pressure (IAP). Measurements were done at first contact, then at 12 and 24 h. Additional parameters recorded included: laboratory tests such as serum bilirubin and total blood count as well as clinical parameters such as urine output, vital signs and peak airway pressure, among others. Frequency, means and standard deviation were used to describe the data. Categorical variables e.g. age, were analysed using Chi square test and continous variables using student 't' test and Mann Whitney test as appropriate RESULT: A total of 113 consecutive surgical patients admitted to the critical care units were recruited. Of our study population, 71.7% (by IAP max) and 67.3% (by IAP mean) had IAH. Abdominal compartment syndrome (ACS) developed in 4.4% of the population. The following factors were significant determinants of risk of IAH : amount of IV fluids over 24 h (3949.6 vs 2931.1, p = 0.003, adjusted OR 1.0 [1.0-1.002]), haemoglobin values at admission (9.9 vs 12.0, p = <0.012, adjusted OR 0.6 [0.4-0.9]), peak airway pressure (28.4 vs 17.3; p = 0.018, adjusted OR 1.6 [1.1-2.4]) and synchronised intermittent mandatory ventilation (SIMV) (60 vs 32; p = 0.041, adjusted OR 1.4 [0.78-2.04]). Of those who had IAH; age, amount of iv fluids over 24 h, fluid balance and ventilator mode were significant determinants of risk of progression to ACS .

CONCLUSION

The prevalence of intraabdominal hypertension and abdominal compartment syndrome at KNH is high. Clinical parameters pertaining to fluids administration and ventilator mode are siginificant determinants.

The respiratory pressure-abdominal volume curve in a porcine model

Background

Increasing intra-abdominal volume (IAV) can lead to intra-abdominal hypertension (IAH) or abdominal compartment syndrome. Both are associated with raised morbidity and mortality. IAH can increase airway pressures and impair ventilation. The relationship between increasing IAV and airway pressures is not known. We therefore assessed the effect of increasing IAV on airway and intra-abdominal pressures (IAP).

Methods

Seven pigs (41.4 +/−8.5 kg) received standardized anesthesia and mechanical ventilation. A latex balloon inserted in the peritoneal cavity was inflated in 1-L increments until IAP exceeded 40 cmH₂O. Peak airway pressure (pP_AW), respiratory compliance, and IAP (bladder pressure) were measured. Abdominal compliance was calculated. Different equations were tested that best described the measured pressure-volume curves.

Results

An exponential equation best described the measured pressure-volume curves. Raising IAV increased pP_AW and IAP in an exponential manner. Increases in IAP were associated with parallel increases in pP_AW with an approximate 40% transmission of IAP to pP_AW. The higher the IAP, the greater IAV effected pP_AW and IAP.

Conclusions

The exponential nature of the effect of IAV on pP_AW and IAP implies that, in the presence of high grades of IAH, small reductions in IAV can lead to significant reductions in airway and abdominal pressures. Conversely, in the presence of normal IAP levels, large increases in IAV may not affect airway and abdominal pressures.

Electronic supplementary material

The online version of this article (doi:10.1186/s40635-017-0124-7) contains supplementary material, which is available to authorized users.

Increased pressure within the abdominal compartment: intra-abdominal hypertension and the abdominal compartment syndrome

PURPOSE OF REVIEW

This article reviews recent developments related to intra-abdominal hypertension (IAH)/abdominal compartment syndrome (ACS) and clinical practice guidelines published in 2013.

RECENT FINDINGS

IAH/ACS often develops because of the acute intestinal distress syndrome. Although the incidence of postinjury ACS is decreasing, IAH remains common and associated with significant morbidity and mortality among critically ill/injured patients. Many risk factors for IAH include those findings suggested to be indications for use of damage control surgery in trauma patients. Medical management strategies for IAH/ACS include sedation/analgesia, neuromuscular blocking and prokinetic agents, enteral decompression tubes, interventions that decrease fluid balance, and percutaneous catheter drainage. IAH/ACS may be prevented in patients undergoing laparotomy by leaving the abdomen open where appropriate. If ACS cannot be prevented with medical or surgical management strategies or treated with percutaneous catheter drainage, guidelines recommend urgent decompressive laparotomy. Use of negative pressure peritoneal therapy for temporary closure of the open abdomen may improve the systemic inflammatory response and patient-important outcomes.

SUMMARY

In the last 15 years, investigators have better clarified the pathogenesis, epidemiology, diagnosis, and appropriate prevention of IAH/ACS. Subsequent study should be aimed at understanding which treatments effectively lower intra-abdominal pressure and whether these treatments ultimately affect patient-important outcomes.

Influence of Ventilation Parameters on Intraabdominal Pressure

Introduction

Intraabdominal pressure monitoring is not routinely performed because the procedure assumes some invasiveness and, like other invasive procedures, it needs to have a clear indication to be performed. The causes of IAH are various. Mechanically ventilated patients have numerous parameters set in order to be optimally ventilated and it is important to identify the ones with the biggest interference in abdominal pressure. Although it was stated that mechanical ventilation is a potential factor of high intraabdominal pressure the set parameters which may lead to this diagnostic are not clearly named.

Objectives

To evaluate the relation between intraabdominal pressure and ventilator parameters in patients with mechanical ventilation and to determine the correlation between intraabdominal pressure and body mass index.

Material and method

This is an observational study which enrolled 16 invasive ventilated patients from which we obtained 61 record sheets. The following parameters were recorded twice daily: ventilator parameters, intraabdominal pressure, SpO2, Partial Oxygen pressure of arterial blood. We calculated the Body Mass Index (BMI) for each patient and the volume tidal/body weight ratio for every recorded data point.

