Pulmonary densities during anesthesia with muscular relaxation--a proposal of atelectasis

Electrical Impedance Tomography-guided PEEP Titration in Patients Undergoing Laparoscopic Abdominal Surgery

The aim of the study is to utilize electrical impedance tomography (EIT) to guide positive end-expiratory pressure (PEEP) and to optimize oxygenation in patients undergoing laparoscopic abdominal surgery.Fifty patients were randomly assigned to the control (C) group and the EIT (E) group (n = 25 each). We set the fraction of inspired oxygen (FiO2) at 0.30. The PEEP was titrated and increased in a 2-cm H2O stepwise manner, from 6 to 14 cm H2O. Hemodynamic variables, respiratory mechanics, EIT images, analysis of blood gas, and regional cerebral oxygen saturation were recorded. The postoperative pulmonary complications within the first 5 days were also observed.We chose 10 cm H2O and 8 cm H2O as the "ideal" PEEP for the C and the E groups, respectively. EIT-guided PEEP titration led to a more dorsal shift of ventilation. The PaO2/FiO2 ratio in the E group was superior to that in the C group in the pneumoperitoneum period, though the difference was not significant (330 ± 10 vs 305.56 ± 4 mm Hg; P = 0.09). The C group patients experienced 8.7% postoperative pulmonary complications versus 5.3% among the E group patients (relative risk 1.27, 95% confidence interval 0.31-5.3, P = 0.75).Electrical impedance tomography represents a new promising technique that could enable anesthesiologists to assess regional ventilation of the lungs and optimize global oxygenation for patients undergoing laparoscopic abdominal surgery.

Mechanical ventilation strategies for the surgical patient

PURPOSE OF REVIEW

To summarize clinical evidence for intraoperative ventilation settings, which could protect against postoperative pulmonary complications (PPCs) in surgical patients with uninjured lungs.

RECENT FINDINGS

There is convincing evidence for protection against PPCs by low tidal volumes: benefit was found in several randomized controlled trials, and was recently confirmed in meta-analyses. Evidence for protection against PPCs by high levels of positive end-expiratory pressure (PEEP) is less definite. Although benefit was found in several randomized controlled trials, most of them compared a bundle of low tidal volume and high level of PEEP with conventional ventilation; one recent large randomized controlled trial that compared high with low levels of PEEP showed that ventilation with high level of PEEP did not protect against PPCs but caused intraoperative complications instead. A recent individual patient data meta-analysis of trials comparing bundles of low tidal volume and high levels of PEEP to conventional intraoperative ventilation suggested that protection against PPCs comes from tidal volume reductions, and not from increasing levels of PEEP.

SUMMARY

The understanding on the protective roles of tidal volume and PEEP settings against PPCs has rapidly expanded. During intraoperative ventilation, low tidal volumes are protective, the protective role of high levels of PEEP is uncertain.

Real-time images of tidal recruitment using lung ultrasound

Background

Ventilator-induced lung injury is a form of mechanical damage leading to a pulmonary inflammatory response related to the use of mechanical ventilation enhanced by the presence of atelectasis. One proposed mechanism of this injury is the repetitive opening and closing of collapsed alveoli and small airways within these atelectatic areas—a phenomenon called tidal recruitment. The presence of tidal recruitment is difficult to detect, even with high-resolution images of the lungs like CT scan. The purpose of this article is to give evidence of tidal recruitment by lung ultrasound.

Findings

A standard lung ultrasound inspection detected lung zones of atelectasis in mechanically ventilated patients. With a linear probe placed in the intercostal oblique position. We observed tidal recruitment within atelectasis as an improvement in aeration at the end of inspiration followed by the re-collapse at the end of expiration. This mechanism disappeared after the performance of a lung recruitment maneuver.

Conclusions

Lung ultrasound was helpful in detecting the presence of atelectasis and tidal recruitment and in confirming their resolution after a lung recruitment maneuver.

Electronic supplementary material

The online version of this article (doi:10.1186/s13089-015-0036-2) contains supplementary material, which is available to authorized users.

Intraoperative use of low volume ventilation to decrease postoperative mortality, mechanical ventilation, lengths of stay and lung injury in patients without acute lung injury

BACKGROUND

During the last decade, there has been a trend towards decreasing tidal volumes for positive pressure ventilation during surgery. It is not known whether this new trend is beneficial or harmful for patients.

OBJECTIVES

To assess the benefit of intraoperative use of low tidal volume ventilation (< 10 mL/kg of predicted body weight) to decrease postoperative complications.

SEARCH METHODS

We searched the Cochrane Central Register of Controlled Trials (CENTRAL 2014, Issue 9), MEDLINE (OvidSP) (from 1946 to 5 September 2014) and EMBASE (OvidSP) (from 1974 to 5 September 2014).

SELECTION CRITERIA

We included all parallel randomized controlled trials (RCTs) that evaluated the effect of low tidal volumes (defined as < 10 mL/kg) on any of our selected outcomes in adult participants undergoing any type of surgery. We did not retain studies with participants requiring one-lung ventilation.

DATA COLLECTION AND ANALYSIS

Two authors independently assessed the quality of the retained studies with the Cochrane 'Risk of bias' tool. We analysed data with both fixed-effect (I(2) statistic < 25%) or random-effects (I(2) statistic > 25%) models based on the degree of heterogeneity. When there was an effect, we calculated a number needed to treat for an additional beneficial outcome (NNTB) using the odds ratio. When there was no effect, we calculated the optimal size information.

MAIN RESULTS

We included 12 studies in the review. In total these studies detailed 1012 participants (499 participants in the low tidal volume group and 513 in the high volume group). All studies included were at risk of bias as defined by the Cochrane tool. Based on nine studies including 899 participants, we found no difference in 0- to 30-day mortality between low and high tidal volume groups (risk ratio (RR) 0.79, 95% confidence interval (CI) 0.40 to 1.54; I(2) statistic 0%; low quality evidence). Based on four studies including 601 participants undergoing abdominal or spinal surgery, we found a lower incidence of postoperative pneumonia in the lower tidal volume group (RR 0.44, 95% CI 0.20 to 0.99; I(2) statistic 19%; moderate quality evidence; NNTB 19, 95% CI 14 to 169). Based on two studies including 428 participants, low tidal volumes decreased the need for non-invasive postoperative ventilatory support (RR 0.31, 95% CI 0.15 to 0.64; moderate quality evidence; NNTB 11, 95% CI 9 to 19). Based on eight studies including 814 participants, low tidal volumes during surgery decreased the need for postoperative invasive ventilatory support (RR 0.33, 95% CI 0.14 to 0.80; I(2) statistic 0%; NNTB 36, 95% CI 27 to 202; moderate quality evidence). Based on three studies including 650 participants, we found no difference in the intensive care unit length of stay (standardized mean difference (SMD) -0.01, 95% CI -0.22 to 0.20; I(2) statistic = 42%; moderate quality evidence). Based on eight studies including 846 participants, we did not find a difference in hospital length of stay (SMD -0.16, 95% CI -0.40 to 0.07; I(2) statistic 52%; moderate quality evidence). A meta-regression showed that the effect size increased proportionally to the peak pressure measured at the end of surgery in the high volume group. We did not find a difference in the risk of pneumothorax (RR 2.01, 95% CI 0.51 to 7.95; I(2) statistic 0%; low quality evidence).