Results

We observed a significant correlation between intraabdominal pressure (IAP) and the value of PEEP (p=0.0006). A significant statistical correlation was noted regarding the tidal volumes used for patient ventilation. The mean tidal volume was 5.18 ml/kg. Another significant correlation was noted between IAP and tidal volume per kilogram (p=0.0022). A positive correlation was found between BMI and IAP (p=0.0049), and another one related to the age of the enrolled patients. (p=0.0045).

Conclusions

The use of positive end-expiratory pressures and high tidal volumes during mechanical ventilation may lead to the elevation of intraabdominal pressure, a possible way of reducing this risk would be using low values of PEEP and also low volumes for the setting of ventilation parameters. There is a close positive correlation between the intraabdominal pressure levels and body mass index.

Low Transmission of Airway Pressures to the Abdomen in Mechanically Ventilated Patients With or Without Acute Respiratory Failure and Intra-Abdominal Hypertension

PURPOSE

Intra-abdominal pressure, measured at end expiration, may depend on ventilator settings and transmission of intrathoracic pressure. We determined the transmission of positive intrathoracic pressure during mechanical ventilation at inspiration and expiration into the abdominal compartment.

METHODS AND RESULTS

We included 9 patients after uncomplicated cardiac surgery and 9 with acute respiratory failure. Intravesical pressures were measured thrice (reproducibility of 1.8%) and averaged, at the end of each inspiratory and expiratory hold maneuvers of 5 seconds. Transmission, the change in intra-abdominal over intrathoracic pressures from end inspiration to end expiration, was about 8%. End-expiratory intra-abdominal pressure was lower than "total" intra-abdominal pressure over the entire respiratory cycle by 0.34 cm H₂O. It was 0.73 cm H₂O higher than "true" intra-abdominal pressure over the entire respiratory cycle, taking transmission into account. The percentage error was 3% for total and 10% for true pressure. Results did not differ among patients with or without acute respiratory failure and decreased respiratory compliance or between those with (≥12 mm Hg, n = 5) or without intra-abdominal hypertension.

CONCLUSIONS

Transmitted airway pressure only slightly affects intra-abdominal pressure in mechanically ventilated patients, irrespective of respiratory compliance and baseline intra-abdominal pressure values. End-expiratory measurements referenced against atmospheric pressure may suffice for clinical practice.

Impact of Chest Wall Modifications and Lung Injury on the Correspondence Between Airway and Transpulmonary Driving Pressures

OBJECTIVE

Recent interest has arisen in airway driving pressure (DP(AW)), the quotient of tidal volume (V(T)), and respiratory system compliance (C(RS)), which could serve as a direct and easily measured marker for ventilator-induced lung injury risk. We aimed to test the correspondence between DP(AW) and transpulmonary driving pressure (DP(TP))-the quotient of V(T) and lung compliance (C(L)), in response to intra-abdominal hypertension and changes in positive end-expiratory pressure during different models of lung pathology.

DESIGN

Well-controlled experimental setting that allowed reversible modification of chest wall compliance (C(CW)) in a variety of models of lung pathology.

SETTING

Large animal laboratory of a university-affiliated hospital.

SUBJECTS

Ten deeply anesthetized swine.

INTERVENTIONS

Application of intra-abdominal pressures of 0 and 20 cm H2O at positive end-expiratory pressure of 1 and 10 cm H2O, under volume-controlled mechanical ventilation in the settings of normal lungs (baseline), unilateral whole-lung atelectasis, and unilateral and bilateral lung injuries caused by saline lavage.

MEASUREMENTS AND MAIN RESULTS

Pulmonary mechanics including esophageal pressure and calculations of DP(AW), DP(TP), C(RS), C(L), and C(CW). When compared with normal intra-abdominal pressures, intra-abdominal hypertension increased DP(AW), during both "normal lung conditions" (p < 0.0001) and "unilateral atelectasis" (p = 0.0026). In contrast, DP(TP) remained virtually unaffected by changes in positive end-expiratory pressure or intra-abdominal pressures in both conditions. During unilateral lung injury, both DPA(W) and DP(TP) were increased by the presence of intra-abdominal hypertension (p < 0.0001 and p = 0.0222, respectively). During bilateral lung injury, intra-abdominal hypertension increased both DP(AW) (at positive end-expiratory pressure of 1 cm H2O, p < 0.0001; and at positive end-expiratory pressure of 10 cm H2O, p = 0.0091) and DP(TP) (at positive end-expiratory pressure of 1 cm H2O, p = 0.0510; and at positive end-expiratory pressure of 10 cm H2O, p = 0.0335).

CONCLUSIONS

Our data indicate that DP(AW) is influenced by reductions in chest wall compliance and by underlying lung properties. As with other measures of pulmonary mechanics that are based on unmodified P(AW), caution is advised in attempting to attribute hazard or safety to any specific absolute value of DP(AW).

Adjusting tidal volume to stress index in an open lung condition optimizes ventilation and prevents overdistension in an experimental model of lung injury and reduced chest wall compliance

INTRODUCTION

The stress index (SI), a parameter derived from the shape of the pressure-time curve, can identify injurious mechanical ventilation. We tested the hypothesis that adjusting tidal volume (VT) to a non-injurious SI in an open lung condition avoids hypoventilation while preventing overdistension in an experimental model of combined lung injury and low chest-wall compliance (Ccw).

METHODS

Lung injury was induced by repeated lung lavages using warm saline solution, and Ccw was reduced by controlled intra-abdominal air-insufflation in 22 anesthetized, paralyzed and mechanically ventilated pigs. After injury animals were recruited and submitted to a positive end-expiratory pressure (PEEP) titration trial to find the PEEP level resulting in maximum compliance. During a subsequent four hours of mechanical ventilation, VT was adjusted to keep a plateau pressure (Pplat) of 30 cmH2O (Pplat-group, n = 11) or to a SI between 0.95 and 1.05 (SI-group, n = 11). Respiratory rate was adjusted to maintain a 'normal' PaCO2 (35 to 65 mmHg). SI, lung mechanics, arterial-blood gases haemodynamics pro-inflammatory cytokines and histopathology were analyzed. In addition Computed Tomography (CT) data were acquired at end expiration and end inspiration in six animals.

RESULTS

PaCO2 was significantly higher in the Pplat-group (82 versus 53 mmHg, P = 0.01), with a resulting lower pH (7.19 versus 7.34, P = 0.01). We observed significant differences in VT (7.3 versus 5.4 mlKg(-1), P = 0.002) and Pplat values (30 versus 35 cmH2O, P = 0.001) between the Pplat-group and SI-group respectively. SI (1.03 versus 0.99, P = 0.42) and end-inspiratory transpulmonary pressure (PTP) (17 versus 18 cmH2O, P = 0.42) were similar in the Pplat- and SI-groups respectively, without differences in overinflated lung areas at end- inspiration in both groups. Cytokines and histopathology showed no differences.

CONCLUSIONS

Setting tidal volume to a non-injurious stress index in an open lung condition improves alveolar ventilation and prevents overdistension without increasing lung injury. This is in comparison with limited Pplat protective ventilation in a model of lung injury with low chest-wall compliance.

Airway Pressures as Surrogate Estimates of Intra-abdominal Pressure

Intra-abdominal pressure (IAP) measurements are essential to the diagnosis and management of patients with intra-abdominal hypertension (IAH) and abdominal compartment syndrome (ACS). Peak inspiratory pressure (PIP), plateau pressure (Pplat), and mean airway pressure (Paw) are used by some surgeons as surrogate estimates of IAP during abdominal closure. Thirty mechanically ventilated surgical/trauma patients with risk factors for IAH/ACS underwent simultaneous triplicate measurements of PIP, Pplat, Paw, and IAP. PIP, Pplat, and Paw were compared with IAP using both coefficient of determination and Bland and Altman analysis. The coefficient of determination for each airway pressure in predicting change in IAP was: PIP 5 per cent ( P = 0.24), Pplat 17 per cent ( P = 0.02), and Paw 15 per cent ( P = 0.03). Bland and Altman analysis identified that marked variability exists between airway pressure and IAP measurements: PIP 19.3 ± 18.7 mmHg, Pplat 11.1 ± 13.7 mmHg, and Paw 2.0 ± 9.8 mmHg. Airway pressures do not accurately reflect IAP and cannot be substituted for IAP measurements in patients at risk for IAH/ACS.

The assessment of transpulmonary pressure in mechanically ventilated ARDS patients

PURPOSE

The optimal method for estimating transpulmonary pressure (i.e. the fraction of the airway pressure transmitted to the lung) has not yet been established.

METHODS

In this study on 44 patients with acute respiratory distress syndrome (ARDS), we computed the end-inspiratory transpulmonary pressure as the change in airway and esophageal pressure from end-inspiration to atmospheric pressure (i.e. release derived) and as the product of the end-inspiratory airway pressure and the ratio of lung to respiratory system elastance (i.e. elastance derived). The end-expiratory transpulmonary pressure was estimated as the product of positive end-expiratory pressure (PEEP) minus the direct measurement of esophageal pressure and by the release method.

RESULTS

The mean elastance- and release-derived transpulmonary pressure were 14.4 ± 3.7 and 14.4 ± 3.8 cmH₂O at 5 cmH₂O of PEEP and 21.8 ± 5.1 and 21.8 ± 4.9 cmH₂O at 15 cmH₂O of PEEP, respectively (P = 0.32, P = 0.98, respectively), indicating that these parameters were significantly related (r(2) = 0.98, P < 0.001 at 5 cmH₂O of PEEP; r(2) = 0.93, P < 0.001 at 15 cmH₂O of PEEP). The percentage error was 5.6 and 12.0 %, respectively. The mean directly measured and release-derived transpulmonary pressure were -8.0 ± 3.8 and 3.9 ± 0.9 cmH₂O at 5 cmH₂O of PEEP and -1.2 ± 3.2 and 10.6 ± 2.2 cmH₂O at 15 cmH₂O of PEEP, respectively, indicating that these parameters were not related (r(2) = 0.07, P = 0.08 at 5 cmH₂O of PEEP; r (2) = 0.10, P = 0.53 at 15 cmH₂O of PEEP).

CONCLUSIONS

Based on our observations, elastance-derived transpulmonary pressure can be considered to be an adequate surrogate of the release-derived transpulmonary pressure, while the release-derived and directly measured end-expiratory transpulmonary pressure are not related.

The biological effects of higher and lower positive end-expiratory pressure in pulmonary and extrapulmonary acute lung injury with intra-abdominal hypertension

Introduction

Mechanical ventilation with high positive end-expiratory pressure (PEEP) has been used in patients with acute respiratory distress syndrome (ARDS) and intra-abdominal hypertension (IAH), but the role of PEEP in minimizing lung injury remains controversial. We hypothesized that in the presence of acute lung injury (ALI) with IAH: 1) higher PEEP levels improve pulmonary morphofunction and minimize lung injury; and 2) the biological effects of higher PEEP are more effective in extrapulmonary (exp) than pulmonary (p) ALI.