AUTHORS' CONCLUSIONS

Low tidal volumes (defined as < 10 mL/kg) should be used preferentially during surgery. They decrease the need for postoperative ventilatory support (invasive and non-invasive). Further research is required to determine the maximum peak pressure of ventilation that should be allowed during surgery.

Intraoperative protective mechanical ventilation for prevention of postoperative pulmonary complications: a comprehensive review of the role of tidal volume, positive end-expiratory pressure, and lung recruitment maneuvers

Postoperative pulmonary complications are associated with increased morbidity, length of hospital stay, and mortality after major surgery. Intraoperative lung-protective mechanical ventilation has the potential to reduce the incidence of postoperative pulmonary complications. This review discusses the relevant literature on definition and methods to predict the occurrence of postoperative pulmonary complication, the pathophysiology of ventilator-induced lung injury with emphasis on the noninjured lung, and protective ventilation strategies, including the respective roles of tidal volumes, positive end-expiratory pressure, and recruitment maneuvers. The authors propose an algorithm for protective intraoperative mechanical ventilation based on evidence from recent randomized controlled trials.

Lung Injury After One-Lung Ventilation: A Review of the Pathophysiologic Mechanisms Affecting the Ventilated and the Collapsed Lung

Lung injury is the leading cause of death after thoracic surgery. Initially recognized after pneumonectomy, it has since been described after any period of 1-lung ventilation (OLV), even in the absence of lung resection. Overhydration and high tidal volumes were thought to be responsible at various points; however, it is now recognized that the pathophysiology is more complex and multifactorial. All causative mechanisms known to trigger ventilator-induced lung injury have been described in the OLV setting. The ventilated lung is exposed to high strain secondary to large, nonphysiologic tidal volumes and loss of the normal functional residual capacity. In addition, the ventilated lung experiences oxidative stress, as well as capillary shear stress because of hyperperfusion. Surgical manipulation and/or resection of the collapsed lung may induce lung injury. Re-expansion of the collapsed lung at the conclusion of OLV invariably induces duration-dependent, ischemia-reperfusion injury. Inflammatory cytokines are released in response to localized injury and may promote local and contralateral lung injury. Protective ventilation and volatile anesthesia lessen the degree of injury; however, increases in biochemical and histologic markers of lung injury appear unavoidable. The endothelial glycocalyx may represent a common pathway for lung injury creation during OLV, because it is damaged by most of the recognized lung injurious mechanisms. Experimental therapies to stabilize the endothelial glycocalyx may afford the ability to reduce lung injury in the future. In the interim, protective ventilation with tidal volumes of 4 to 5 mL/kg predicted body weight, positive end-expiratory pressure of 5 to 10 cm H2O, and routine lung recruitment should be used during OLV in an attempt to minimize harmful lung stress and strain. Additional strategies to reduce lung injury include routine volatile anesthesia and efforts to minimize OLV duration and hyperoxia.

Anatomy and physiology of respiratory system relevant to anaesthesia

Clinical application of anatomical and physiological knowledge of respiratory system improves patient's safety during anaesthesia. It also optimises patient's ventilatory condition and airway patency. Such knowledge has influence on airway management, lung isolation during anaesthesia, management of cases with respiratory disorders, respiratory endoluminal procedures and optimising ventilator strategies in the perioperative period. Understanding of ventilation, perfusion and their relation with each other is important for understanding respiratory physiology. Ventilation to perfusion ratio alters with anaesthesia, body position and with one-lung anaesthesia. Hypoxic pulmonary vasoconstriction, an important safety mechanism, is inhibited by majority of the anaesthetic drugs. Ventilation perfusion mismatch leads to reduced arterial oxygen concentration mainly because of early closure of airway, thus leading to decreased ventilation and atelectasis during anaesthesia. Various anaesthetic drugs alter neuronal control of the breathing and bronchomotor tone.

Effect of intraoperative PEEP application on colonic anastomoses healing: An experimental animal study

PURPOSE

This study aimed to assess the effect of intraoperative PEEP intervention on the healing of colonic anastomoses in rabbits.

MATERIALS AND METHODS

Thirty-two New Zealand type male rabbits were divided into two groups of sixteen animals each. Following ventilation with tracheostomy, colonic resection and anastomosis were performed in both groups. While 10 cm H2O PEEP level was applied in Group I (PEEP), Group II (ZEEP) was ventilated without PEEP throughout the surgery. Half of the both PEEP and ZEEP group animals were killed on the third postoperative day, while the remaining half on the seventh. Anastomotic bursting pressures, the tissue concentrations in hydroxyproline, and histological assessments were performed. Besides, intraoperative oxygen saturation and postoperative arterial blood gas parameters were also compared.

RESULTS

On the first postoperative day, both arterial oxygen tension (PO2) and oxygen saturation (SO2) in the PEEP group were significantly higher than in the ZEEP group. On the seventh postoperative day, the bursting pressures of the anastomoses were significantly higher in the PEEP group, however the hydroxyproline content was significantly lower in the PEEP group than that in the ZEEP group. At day 7, PEEP group was significantly associated with increased neoangiogenesis compared with the ZEEP group.

CONCLUSION

The anastomotic healing process is positively influenced by the intraoperative PEEP application.

Respiratory management of the obese patient undergoing surgery

As a reflection of the increasing global incidence of obesity, there has been a corresponding rise in the proportion of obese patients undergoing major surgery. This review reports the physiological effect of these changes in body composition on the respiratory system and discusses the clinical approach required to maximize safety and minimize the risk to the patient. The changes in respiratory system compliance and lung volumes, which can adversely affect pulmonary gas exchange, combined with upper airways obstruction and sleep-disordered breathing need to be considered carefully in the peri-operative period. Indeed, these challenges in the obese patient have led to a clear focus on the clinical management strategy and development of peri-operative pathways, including pre-operative risk assessment, patient positioning at induction and under anesthesia, modified approach to intraoperative ventilation and the peri-operative use of non-invasive ventilation (NIV) and continuous positive airways pressure.

Comparison of radiographic and computed tomographic images of the lungs in healthy neonatal foals

OBJECTIVE

To compare CT and radiographic images of the lungs in sedated healthy foals positioned in sternal recumbency and to investigate whether a relationship exists between CT-derived measurements of lung attenuation and Paco2 and Pao2.

ANIMALS

6 healthy Standardbred foals < 14 days of age.

PROCEDURES

Thoracic CT images were acquired followed by radiographic views with each foal sedated and positioned in sternal recumbency. For each foal, both CT and radiographic images were evaluated for severity and extent of changes by lung regions on the basis of a subjective scoring system by 3 investigators. Quantitative analysis of CT images was also performed. Assessments of Pao2 and Paco2 were performed before sedation, following sedation prior to CT, and after CT prior to radiography.