Methods

In 48 adult male Wistar rats, ALIp and ALIexp were induced by Escherichia coli lipopolysaccharide intratracheally and intraperitoneally, respectively. After 24 hours, animals were anesthetized and mechanically ventilated (tidal volume of 6 mL/kg). IAH (15 mmHg) was induced and rats randomly assigned to PEEP of 5 (PEEP5), 7 (PEEP7) or 10 (PEEP10) cmH₂O for 1 hour.

Results

In both ALIp and ALIexp, higher PEEP levels improved oxygenation. PEEP10 increased alveolar hyperinflation and epithelial cell damage compared to PEEP5, independent of ALI etiology. In ALIp, PEEP7 and PEEP10 increased lung elastance compared to PEEP5 (4.3 ± 0.7 and 4.3 ± 0.9 versus 3.1 ± 0.3 cmH₂O/mL, respectively, P <0.01), without changes in alveolar collapse, interleukin-6, caspase-3, type III procollagen, receptor for advanced glycation end-products, and vascular cell adhesion molecule-1 expressions. Moreover, PEEP10 increased diaphragmatic injury compared to PEEP5. In ALIexp, PEEP7 decreased lung elastance and alveolar collapse compared to PEEP5 (2.3 ± 0.5 versus 3.6 ± 0.7 cmH₂O/mL, P <0.02, and 27.2 (24.7 to 36.8) versus 44.2 (39.7 to 56.9)%, P <0.05, respectively), while PEEP7 and PEEP10 increased interleukin-6 and type III procollagen expressions, as well as type II epithelial cell damage compared to PEEP5.

Conclusions

In the current models of ALI with IAH, in contrast to our primary hypothesis, higher PEEP is more effective in ALIp than ALIexp as demonstrated by the activation of biological markers. Therefore, higher PEEP should be used cautiously in the presence of IAH and ALI, mainly in ALIexp.

Year in review 2012: Critical Care--Respirology

Acute respiratory failure is a dominant feature of critical illness. In this review, we discuss 17 studies published last year in Critical Care. The discussion focuses on articles on several topics: respiratory monitoring, acute respiratory distress syndrome, noninvasive ventilation, airway management, secretion management and weaning.

Value and limitations of transpulmonary pressure calculations during intra-abdominal hypertension

OBJECTIVE

To clarify the effect of progressively increasing intra-abdominal pressure on esophageal pressure, transpulmonary pressure, and functional residual capacity.

DESIGN

Controlled application of increased intra-abdominal pressure at two positive end-expiratory pressure levels (1 and 10 cm H2O) in an anesthetized porcine model of controlled ventilation.

SETTING

Large animal laboratory of a university-affiliated hospital.

SUBJECTS

Eleven deeply anesthetized swine (weight 46.2 ± 6.2 kg).

INTERVENTIONS

Air-regulated intra-abdominal hypertension (0-25 mm Hg).

MEASUREMENTS

Esophageal pressure, tidal compliance, bladder pressure, and end-expiratory lung aeration by gas dilution.

MAIN RESULTS

Functional residual capacity was significantly reduced by increasing intra-abdominal pressure at both positive end-expiratory pressure levels (p ≤ 0.0001) without corresponding changes of end-expiratory esophageal pressure. Above intra-abdominal pressure 5 mm Hg, plateau airway pressure increased linearly by ~ 50% of the applied intra-abdominal pressure value, associated with commensurate changes of esophageal pressure. With tidal volume held constant, negligible changes occurred in transpulmonary pressure due to intra-abdominal pressure. Driving pressures calculated from airway pressures alone (plateau airway pressure--positive end-expiratory pressure) did not equate to those computed from transpulmonary pressure (tidal changes in transpulmonary pressure). Increasing positive end-expiratory pressure shifted the predominantly negative end-expiratory transpulmonary pressure at positive end-expiratory pressure 1 cm H2O (mean -3.5 ± 0.4 cm H2O) into the positive range at positive end-expiratory pressure 10 cm H2O (mean 0.58 ± 1.2 cm H2O).

CONCLUSIONS

Despite its insensitivity to changes in functional residual capacity, measuring transpulmonary pressure may be helpful in explaining how different levels of positive end-expiratory pressure influence recruitment and collapse during tidal ventilation in the presence of increased intra-abdominal pressure and in calculating true transpulmonary driving pressure (tidal changes of transpulmonary pressure). Traditional interpretations of respiratory mechanics based on unmodified airway pressure were misleading regarding lung behavior in this setting.

Pleural pressure and optimal positive end-expiratory pressure based on esophageal pressure versus chest wall elastance: incompatible results*

OBJECTIVES

1) To compare two published methods for estimating pleural pressure, one based on directly measured esophageal pressure and the other based on chest wall elastance. 2) To evaluate the agreement between two published positive end-expiratory pressure optimization strategies based on these methods, one targeting an end-expiratory esophageal pressure-based transpulmonary pressure of 0 cm H2O and the other targeting an end-inspiratory elastance-based transpulmonary pressure of 26 cm H2O.

DESIGN

Retrospective study using clinical data.

SETTING

Medical and surgical ICUs.

PATIENTS

Sixty-four patients mechanically ventilated for acute respiratory failure with esophageal balloons placed for clinical management.

METHODS

Esophageal pressure and chest wall elastance-based methods for estimating pleural pressure and setting positive end-expiratory pressure were retrospectively applied to each of the 64 patients. In patients who were ventilated at two positive end-expiratory pressure levels, chest wall and respiratory system elastances were calculated at each positive end-expiratory pressure level.

MEASUREMENTS AND MAIN RESULTS

The pleural pressure estimates using both methods were discordant and differed by as much as 10 cm H2O for a given patient. The two positive end-expiratory pressure optimization strategies recommended positive end-expiratory pressure changes in opposite directions in 33% of patients. The ideal positive end-expiratory pressure levels recommended by the two methods for each patient were discordant and uncorrelated (R = 0.05). Chest wall and respiratory system elastances grew with increases in positive end-expiratory pressure in patients with positive end-expiratory esophageal pressure-based transpulmonary pressures (p < 0.05).

CONCLUSIONS

Esophageal pressure and chest wall elastance-based methods for estimating pleural pressure do not yield similar results. The strategies of targeting an end-expiratory esophageal pressure-based transpulmonary pressure of 0 cm H2O and targeting an end-inspiratory elastance-based transpulmonary pressure of 26 cm H2O cannot be considered interchangeable. Finally, chest wall and respiratory system elastances may vary unpredictably with changes in positive end-expiratory pressure.

Experimental intra-abdominal hypertension influences airway pressure limits for lung protective mechanical ventilation

BACKGROUND

Intra-abdominal hypertension (IAH) and abdominal compartment syndrome (ACS) may complicate monitoring of pulmonary mechanics owing to their impact on the respiratory system. However, recommendations for mechanical ventilation of patients with IAH/ACS and the interpretation of thoracoabdominal interactions remain unclear. Our study aimed to characterize the influence of elevated intra-abdominal pressure (IAP) and positive end-expiratory pressure (PEEP) on airway plateau pressure (PPLAT) and bladder pressure (PBLAD).

METHODS

Nine deeply anesthetized swine were mechanically ventilated via tracheostomy: volume-controlled mode at tidal volume (VT) of 10 mL/kg, frequency of 15, inspiratory-expiratory ratio of 1:2, and PEEP of 1 and 10 cm H2O (PEEP1 and PEEP10, respectively). A tracheostomy tube was placed in the peritoneal cavity, and IAP levels of 5, 10, 15, 20, and 25 mm Hg were applied, using a continuous positive airway pressure system. At each IAP level, PBLAD and airway pressure measurements were performed during both PEEP1 and PEEP10.

RESULTS

PBLAD increased as experimental IAP rose (y = 0.83x + 0.5; R = 0.98; p < 0.001 at PEEP1). Minimal underestimation of IAP by PBLAD was observed (-2.5 ± 0.8 mm Hg at an IAP of 10-25 mm Hg). Applying PEEP10 did not significantly affect the correlation between experimental IAP and PBLAD. Approximately 50% of the PBLAD (in cm H2O) was reflected by changes in PPLAT, regardless of the PEEP level applied. Increasing IAP did not influence hemodynamics at any level of IAP generated.

CONCLUSION

With minimal underestimation, PBLAD measurements closely correlated with experimentally regulated IAP, independent of the PEEP level applied. For each PEEP level applied, a constant proportion (approximately 50%) of measured PBLAD (in cm H2O) was reflected in PPLAT. A higher safety threshold for PPLAT should be considered in the setting of IAH/ACS as the clinician considers changes in VT. A strategy of reducing VT to cap PPLAT at widely recommended values may not be warranted in the setting of increased IAP.

Respiratory functions of burn patients undergoing decompressive laparotomy due to secondary abdominal compartment syndrome

INTRODUCTION

The development of secondary abdominal compartment syndrome (ACS) is associated with multiple organ dysfunction. There is little information about the effects of decompressive laparotomy (DL) on respiratory function (RF) in burn patients developing ACS.

PATIENTS AND METHODS

We retrospectively obtained data characterising RF from the database of an adult burn intensive care unit (BICU). Peak inspiratory pressure (Pip), PaO2/FiO2-ratio (P/F), static compliance (Cstat) and airway resistance (Raw) were analysed over the course of 60 h at 8 time points relative to DL.

RESULTS

Thirty-five patients with ACS underwent DL with a mean percentage of total burned body surface area (TBSA) 39 ± 23% and mean intra-abdominal pressure 33 ± 7 mmHg. All patients presented with significantly deteriorating RF within 12h of DL (Pip 33 ± 4 to 39 ± 7 cm/H2O, p=0.003; P/F 232 ± 59 to 160 ± 55 mmHg, p<0.001, Cstat 34 ± 5 to 26 ± 6 mL/cmH2O, p<0.001; Raw 18 ± 3 to 24 ± 9 cm H2O/L/s, p=0.02). All these parameters improved significantly (p<0.001) after DL, regardless of the presence of inhalation injury or torso burns. Mortality was 71.4%.

CONCLUSIONS

Variables characterising RF demonstrated a rapid deterioration before and a significant and sustained improvement after DL in burn patients developing ACS. Despite these respiratory improvements, DL was associated with low survival rates. Secondary ACS remains a challenge in burn patients and thus warrants particular attention during intensive care treatment.