RESULTS

Interobserver agreement for CT and radiographic image scoring was strong (0.73) and fair (0.65), respectively; intraobserver agreement was near perfect for CT (0.97) and radiographic (0.94) image scoring. Increased CT attenuation and radiographic changes were identified for all foals and were preferentially distributed in the caudoventral portion of the lungs. Radiographic scores were significantly lower than CT image scores. A positive correlation (r = 0.872) between lung attenuation and CT image score was identified. A significant increase in Paco2 was not considered clinically relevant. Significant changes in Pao2 were not observed.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggested that interpretation of CT images may be less subjective, compared with interpretation of radiographic images. These findings may aid in the evaluation of CT and radiographic images of neonatal foals with respiratory tract disease.

Impact of Extraperitoneal Dioxyde Carbon Insufflation on Respiratory Function in Anesthetized Adults: A Preliminary Study Using Electrical Impedance Tomography and Wash-out/Wash-in Technic

Background:

Extraperitoneal laparoscopy has become a common technique for many surgical procedures, especially for inguinal hernia surgery. Investigations of physiological changes occurring during extraperitoneal carbon dioxide (CO₂) insufflation mostly focused on blood gas changes. To date, the impact of extraperitoneal CO₂ insufflation on respiratory mechanics remains unknown, whereas changes in respiratory mechanics have been extensively studied in intraperitoneal insufflation.

Objectives:

The aim of this study was to investigate the effects of extraperitoneal CO₂ insufflation on respiratory mechanics.

Patients and Methods:

A prospective and observational study was performed on nine patients undergoing laparoscopic inguinal hernia repair. Anesthetic management and intraoperative care were standardized. All patients were mechanically ventilated with a tidal volume of 8 mL/kg using an Engström Carestation ventilator (GE Healthcare). Ventilation distribution was assessed by electrical impedance tomography (EIT). End-expiratory lung volume (EELV) was measured by a nitrogen wash-out/wash-in method. Ventilation distribution, EELV, ventilator pressures and hemodynamic parameters were assessed before extraperitoneal insufflation, and during insufflation with a PEEP of 0 cmH₂O, 5 cmH₂0 and of 10 cmH₂0.

Results:

EELV and thoracopulmonary compliance were significantly decreased after extraperitoneal insufflation. Ventilation distribution was significantly higher in ventral lung regions during general anesthesia and was not modified after insufflation. A 10 cmH₂0 PEEP application resulted in a significant increase in EELV, and a shift of ventilation toward the dorsal lung regions.

Conclusions:

Extraperitoneal insufflation decreased EELV and thoracopulmonary compliance. Application of a 10 cmH₂0 PEEP increased EELV and homogenized ventilation distribution. This preliminary clinical study showed that extraperitoneal insufflation worsened respiratory mechanics, which may justify further investigations to evaluate the clinical impact.

Monitoring Lung Volumes During Mechanical Ventilation

Respiratory inductive plethysmography (RIP) is a non-invasive method of measuring change in lung volume which is well-established as a monitor of tidal ventilation and thus respiratory patterns in sleep medicine. As RIP is leak independent, can measure end-expiratory lung volume as well as tidal volume and is applicable to both the ventilated and spontaneously breathing patient, there has been a recent interest in its use as a bedside tool in the intensive care unit.

Intraoperative positive end-expiratory pressure evaluation using the intratidal compliance-volume profile

BACKGROUND

Lung-protective mechanical ventilation during general surgery including the application of PEEP can reduce postoperative pulmonary complications. In a prospective clinical observation study, we evaluated volume-dependent respiratory system compliance in adult patients undergoing ear-nose-throat surgery with ventilation settings chosen empirically by the attending anaesthetist.

METHODS

In 40 patients, we measured the respiratory variables during intraoperative mechanical ventilation. All measurements were subdivided into 5 min intervals. Dynamic compliance (CRS) and the intratidal volume-dependent CRS curve was calculated for each interval and classified into one of the six specific compliance profiles indicating intratidal recruitment/derecruitment, overdistension or all. We retrospectively compared the occurrences of the respective compliance profiles at PEEP levels of 5 cm H2O and at higher levels.

RESULTS

The attending anaesthetists set the PEEP level initially to 5 cm H2O in 29 patients (83%), to 7 cm H2O in 5 patients (14%), and to 8 cm H2O in 2 patients (6%). Across all measurements the mean CRS was 61 (11) ml cm H2O(-1) (40-86 ml cm H2O(-1)) and decreased continuously during the procedure. At PEEP of 5 cm H2O the compliance profile indicating strong intratidal recruitment/derecruitment occurred more often (18.6%) compared with higher PEEP levels (5.5%, P<0.01). Overdistension was practically never observed.

CONCLUSIONS

In most patients, a PEEP of 5 cm H2O during intraoperative mechanical ventilation is too low to prevent intratidal recruitment/derecruitment. The analysis of the intratidal compliance profile provides the rationale to individually titrate a PEEP level that stabilizes the alveolar recruitment status of the lung during intraoperative mechanical ventilation.

TRIAL REGISTRATION NUMBER

DRKS00004286.

Pressure safety range of barotrauma with lung recruitment manoeuvres: a randomised experimental study in a healthy animal model

CONTEXT

Recruitment manoeuvres aim at reversing atelectasis during general anaesthesia but are associated with potential risks such as barotrauma.

OBJECTIVE

To explore the range of pressures that can be used safely to fully recruit the lung without causing barotrauma in an ex-vivo healthy lung rabbit model.

DESIGN

Prospective, randomised, experimental study.

SETTING

Experimental Unit, La Paz University Hospital, Madrid, Spain.

ANIMALS

Fourteen healthy young New Zealand rabbits of 12 weeks of age.

INTERVENTIONS

Animals were euthanised, the thorax and both pleural spaces were opened and the animals were allocated randomly into one of two groups submitted to two distinct recruitment manoeuvre strategies: PEEP-20 group, in which positive end-expiratory pressure (PEEP) was increased in 5-cmH2O steps from 0 to 20 cmH2O and PEEP-50 group, in which PEEP was increased in 5-cmH2O steps from 0 to 50 cmH2O. In both groups, a driving pressure of 15 cmH2O was maintained until maximal PEEP and its corresponding maximal inspiratory pressures (MIPs) were reached. From there on, driving pressure was progressively increased in 5-cmH2O steps until detectable barotrauma occurred. Two macroscopic conditions were defined: anatomically open lung and barotrauma.

MAIN OUTCOME MEASURES

We measured open lung and barotrauma MIP, PEEP and driving pressure obtained using each strategy. A pressure safety range, defined as the difference between barotrauma MIP and anatomically open lung MIP, was also determined in both groups.

RESULTS

Open lung MIP was similar in both groups: 23.6 ± 3.8 and 23.3 ± 4.1 cmH2O in the PEEP-50 and PEEP-20 groups, respectively (P = 0.91). However, barotrauma MIP in the PEEP-50 group was higher (65.7 ± 3.4 cmH2O) than in the PEEP-20 group (56.7 ± 5 0.2 cmH2O) (P = 0.003) resulting in a safety range of pressures of respectively 33.3 ± 8.7 and 42.1 ± 3.9 cmH2O (P = 0.035).

CONCLUSION

In this ex-vivo model, we found a substantial difference between recruitment and barotrauma pressures using both recruitment strategies. However, a higher margin of safety was obtained when a higher PEEP and lower driving pressure strategy was used for recruiting the lung.