The importance of timing of decompression in severe acute pancreatitis combined with abdominal compartment syndrome

BACKGROUND

Surgical decompression is widely considered as an important treatment in patients with severe acute pancreatitis (SAP) and abdominal compartment syndrome (ACS). Until now, the indication and optimal time of decompression remain unknown, and no experimental data exist, although extremely high mortality has been repeatedly reported in these patients. The aim of this study was to evaluate the effects of three different time points for decompression in a 24-hour lasting porcine model.

METHODS

Following baseline registrations, 32 animals were divided into four groups (8 animals each group) as follows: one SAP-alone group and three SAP + ACS groups, which received decompression at 6, 9, and 12 hours. We used a N2 pneumoperitoneum to increase the intra-abdominal pressure to 25 mm Hg and retrograde intra-ductal infusion of sodium taurocholate to induce SAP. Global hemodynamic profiles, urine output, systemic oxygenation, and serum biochemical parameters of the animals were studied. At the end of the experiment, histologic examination of the intestine and lung was performed.

RESULTS

The survival time of the 12-hour group was significantly shortened (p = 0.037 vs. 9 hours and p = 0.008 vs. 6 hours). In SAP + ACS animals, decompression at 6 hours restored systemic hemodynamics, oxygen-derived parameters, organ function, and inflammatory intensity to a level comparable with that of the SAP-alone group. In contrast, animals in the 9 hours and 12 hours developed more severe hemodynamic and organ dysfunction. The histopathologic analyses also revealed higher grade injury of the intestine and lung in animals receiving delayed decompression.

CONCLUSION

Well-timed decompression in a porcine model of SAP incorporating 25-mm Hg intra-abdominal hypertension/ACS was associated with significantly reduced mortality, improved systemic hemodynamics and organ function, as well as alleviated histologic injury and inflammatory intensity. According to our results and previous reports, both too early and too late decompression should be avoided owing to significant morbidity for the former and unfavorable outcomes for the latter.

Can selection of mechanical ventilation mode prevent increased intra-abdominal pressure in patients admitted to the intensive care unit?

BACKGROUND

Increased intra-abdominal pressure (IAP) results in dysfunction of vital organs. The aim of the present study was to evaluate the effect of mechanical ventilation mode on IAP.

METHODS

In a cohort study, a total of 60 patients aged 20-70 years who were admitted to the ICU and underwent mechanical ventilation were recruited. Mechanical ventilation included one of the three modes: Biphasic positive airway pressure (BIPAP) group, synchronize intermittent mandatory ventilation (SIMV) group, or continuous positive airway pressure (CPAP) group. For each patient, mechanical ventilation mode and its parameters, blood pressure, SpO2, and status of tube feeding and IAP were recorded.

RESULTS

Our findings indicate that the study groups were not significantly different in terms of anthropometric characteristics including age (64.5 ± 4, P = 0.1), gender (male/female 31/29, P = 0.63), and body mass index (24 ± 1.2, P = 0.11). Increase IAP was related to the type of respiratory mode with the more increased IAP observed in SIMV mode, followed by BIPAP and CPAP modes (P = 0.01). There were significant correlations between increased IAP and respiratory variables including respiratory rate, pressure support ventilation, and inspiratory pressure (P < 0.05). Tube feeding tolerance through NG-tube was lower in SIMV group, followed by BIPAP and CPAP groups (P < 0.05).

CONCLUSIONS

There is a significant relationship between respiratory modes and IAP; therefore, it is better to utilize those types of mechanical ventilation like CPAP and BIPAP mode in patients who are prone to Intra-abdominal hypertension.

Combined effects of ventilation mode and positive end-expiratory pressure on mechanics, gas exchange and the epithelium in mice with acute lung injury

The accepted protocol to ventilate patients with acute lung injury is to use low tidal volume (V(T)) in combination with recruitment maneuvers or positive end-expiratory pressure (PEEP). However, an important aspect of mechanical ventilation has not been considered: the combined effects of PEEP and ventilation modes on the integrity of the epithelium. Additionally, it is implicitly assumed that the best PEEP-V(T) combination also protects the epithelium. We aimed to investigate the effects of ventilation mode and PEEP on respiratory mechanics, peak airway pressures and gas exchange as well as on lung surfactant and epithelial cell integrity in mice with acute lung injury. HCl-injured mice were ventilated at PEEPs of 3 and 6 cmH(2)O with conventional ventilation (CV), CV with intermittent large breaths (CV(LB)) to promote recruitment, and a new mode, variable ventilation, optimized for mice (VV(N)). Mechanics and gas exchange were measured during ventilation and surfactant protein (SP)-B, proSP-B and E-cadherin levels were determined from lavage and lung homogenate. PEEP had a significant effect on mechanics, gas exchange and the epithelium. The higher PEEP reduced lung collapse and improved mechanics and gas exchange but it also down regulated surfactant release and production and increased epithelial cell injury. While CV(LB) was better than CV, VV(N) outperformed CV(LB) in recruitment, reduced epithelial injury and, via a dynamic mechanotransduction, it also triggered increased release and production of surfactant. For long-term outcome, selection of optimal PEEP and ventilation mode may be based on balancing lung physiology with epithelial injury.

Matching positive end-expiratory pressure to intra-abdominal pressure improves oxygenation in a porcine sick lung model of intra-abdominal hypertension

INTRODUCTION

Intra-abdominal hypertension (IAH) causes atelectasis, reduces lung volumes and increases respiratory system elastance. Positive end-expiratory pressure (PEEP) in the setting of IAH and healthy lungs improves lung volumes but not oxygenation. However, critically ill patients with IAH often suffer from acute lung injury (ALI). This study, therefore, examined the respiratory and cardiac effects of positive end-expiratory pressure in an animal model of IAH, with sick lungs.