Two levels of the inspired oxygen fraction in propofol-anesthetized dogs with high intracranial pressure: cardiopulmonary function

In the initial stage of traumatic brain injury, the use of 1.0 inspired oxygen fraction (FiO2) is indicated. However, high FiO2 has been correlated with atelectasis. Thus, the effects of FiO2= 1.0 and FiO2= 0.6 on the cardiopulmonary function in propofol-anesthetized dogs with high intracranial pressure (ICP) were evaluated. Eight dogs were anesthetized on two occasions, receiving, during controlled ventilation, an FiO2= 1 (G100) or an FiO2= 0.6 (G60). Propofol was used for induction (10mg.kg-1) followed by a continuous rate infusion (0.6mg.kg-1.minute-1). An increase in the ICP was induced by temporary obliteration of the right jugular vein (OJv) 50 minutes after induction of anesthesia. The measurement was taken twenty minutes after OJv (T0) and then at 15-minute intervals (T15 to T60). Alveolar oxygen partial pressure in G60 was lower than in G100 during the whole procedure. Alveolar-arterial oxygen gradient in G100 was greater than in G60 at T0 and at T60. No differences were observed for arterial oxygen partial pressure/inspired oxygen fraction ratio, arterial-to-alveolar oxygen pressure ratio, respiratory index, venous admixture, oxygen delivery, oxygen consumption, oxygen extraction, heart rate, mean pulmonary arterial pressure, pulmonary arterial occlusion pressure, cardiac index, stroke index and systemic vascular resistance index. In G100, mean arterial pressure at T0 was higher than at T45. In dogs with high ICP, the cardiopulmonary function was not influenced by the different FiO2 used.

High versus low positive end-expiratory pressure during general anaesthesia for open abdominal surgery (PROVHILO trial): a multicentre randomised controlled trial

BACKGROUND

The role of positive end-expiratory pressure in mechanical ventilation during general anaesthesia for surgery remains uncertain. Levels of pressure higher than 0 cm H2O might protect against postoperative pulmonary complications but could also cause intraoperative circulatory depression and lung injury from overdistension. We tested the hypothesis that a high level of positive end-expiratory pressure with recruitment manoeuvres protects against postoperative pulmonary complications in patients at risk of complications who are receiving mechanical ventilation with low tidal volumes during general anaesthesia for open abdominal surgery.

METHODS

In this randomised controlled trial at 30 centres in Europe and North and South America, we recruited 900 patients at risk for postoperative pulmonary complications who were planned for open abdominal surgery under general anaesthesia and ventilation at tidal volumes of 8 mL/kg. We randomly allocated patients to either a high level of positive end-expiratory pressure (12 cm H2O) with recruitment manoeuvres (higher PEEP group) or a low level of pressure (≤2 cm H2O) without recruitment manoeuvres (lower PEEP group). We used a centralised computer-generated randomisation system. Patients and outcome assessors were masked to the intervention. Primary endpoint was a composite of postoperative pulmonary complications by postoperative day 5. Analysis was by intention-to-treat. The study is registered at Controlled-Trials.com, number ISRCTN70332574.

FINDINGS

From February, 2011, to January, 2013, 447 patients were randomly allocated to the higher PEEP group and 453 to the lower PEEP group. Six patients were excluded from the analysis, four because they withdrew consent and two for violation of inclusion criteria. Median levels of positive end-expiratory pressure were 12 cm H2O (IQR 12-12) in the higher PEEP group and 2 cm H2O (0-2) in the lower PEEP group. Postoperative pulmonary complications were reported in 174 (40%) of 445 patients in the higher PEEP group versus 172 (39%) of 449 patients in the lower PEEP group (relative risk 1·01; 95% CI 0·86-1·20; p=0·86). Compared with patients in the lower PEEP group, those in the higher PEEP group developed intraoperative hypotension and needed more vasoactive drugs.

INTERPRETATION

A strategy with a high level of positive end-expiratory pressure and recruitment manoeuvres during open abdominal surgery does not protect against postoperative pulmonary complications. An intraoperative protective ventilation strategy should include a low tidal volume and low positive end-expiratory pressure, without recruitment manoeuvres.

FUNDING

Academic Medical Center (Amsterdam, Netherlands), European Society of Anaesthesiology.

Continuous positive airway pressure (CPAP) during the postoperative period for prevention of postoperative morbidity and mortality following major abdominal surgery

BACKGROUND

Major abdominal surgery can be associated with a number of serious complications that may impair patient recovery. In particular, postoperative pulmonary complications (PPCs), including respiratory complications such as atelectasis and pneumonia, are a major contributor to postoperative morbidity and may even contribute to increased mortality. Continuous positive airway pressure (CPAP) is a type of therapy that uses a high-pressure gas source to deliver constant positive pressure to the airways throughout both inspiration and expiration. This approach is expected to prevent some pulmonary complications, thus reducing mortality.

OBJECTIVES

To determine whether any difference can be found in the rate of mortality and adverse events following major abdominal surgery in patients treated postoperatively with CPAP versus standard care, which may include traditional oxygen delivery systems, physiotherapy and incentive spirometry.

SEARCH METHODS

We searched the Cochrane Central Register of Controlled Trials (CENTRAL) 2013, Issue 9; Ovid MEDLINE (1966 to 15 September 2013); EMBASE (1988 to 15 September 2013); Web of Science (to September 2013) and the Cumulative Index to Nursing and Allied Health Literature (CINAHL) (to September 2013).

SELECTION CRITERIA

We included all randomized controlled trials (RCTs) in which CPAP was compared with standard care for prevention of postoperative mortality and adverse events following major abdominal surgery. We included all adults (adults as defined by individual studies) of both sexes. The intervention of CPAP was applied during the postoperative period. We excluded studies in which participants had received PEEP during surgery.

DATA COLLECTION AND ANALYSIS

Two review authors independently selected studies that met the selection criteria from all studies identified by the search strategy. Two review authors extracted the data and assessed risk of bias separately, using a data extraction form. Data entry into RevMan was performed by one review author and was checked by another for accuracy. We performed a limited meta-analysis and constructed a summary of findings table.

MAIN RESULTS

We selected 10 studies for inclusion in the review from 5236 studies identified in the search. These 10 studies included a total of 709 participants. Risk of bias for the included studies was assessed as high in six studies and as unclear in four studies.Two RCTs reported all-cause mortality. Among 413 participants, there was no clear evidence of a difference in mortality between CPAP and control groups, and considerable heterogeneity between trials was noted (risk ratio (RR) 1.28, 95% confidence interval (CI) 0.35 to 4.66; I(2) = 75%).Six studies reported demonstrable atelectasis in the study population. A reduction in atelectasis was observed in the CPAP group, although heterogeneity between studies was substantial (RR 0.62, 95% CI 0.45 to 0.86; I(2) = 61%). Pneumonia was reported in five studies, including 563 participants; CPAP reduced the rate of pneumonia, and no important heterogeneity was noted (RR 0.43, 95% CI 0.21 to 0.84; I(2) = 0%). The number of participants identified as having serious hypoxia was reported in two studies, with no clear difference between CPAP and control groups, given imprecise results and substantial heterogeneity between trials (RR 0.48, 95% CI 0.22 to 1.02; I(2) = 67%). A reduced rate of reintubation was reported in the CPAP group compared with the control group in two studies, and no important heterogeneity was identified (RR 0.14, 95% CI 0.03 to 0.58; I(2) = 0%). Admission into the intensive care unit (ICU) for invasive ventilation and supportive care was reduced in the CPAP group, but this finding did not reach statistical significance (RR 0.45, 95% CI 0.18 to 1.14; I(2) = 0).Secondary outcomes such as length of hospital stay and adverse effects were only minimally reported.A summary of findings table was constructed using the GRADE (Grades of Recommendation, Assessment, Development and Evaluation) principle. The quality of evidence was determined to be very low.

AUTHORS' CONCLUSIONS

Very low-quality evidence from this review suggests that CPAP initiated during the postoperative period might reduce postoperative atelectasis, pneumonia and reintubation, but its effects on mortality, hypoxia or invasive ventilation are uncertain. Evidence is not sufficiently strong to confirm the benefits or harms of CPAP during the postoperative period in those undergoing major abdominal surgery. Most of the included studies did not report on adverse effects attributed to CPAP.New, high-quality research is much needed to evaluate the use of CPAP in preventing mortality and morbidity following major abdominal surgery. With increasing availability of CPAP to our surgical patients and its potential to improve outcomes (possibly in conjunction with intraoperative lung protective ventilation strategies), unanswered questions regarding its efficacy and safety need to be addressed. Any future study must report on the adverse effects of CPAP.

Stress concentration around an atelectatic region: a finite element model

Lung parenchyma surrounding an atelectatic region is thought to be subjected to increased stress compared with the rest of the lung. Using 37 hexagonal cells made of linear springs, Mead et al. (1970) measured a stress concentration greater than 30% in the springs surrounding a stiffer central cell. We re-examine the problem using a 2D finite element model of 500 cells made of thin filaments with a non-linear stress-strain relationship. We study the consequences of increasing the central stiff region from one to nine contiguous cells in regular hexagonal honeycombs and random Voronoi honeycombs. The honeycomb structures were uniformly expanded with strains of 15%, 30%, 45% and 55% above their resting, non-deformed geometry. The curve of biaxial stress vs. fractional area change has a similar shape to that of the pressure-volume curve of the lung, showing an initial regime with relatively flat slope and a final regime with decreasing slope, tending toward an asymptote. Regular honeycombs had little variability in the maximum stress in radially oriented filaments adjacent to the central stiff region. In contrast, some filaments in random Voronoi honeycombs were subjected to stress concentration approximately 16 times the average stress concentration in the radially oriented filaments adjacent to the stiff region. These results may have implications in selecting the appropriate strategy for mechanical ventilation in ARDS and defining a "safe" level of alveolar pressure for ventilating atelectatic lungs.

Accuracy of transthoracic lung ultrasound for diagnosing anesthesia-induced atelectasis in children

BACKGROUND

The aim of this study was to test the accuracy of lung sonography (LUS) to diagnose anesthesia-induced atelectasis in children undergoing magnetic resonance imaging (MRI).

METHODS

Fifteen children with American Society of Anesthesiology's physical status classification I and aged 1 to 7 yr old were studied. Sevoflurane anesthesia was performed with the patients breathing spontaneously during the study period. After taking the reference lung MRI images, LUS was carried out using a linear probe of 6 to 12 MHz. Atelectasis was documented in MRI and LUS segmenting the chest into 12 similar anatomical regions. Images were analyzed by four blinded radiologists, two for LUS and two for MRI. The level of agreement for the diagnosis of atelectasis among observers was tested using the κ reliability index.

RESULTS

Fourteen patients developed atelectasis mainly in the most dependent parts of the lungs. LUS showed 88% of sensitivity (95% CI, 74 to 96%), 89% of specificity (95% CI, 83 to 94%), and 88% of accuracy (95% CI, 83 to 92%) for the diagnosis of atelectasis taking MRI as reference. The agreement between the two radiologists for diagnosing atelectasis by MRI was very good (κ, 0.87; 95% CI, 0.72 to 1; P < 0.0001) as was the agreement between the two radiologists for detecting atelectasis by LUS (κ, 0.90; 95% CI, 0.75 to 1; P < 0.0001). MRI and LUS also showed good agreement when data from the four radiologists were pooled and examined together (κ, 0.75; 95% CI, 0.69 to 0.81; P < 0.0001).

CONCLUSION

LUS is an accurate, safe, and simple bedside method for diagnosing anesthesia-induced atelectasis in children.

Positive end-expiratory pressure (PEEP) during anaesthesia for prevention of mortality and postoperative pulmonary complications

BACKGROUND

General anaesthesia causes atelectasis, which can lead to impaired respiratory function. Positive end-expiratory pressure (PEEP) is a mechanical manoeuvre that increases functional residual capacity (FRC) and prevents collapse of the airways, thereby reducing atelectasis. It is not known whether intraoperative PEEP alters the risks of postoperative mortality and pulmonary complications. This review was originally published in 2010 and was updated in 2013.

OBJECTIVES

To assess the benefits and harms of intraoperative PEEP in terms of postoperative mortality and pulmonary outcomes in all adult surgical patients.

SEARCH METHODS

We searched the Cochrane Central Register of Controlled Trials (CENTRAL) 2013, Issue 10, part of The Cochrane Library, as well as MEDLINE (via Ovid) (1966 to October 2013), EMBASE (via Ovid) (1980 to October 2013), CINAHL (via EBSCOhost) (1982 to October 2013), ISI Web of Science (1945 to October 2013) and LILACS (via BIREME interface) (1982 to October 2010). The original search was performed in January 2010.

SELECTION CRITERIA

We included randomized clinical trials assessing the effects of PEEP versus no PEEP during general anaesthesia on postoperative mortality and postoperative respiratory complications in adults, 16 years of age and older.

DATA COLLECTION AND ANALYSIS

Two review authors independently selected papers, assessed trial quality and extracted data. We contacted study authors to ask for additional information, when necessary. We calculated the number of additional participants needed (information size) to make reliable conclusions.

MAIN RESULTS

This updated review includes two new randomized trials. In total, 10 randomized trials with 432 participants and four comparisons are included in this review. One trial had a low risk of bias. No differences were demonstrated in mortality, with risk ratio (RR) of 0.97 (95% confidence interval (CI) 0.20 to 4.59; P value 0.97; 268 participants, six trials, very low quality of evidence (grading of recommendations assessment, development and evaluation (GRADE)), and in pneumonia, with RR of 0.40 (95% CI 0.11 to 1.39; P value 0.15; 120 participants, three trials, very low quality of evidence (GRADE)). Statistically significant results included the following: The PEEP group had higher arterial oxygen pressure (PaO2)/fraction of inspired oxygen (FiO2) on day one postoperatively, with a mean difference of 22.98 (95% CI 4.40 to 41.55; P value 0.02; 80 participants, two trials, very low quality of evidence (GRADE)), and postoperative atelectasis (defined as an area of collapsed lung, quantified by computerized tomography scan) was less in the PEEP group (standard mean difference -1.2, 95% CI -1.78 to -0.79; P value 0.00001; 88 participants, two trials, very low quality of evidence (GRADE)). No adverse events were reported in the three trials that adequately measured these outcomes (barotrauma and cardiac complications). Using information size calculations, we estimated that a further 21,200 participants would have to be randomly assigned to allow a reliable conclusion about PEEP and mortality.