METHODS

Nine pigs were anesthetized and ventilated (48 +/- 6 kg). Lung injury was induced with oleic acid. Three levels of intra-abdominal pressure (baseline, 18, and 22 mmHg) were randomly generated. At each level of intra-abdominal pressure, three levels of PEEP were randomly applied: baseline (5 cmH2O), moderate (0.5 × intra-abdominal pressure), and high (1.0 × intra-abdominal pressure). We measured end-expiratory lung volumes, arterial oxygen levels, respiratory mechanics, and cardiac output 10 minutes after each new IAP and PEEP setting.

RESULTS

At baseline PEEP, IAH (22 mmHg) decreased oxygen levels (-55%, P <0.001) and end-expiratory lung volumes (-45%, P = 0.007). At IAP of 22 mmHg, moderate and high PEEP increased oxygen levels (+60%, P = 0.04 and +162%, P <0.001) and end-expiratory lung volume (+44%, P = 0.02 and +279%, P <0.001) and high PEEP reduced cardiac output (-30%, P = 0.04). Shunt and dead-space fraction inversely correlated with oxygen levels and end-expiratory lung volumes. In the presence of IAH, lung, chest wall and respiratory system elastance increased. Subsequently, PEEP decreased respiratory system elastance by decreasing chest wall elastance.

CONCLUSIONS

In a porcine sick lung model of IAH, PEEP matched to intra-abdominal pressure led to increased lung volumes and oxygenation and decreased chest wall elastance shunt and dead-space fraction. High PEEP decreased cardiac output. The study shows that lung injury influences the effects of IAH and PEEP on oxygenation and respiratory mechanics. Our findings support the application of PEEP in the setting of acute lung injury and IAH.

Matching positive end-expiratory pressure to intra-abdominal pressure prevents end-expiratory lung volume decline in a pig model of intra-abdominal hypertension

OBJECTIVE

Intra-abdominal hypertension is common in critically ill patients and is associated with increased morbidity and mortality. In a previous experimental study, positive end-expiratory pressures of up to 15 cm H2O did not prevent end-expiratory lung volume decline caused by intra-abdominal hypertension. Therefore, we examined the effect of matching positive end-expiratory pressure to the intra-abdominal pressure on cardio-respiratory parameters.

DESIGN

Experimental pig model of intra-abdominal hypertension.

SETTING

Large animal facility, University of Western Australia.

SUBJECTS

Nine anesthetized, nonparalyzed, and ventilated pigs (48 ± 7 kg).

INTERVENTIONS

Four levels of intra-abdominal pressure (baseline, 12, 18, and 22 mm Hg) were generated in a randomized order by inflating an intra-abdominal balloon. At each level of intra-abdominal pressure, three levels of positive end-expiratory pressure were randomly applied with varying degrees of matching the corresponding intra-abdominal pressure: baseline positive end-expiratory pressure (= 5 cm H2O), moderate positive end-expiratory pressure (= half intra-abdominal pressure in cm H2O + 5 cm H2O), and high positive end-expiratory pressure (= intra-abdominal pressure in cm H2O).

MEASUREMENTS

We measured end-expiratory lung volume, arterial oxygen levels, respiratory mechanics, and cardiac output 5 mins after each new intra-abdominal pressure and positive end-expiratory pressure setting.

MAIN RESULTS

Intra-abdominal hypertension decreased end-expiratory lung volume and PaO2 (-49% [p < .001] and -8% [p < .05], respectively, at 22 mm Hg intra-abdominal pressure compared with baseline intra-abdominal pressure) but did not change cardiac output (p = .5). At each level of intra-abdominal pressure, moderate positive end-expiratory pressure increased end-expiratory lung volume (+119% [p < .001] at 22 mm Hg intra-abdominal pressure compared with 5 cm H2O positive end-expiratory pressure) while minimally decreasing cardiac output (-8%, p < .05). High positive end-expiratory pressure further increased end-expiratory lung volume (+233% [p < .001] at 22 mm Hg intra-abdominal pressure compared with 5 cm H2O positive end-expiratory pressure) but led to a greater decrease in cardiac output (-26%, p < .05). Neither moderate nor high positive end-expiratory pressure improved PaO2 (p = .7). Intra-abdominal hypertension decreased end-expiratory transpulmonary pressure but did not alter end-inspiratory transpulmonary pressure. Intra-abdominal hypertension decreased total respiratory compliance through a decrease in chest wall compliance. Positive end-expiratory pressure decreased the respiratory compliance by reducing lung compliance.

CONCLUSIONS

In a pig model of intra-abdominal hypertension, positive end-expiratory pressure matched to intra-abdominal pressure led to a preservation of end-expiratory lung volume, but did not improve arterial oxygen tension and caused a reduction in cardiac output. Therefore, we do not recommend routine application of positive end-expiratory pressure matched to intra-abdominal pressure to prevent intra-abdominal pressure-induced end-expiratory lung volume decline in healthy lungs.