AUTHORS' CONCLUSIONS

Evidence is currently insufficient to permit conclusions about whether intraoperative PEEP alters risks of postoperative mortality and respiratory complications among undifferentiated surgical patients.

Low intraoperative tidal volume ventilation with minimal PEEP is associated with increased mortality

Background

Anaesthetists have traditionally ventilated patients’ lungs with tidal volumes (TVs) between 10 and 15 ml kg⁻¹ of ideal body weight (IBW), without the use of PEEP. Over the past decade, influenced by the results of the Acute Respiratory Distress Syndrome Network trial, many anaesthetists have begun using lower TVs during surgery. It is unclear whether the benefits of low TV ventilation can be extended into the perioperative period.

Methods

We reviewed the records of 29 343 patients who underwent general anaesthesia with mechanical ventilation between January 1, 2008 and December 31, 2011. We calculated TV kg⁻¹ IBW, PEEP, peak inspiratory pressure (PIP), and dynamic compliance. Cox regression analysis with propensity score matching was performed to examine the association between TV and 30-day mortality.

Results

Median TV was 8.6 [7.7–9.6] ml kg⁻¹ IBW with minimal PEEP [4.0 (2.2–5.0) cm H₂O]. A significant reduction in TV occurred over the study period, from 9 ml kg⁻¹ IBW in 2008 to 8.3 ml kg⁻¹ IBW in 2011 (P=0.01). Low TV 6–8 ml kg⁻¹ IBW was associated with a significant increase in 30-day mortality vs TV 8–10 ml kg⁻¹ IBW: hazard ratio (HR) 1.6 [95% confidence interval (CI) [1.25–2.08], P=0.0002]. The association remained significant after matching: HR 1.63 [95% CI (1.22–2.18), P<0.001]. There was only a weak correlation between TV kg⁻¹ IBW and dynamic compliance (r=−0.006, P=0.31) and a weak-to-moderate correlation between TV kg⁻¹ IBW and PIP (r=0.32 P<0.0001).

Conclusions

Use of low intraoperative TV with minimal PEEP is associated with an increased risk of 30-day mortality.

Gas exchange in the respiratory distress syndromes

This article describes the gas exchange abnormalities occurring in the acute respiratory distress syndrome seen in adults and children and in the respiratory distress syndrome that occurs in neonates. Evidence is presented indicating that the major gas exchange abnormality accounting for the hypoxemia in both conditions is shunt, and that approximately 50% of patients also have lungs regions in which low ventilation-to-perfusion ratios contribute to the venous admixture. The various mechanisms by which hypercarbia may develop and by which positive end-expiratory pressure improves gas exchange are reviewed, as are the effects of vascular tone and airway narrowing. The mechanisms by which surfactant abnormalities occur in the two conditions are described, as are the histological findings that have been associated with shunt and low ventilation-to-perfusion.

Respiratory function during anesthesia: effects on gas exchange

Anaesthesia causes a respiratory impairment, whether the patient is breathing spontaneously or is ventilated mechanically. This impairment impedes the matching of alveolar ventilation and perfusion and thus the oxygenation of arterial blood. A triggering factor is loss of muscle tone that causes a fall in the resting lung volume, functional residual capacity. This fall promotes airway closure and gas adsorption, leading eventually to alveolar collapse, that is, atelectasis. The higher the oxygen concentration, the faster will the gas be adsorbed and the aleveoli collapse. Preoxygenation is a major cause of atelectasis and continuing use of high oxygen concentration maintains or increases the lung collapse, that typically is 10% or more of the lung tissue. It can exceed 25% to 40%. Perfusion of the atelectasis causes shunt and cyclic airway closure causes regions with low ventilation/perfusion ratios, that add to impaired oxygenation. Ventilation with positive end-expiratory pressure reduces the atelectasis but oxygenation need not improve, because of shift of blood flow down the lung to any remaining atelectatic tissue. Inflation of the lung to an airway pressure of 40 cmH2O recruits almost all collapsed lung and the lung remains open if ventilation is with moderate oxygen concentration (< 40%) but recollapses within a few minutes if ventilation is with 100% oxygen. Severe obesity increases the lung collapse and obstructive lung disease and one-lung anesthesia increase the mismatch of ventilation and perfusion. CO2 pneumoperitoneum increases atelectasis formation but not shunt, likely explained by enhanced hypoxic pulmonary vasoconstriction by CO2. Atelectasis may persist in the postoperative period and contribute to pneumonia.

Regional lung derecruitment and inflammation during 16 hours of mechanical ventilation in supine healthy sheep

BACKGROUND

Lung derecruitment is common during general anesthesia. Mechanical ventilation with physiological tidal volumes could magnify derecruitment, and produce lung dysfunction and inflammation. The authors used positron emission tomography to study the process of derecruitment in normal lungs ventilated for 16 h and the corresponding changes in regional lung perfusion and inflammation.

METHODS

Six anesthetized supine sheep were ventilated with VT=8 ml/kg and positive end-expiratory pressure=0. Transmission scans were performed at 2-h intervals to assess regional aeration. Emission scans were acquired at baseline and after 16 h for the following tracers: (1) F-fluorodeoxyglucose to evaluate lung inflammation and (2) NN to calculate regional perfusion and shunt fraction.

RESULTS

Gas fraction decreased from baseline to 16 h in dorsal (0.31±0.13 to 0.14±0.12, P<0.01), but not in ventral regions (0.61±0.03 to 0.63±0.07, P=nonsignificant), with time constants of 1.5-44.6 h. Although the vertical distribution of relative perfusion did not change from baseline to 16 h, shunt increased in dorsal regions (0.34±0.23 to 0.63±0.35, P<0.01). The average pulmonary net F-fluorodeoxyglucose uptake rate in six regions of interest along the ventral-dorsal direction increased from 3.4±1.4 at baseline to 4.1±1.5 10(-3)/min after 16 h (P<0.01), and the corresponding average regions of interest F-fluorodeoxyglucose phosphorylation rate increased from 2.0±0.2 to 2.5±0.2 10(-2)/min (P<0.01).

CONCLUSIONS

When normal lungs are mechanically ventilated without positive end-expiratory pressure, loss of aeration occurs continuously for several hours and is preferentially localized to dorsal regions. Progressive lung derecruitment was associated with increased regional shunt, implying an insufficient hypoxic pulmonary vasoconstriction. The increased pulmonary net uptake and phosphorylation rates of F-fluorodeoxyglucose suggest an incipient inflammation in these initially normal lungs.

Limits of normality of quantitative thoracic CT analysis

INTRODUCTION

Although computed tomography (CT) is widely used to investigate different pathologies, quantitative data from normal populations are scarce. Reference values may be useful to estimate the anatomical or physiological changes induced by various diseases.