Lung parenchymal mechanics

The lung parenchyma comprises a large number of thin-walled alveoli, forming an enormous surface area, which serves to maintain proper gas exchange. The alveoli are held open by the transpulmonary pressure, or prestress, which is balanced by tissues forces and alveolar surface film forces. Gas exchange efficiency is thus inextricably linked to three fundamental features of the lung: parenchymal architecture, prestress, and the mechanical properties of the parenchyma. The prestress is a key determinant of lung deformability that influences many phenomena including local ventilation, regional blood flow, tissue stiffness, smooth muscle contractility, and alveolar stability. The main pathway for stress transmission is through the extracellular matrix. Thus, the mechanical properties of the matrix play a key role both in lung function and biology. These mechanical properties in turn are determined by the constituents of the tissue, including elastin, collagen, and proteoglycans. In addition, the macroscopic mechanical properties are also influenced by the surface tension and, to some extent, the contractile state of the adherent cells. This chapter focuses on the biomechanical properties of the main constituents of the parenchyma in the presence of prestress and how these properties define normal function or change in disease. An integrated view of lung mechanics is presented and the utility of parenchymal mechanics at the bedside as well as its possible future role in lung physiology and medicine are discussed.

Chest wall mechanics and abdominal pressure during general anaesthesia in normal and obese individuals and in acute lung injury

PURPOSE OF REVIEW

This article discusses the methods available to evaluate chest wall mechanics and the relationship between intraabdominal pressure (IAP) and chest wall mechanics during general anaesthesia in normal and obese individuals, as well as in acute lung injury/acute respiratory distress syndrome.

RECENT FINDINGS

The interactions between the abdominal and thoracic compartments pose a specific challenge for intensive care physicians. IAP affects respiratory system, lung and chest wall elastance in an unpredictable way. Thus, transpulmonary pressure should be measured if IAP is more than 12 mmHg or if chest wall elastance is compromised for other reasons, even though the absolute values of pleural and transpulmonary pressures are not easily obtained at bedside. We suggest defining intraabdominal hypertension (IAH) as IAP at least 20 mmHg and abdominal compartment syndrome (ACS) as IAP at least 20 mmHg associated with failure of one or more organs, although further studies are required to confirm this hypothesis. Additionally, in the presence of IAH, controlled mechanical ventilation should be applied and positive end-expiratory pressure individually titrated. Prophylactic open abdomen should be considered in the presence of ACS.

SUMMARY

Increased IAP markedly affects respiratory function and complicates patient management. Frequent assessment of IAP is recommended.

Pros and cons of recruitment maneuvers in acute lung injury and acute respiratory distress syndrome

In patients with acute lung injury and acute respiratory distress syndrome, a protective mechanical ventilation strategy characterized by low tidal volumes has been associated with reduced mortality. However, such a strategy may result in alveolar collapse, leading to cyclic opening and closing of atelectatic alveoli and distal airways. Thus, recruitment maneuvers (RMs) have been used to open up collapsed lungs, while adequate positive end-expiratory pressure (PEEP) levels may counteract alveolar derecruitment during low tidal volume ventilation, improving respiratory function and minimizing ventilator-associated lung injury. Nevertheless, considerable uncertainty remains regarding the appropriateness of RMs. The most commonly used RM is conventional sustained inflation, associated with respiratory and cardiovascular side effects, which may be minimized by newly proposed strategies: prolonged or incremental PEEP elevation; pressure-controlled ventilation with fixed PEEP and increased driving pressure; pressure-controlled ventilation applied with escalating PEEP and constant driving pressure; and long and slow increase in pressure. The efficiency of RMs may be affected by different factors, including the nature and extent of lung injury, capability of increasing inspiratory transpulmonary pressures, patient positioning and cardiac preload. Current evidence suggests that RMs can be used before setting PEEP, after ventilator circuit disconnection or as a rescue maneuver to overcome severe hypoxemia; however, their routine use does not seem to be justified at present. The development of new lung recruitment strategies that have fewer hemodynamic and biological effects on the lungs, as well as randomized clinical trials analyzing the impact of RMs on morbidity and mortality of acute lung injury/acute respiratory distress syndrome patients, are warranted.

Commonly applied positive end-expiratory pressures do not prevent functional residual capacity decline in the setting of intra-abdominal hypertension: a pig model

INTRODUCTION

Intra-abdominal hypertension is common in critically ill patients and is associated with increased morbidity and mortality. The optimal ventilation strategy remains unclear in these patients. We examined the effect of positive end-expiratory pressures (PEEP) on functional residual capacity (FRC) and oxygen delivery in a pig model of intra-abdominal hypertension.

METHODS

Thirteen adult pigs received standardised anaesthesia and ventilation. We randomised three levels of intra-abdominal pressure (3 mmHg (baseline), 18 mmHg, and 26 mmHg) and four commonly applied levels of PEEP (5, 8, 12 and 15 cmH2O). Intra-abdominal pressures were generated by inflating an intra-abdominal balloon. We measured intra-abdominal (bladder) pressure, functional residual capacity, cardiac output, haemoglobin and oxygen saturation, and calculated oxygen delivery.

RESULTS

Raised intra-abdominal pressure decreased FRC but did not change cardiac output. PEEP increased FRC at baseline intra-abdominal pressure. The decline in FRC with raised intra-abdominal pressure was partly reversed by PEEP at 18 mmHg intra-abdominal pressure and not at all at 26 mmHg intra-abdominal pressure. PEEP significantly decreased cardiac output and oxygen delivery at baseline and at 26 mmHg intra-abdominal pressure but not at 18 mmHg intra-abdominal pressure.

CONCLUSIONS

In a pig model of intra-abdominal hypertension, PEEP up to 15 cmH2O did not prevent the FRC decline caused by intra-abdominal hypertension and was associated with reduced oxygen delivery as a consequence of reduced cardiac output. This implies that PEEP levels inferior to the corresponding intra-abdominal pressures cannot be recommended to prevent FRC decline in the setting of intra-abdominal hypertension.