METHODS

We analyzed 100 helical CT scans taken for clinical purposes and referred as nonpathological by the radiologist. Profiles were manually outlined on each CT scan slice and each voxel was classified according to its gas/tissue ratio. For regional analysis, the lungs were divided into 10 sterno-vertebral levels.

RESULTS

We studied 53 males and 47 females (age 64 ± 13 years); males had a greater total lung volume, lung gas volume and lung tissue. Noninflated tissue averaged 7 ± 4% of the total lung weight, poorly inflated tissue averaged 18 ± 3%, normally inflated tissue averaged 65 ± 8% and overinflated tissue averaged 11 ± 7%. We found a significant correlation between lung weight and subject's height (P <0.0001, r2 = 0.49); the total lung capacity in a supine position was 4,066 ± 1,190 ml, ~1,800 ml less than the predicted total lung capacity in a sitting position. Superimposed pressure averaged 2.6 ± 0.5 cmH2O.

CONCLUSION

Subjects without lung disease present significant amounts of poorly inflated and overinflated tissue. Normal lung weight can be predicted from patient's height with reasonable confidence.

Reference values for volumetric capnography-derived non-invasive parameters in healthy individuals

The aim of this study was to determine typical values for non-invasive volumetric capnography (VCap) parameters for healthy volunteers and anesthetized individuals. VCap was obtained by a capnograph connected to the airway opening. We prospectively studied 33 healthy volunteers 32 ± 6 years of age weighing 70 ± 13 kg at a height of 171 ± 11 cm in the supine position. Data from these volunteers were compared with a cohort of similar healthy anesthetized patients ventilated with the following settings: tidal volume (VT) of 6-8 mL/kg, respiratory rate 10-15 bpm, PEEP of 5-6 cmH₂O and FiO₂ of 0.5. Volunteers showed better clearance of CO₂ compared to anesthetized patients as indicated by (median and interquartile range): (1) an increased elimination of CO₂ per mL of VT of 0.028 (0.005) in volunteers versus 0.023 (0.003) in anesthetized patients, p < 0.05; (2) a lower normalized slope of phase III of 0.26 (0.17) in volunteers versus 0.39 (0.38) in anesthetized patients, p < 0.05; and (3) a lower Bohr dead space ratio of 0.23 (0.05) in volunteers versus 0.28 (0.05) in anesthetized patients, p < 0.05. This study presents reference values for non-invasive volumetric capnography-derived parameters in healthy individuals. Mechanical ventilation and anesthesia altered these values significantly.

The role of neuromuscular blockers in ARDS: benefits and risks

PURPOSE OF REVIEW

Neuromuscular blocking agents (NMBAs) are part of the pharmaceutical arsenal employed to treat acute respiratory distress syndrome (ARDS). However, their use remains controversial because the potential benefits of these agents are counterbalanced by possible adverse effects. This review summarizes advantages and risks of NMBAs based on the most recent literature.

RECENT FINDINGS

NMBAs have been shown to improve oxygenation during severe ARDS in three randomized controlled trials. The most recent results demonstrated that NMBAs decrease 90-day in-hospital mortality, particularly in the most hypoxaemic patients. NMBAs have not been shown to be an independent risk factor of neuromyopathy in most studies.

SUMMARY

NMBAs are commonly used in ARDS (25-55% of patients), but the benefits and the risks of using these agents are controversial. Recent data suggest that a continuous infusion of cisatracurium during the first 48  h of ARDS, particularly for patients with a P(a)O(2)/F(i)O(2) ratio less than 120, can decrease 90-day in-hospital mortality. NMBAs do not appear to be an independent risk factor for ICU-acquired weakness if they are not given with corticosteroids or in patients with hyperglycaemia.

Pulmonary pathophysiology and lung mechanics in anesthesiology: a case-based overview

Anesthesia, surgical requirements, and patients' unique pathophysiology all combine to make the accumulated knowledge of respiratory physiology and lung mechanics vital in patient management. This article take a case-based approach to discuss how the complex interactions between anesthesia, surgery, and patient disease affect patient care with respect to pulmonary pathophysiology and clinical decision making. Two disparate scenarios are examined: a patient with chronic obstructive pulmonary disease undergoing a lung resection, and a patient with coronary artery disease undergoing cardiopulmonary bypass. The impacts of important concepts in pulmonary physiology and respiratory mechanics on clinical management decisions are discussed.

Sudden generalized lung atelectasis during thoracotomy following thoracic lavage in 3 dogs

OBJECTIVE

To describe sudden onset of generalized pulmonary atelectasis following thoracic lavage in 3 dogs.

SERIES SUMMARY

Thoracic lavage was performed following ligation of a patent ductus arteriosus in case 1, prior to closure of a large traumatic full thickness wound in the chest wall in case 2, and during investigation of an idiopathic spontaneous pneumothorax in case 3. In each case anesthesia and surgery were uneventful until thoracic lavage was performed, after which sudden generalized pulmonary atelectasis was observed. The atelectasis was visualized and was associated with oxyhemoglobin desaturation, decreased end-tidal carbon dioxide partial pressure (ETCO(2)), and a marked increase in the peak inspiratory pressure (PIP) required to achieve visible lung inflation. Occlusion of the endotracheal tube and cervical trachea was directly eliminated as the cause of atelectasis in cases 1 and 2, and indirectly eliminated in case 3. Improvement in pulmonary function occurred in all cases in response to increased PIP ± positive end expiratory pressure (PEEP).

NEW OR UNIQUE INFORMATION PROVIDED

Generalized atelectasis should be considered a possible complication of thoracic lavage performed during thoracotomy. In the cases presented here, it is suspected that pre-existing reduction in lung volume (due to inadequate ventilation, surgical compression, absorption atelectasis) was exacerbated by the addition of the lavage fluid to the thoracic cavity. This pre-existing lung collapse is believed to have resulted in reduction of lung volume and that further reduction below the critical closing volume occurred following instillation of saline into the thorax resulting in the subsequent development of generalized atelectasis. The performance of regular arterial blood gas analyses and different ventilation protocols may have prevented the marked atelectasis that was observed in these cases.

Oxygen and anesthesia: what lung do we deliver to the post-operative ward?

Anesthesia is safe in most patients. However, anesthetics reduce functional residual capacity (FRC) and promote airway closure. Oxygen is breathed during the induction of anesthesia, and increased concentration of oxygen (O(2) ) is given during the surgery to reduce the risk of hypoxemia. However, oxygen is rapidly adsorbed behind closed airways, causing lung collapse (atelectasis) and shunt. Atelectasis may be a locus for infection and may cause pneumonia. Measures to prevent atelectasis and possibly reduce post-operative pulmonary complications are based on moderate use of oxygen and preservation or restoration of FRC. Pre-oxygenation with 100% O(2) causes atelectasis and should be followed by a recruitment maneuver (inflation to an airway pressure of 40 cm H(2) O for 10 s and to higher airway pressures in patients with reduced abdominal compliance (obese and patients with abdominal disorders). Pre-oxygenation with 80% O(2) may be sufficient in most patients with no anticipated difficulty in managing the airway, but time to hypoxemia during apnea decreases from mean 7 to 5 min. An alternative, possibly challenging, procedure is induction of anesthesia with continuous positive airway pressure/positive end-expiratory pressure to prevent fall in FRC enabling use of 100% O(2) . A continuous PEEP of 7-10 cm H(2) O may not necessarily improve oxygenation but should keep the lung open until the end of anesthesia. Inspired oxygen concentration of 30-40%, or even less, should suffice if the lung is kept open. The goal of the anesthetic regime should be to deliver a patient with no atelectasis to the post-operative ward and to keep the lung open.

Lung aeration during ventilation after recruitment guided by tidal elimination of carbon dioxide and dynamic compliance was better than after end-tidal carbon dioxide targeted ventilation: a computed tomography study in surfactant-depleted piglets

OBJECTIVE

To test the hypothesis that tidal elimination of carbon dioxide and dynamic compliance guided lung recruitment and positive end-expiratory pressure titration in surfactant-depleted piglets result in improved aeration (repeated computed tomography scans) and reduced ventilation pressures compared to those of a control group with conventional end-tidal carbon dioxide targeted ventilation.

DESIGN

Prospective animal investigation.

SETTING

Clinical physiology research laboratory.

SUBJECTS

Seventeen saline-lavaged piglets.

INTERVENTIONS

The piglets were initially ventilated at an end-inspiratory pressure of 20 cm H2O, a positive end-expiratory pressure of 5 cm H2O, and a tidal volume of 10 mL kg for an end-tidal carbon dioxide target of 30-45 torr followed by 5 mins of ventilation without positive end-expiratory pressure. After this, the control group was ventilated for the same end-tidal carbon dioxide target during the study period. In the recruitment group, the protocol started with an increase of the positive end-expiratory pressure to 15 cm H2O. The end-inspiratory pressure was then increased in steps of 3 cm H2O to a tidal elimination of carbon dioxide peak/plateau in one recruitment group and further increased in two steps in a second recruitment group. A downward positive end-expiratory pressure titration was followed by continuous dynamic compliance monitoring. The "open lung positive end-expiratory pressure" was set 2 cm H2O above the positive end-expiratory pressure at the first dynamic compliance decline and used for a final "open lung ventilation" period.

MEASUREMENTS AND MAIN RESULTS

The recruitment groups showed better aeration, lower ventilatory pressure amplitude, and better dynamic compliance than the control group at the end of the study. Recruitment using airway pressures above the tidal elimination of carbon dioxide peak/plateau did not improve aeration. Using end-tidal carbon dioxide targeted ventilation in the control group restored aeration after the ventilation without positive end-expiratory pressure, but no recruitment or improvement of dynamic compliance was measured.

CONCLUSIONS

Aeration was significantly better after recruitment and positive end-expiratory pressure titration than in a control group managed by "conventional" end-tidal carbon dioxide targeted ventilation. An increase of the end-inspiratory pressure above the tidal elimination of carbon dioxide peak/plateau did not result in an increased amount of normally aerated lung. A recruitment maneuver resulted in a lower ventilatory amplitude for achieving a target tidal volume and better dynamic compliance at the end of the study period compared to those of the control group.

Effect of PEEP on regional ventilation during laparoscopic surgery monitored by electrical impedance tomography

BACKGROUND

Anesthesia per se and pneumoperitoneum during laparoscopic surgery lead to atelectasis and impairment of oxygenation. We hypothesized that a ventilation with positive end-expiratory pressure (PEEP) during general anesthesia and laparoscopic surgery leads to a more homogeneous ventilation distribution as determined by electrical impedance tomography (EIT). Furthermore, we supposed that PEEP ventilation in lung-healthy patients would improve the parameters of oxygenation and respiratory compliance.

METHODS

Thirty-two patients scheduled to undergo laparoscopic cholecystectomy were randomly assigned to be ventilated with ZEEP (0 cmH(2)O) or with PEEP (10 cmH(2)O) and a subsequent recruitment maneuver. Differences in regional ventilation were analyzed by the EIT-based center-of-ventilation index (COV), which quantifies the distribution of ventilation and indicates ventilation shifts.

RESULTS

Higher amount of ventilation was examined in the dorsal parts of the lungs in the PEEP group. Throughout the application of PEEP, a lower shift of ventilation was found, whereas after the induction of anesthesia, a remarkable ventral shift of ventilation in ZEEP-ventilated patients (COV: ZEEP, 40.6 ± 2.4%; PEEP, 46.5 ± 3.5%; P<0.001) was observed. Compared with the PEEP group, ZEEP caused a ventral misalignment of ventilation during pneumoperitoneum (COV: ZEEP, 41.6 ± 2.4%; PEEP, 44 ± 2.7%; P=0.013). Throughout the study, there were significant differences in the parameters of oxygenation and respiratory compliance with improved values in PEEP-ventilated patients.

CONCLUSION

The effect of anesthesia, pneumoperitoneum, and different PEEP levels can be evaluated by EIT-based COV monitoring. An initial recruitment maneuver and a PEEP of 10 cmH(2)O preserved homogeneous regional ventilation during laparoscopic surgery in most, but not all, patients and improved oxygenation and respiratory compliance.

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.

Dependence of the gradient between arterial and end-tidal P(CO(2)) on the fraction of inspired oxygen

BACKGROUND

End-tidal P(CO(2)) (Pe'(CO(2))) is routinely used in the clinical assessment of the adequacy of ventilation because it provides an estimate of Pa(CO(2)). How well Pe'(CO(2)) reflects Pa(CO(2)) depends on the gradient between them, expressed as ΔPa-e'(CO(2)). The major determinant of ΔPa-e'(CO(2)) is alveolar dead space (Vd(alv)). The fraction of inspired O(2) (Fi(O(2))) is not thought to substantially affect ΔPa-e'(CO(2)) in anaesthetized patients. We hypothesized that a high Fi(O(2)) may indeed increase ΔPa-e'(CO(2)) by preferentially vasodilating well-perfused alveoli, resulting in the redistribution of blood flow to these alveoli from poorly perfused alveoli and an increase in Vd(alv). We therefore investigated the effects of changes in Fi(O(2)) on ΔPa-e'(CO(2)) and Vd(alv).

METHODS

With Institutional Review Board approval and informed consent, we studied 20 ASA I-II supine patients undergoing elective lower abdominal surgery under combined general and epidural anaesthesia. At constant levels of ventilation, Fi(O(2)) levels of 0.21, 0.33, 0.5, 0.75, and 0.97 were applied in a random order and ΔPa-e'(CO(2)) and Vd(alv) were calculated.

RESULTS

The ΔPa-e'(CO(2)) values were, in order of ascending Fi(O(2)), {mean [standard error of the mean (SEM)]} 0.13 (0.04), 0.28 (0.08), 0.29 (0.09), 0.44 (0.11), and 0.53 (0.09) kPa. The corresponding values of Vd(alv) were 25.5, 33.8, 35.8, 48.9, and 47.4 ml. Each successive hyperoxic level showed a significant increase in ΔPa-e'(CO(2)) except between the 0.33-0.5 and 0.75-0.97 Fi(O(2)) levels.

CONCLUSIONS

These data demonstrate that ΔPa-e'(CO(2)), in anaesthetized patients depends on Fi(O(2)).