Reply: Intrapulmonary shunt and alveolar dead space in a cohort of patients with acute COVID-19 pneumonitis and early recovery

We thank M. Ackermann and co-workers for their interest in our recent publication [1]. These authors highlight our report of persistent increased alveolar dead space in 30% of 17 patients studied within ∼2 months after an acute episode of mild–moderate COVID-19. After outlining their own work, demonstrating secondary pulmonary lobule pathologies found in the lungs of deceased COVID-19 patients, they then hypothesise that secondary lobular micro-ischaemia may be responsible for the elevated alveolar dead space found in our study.

Increased dead space following COVID-19 may be due to microvascular injury or secondary micro-ischaemia https://bit.ly/3Fypdwz

Intrapulmonary shunt and alveolar dead space in a cohort of patients with acute COVID-19 pneumonitis and early recovery

BACKGROUND

Pathological evidence suggests that coronavirus disease 2019 (COVID-19) pulmonary infection involves both alveolar damage (causing shunt) and diffuse microvascular thrombus formation (causing alveolar dead space). We propose that measuring respiratory gas exchange enables detection and quantification of these abnormalities. We aimed to measure shunt and alveolar dead space in moderate COVID-19 during acute illness and recovery.

METHODS

We studied 30 patients (22 males; mean±sd age 49.9±13.5 years) 3-15 days from symptom onset and again during recovery, 55±10 days later (n=17). Arterial blood (breathing ambient air) was collected while exhaled oxygen and carbon dioxide concentrations were measured, yielding alveolar-arterial differences for each gas (P _A-aO2 and P _a-ACO2 , respectively) from which shunt and alveolar dead space were computed.

RESULTS

For acute COVID-19 patients, group mean (range) for P _A-aO2 was 41.4 (-3.5-69.3) mmHg and for P _a-ACO2 was 6.0 (-2.3-13.4) mmHg. Both shunt (% cardiac output) at 10.4% (0-22.0%) and alveolar dead space (% tidal volume) at 14.9% (0-32.3%) were elevated (normal: <5% and <10%, respectively), but not correlated (p=0.27). At recovery, shunt was 2.4% (0-6.1%) and alveolar dead space was 8.5% (0-22.4%) (both p<0.05 versus acute). Shunt was marginally elevated for two patients; however, five patients (30%) had elevated alveolar dead space.

CONCLUSIONS

We speculate impaired pulmonary gas exchange in early COVID-19 pneumonitis arises from two concurrent, independent and variable processes (alveolar filling and pulmonary vascular obstruction). For most patients these resolve within weeks; however, high alveolar dead space in ∼30% of recovered patients suggests persistent pulmonary vascular pathology.

Geo-economic variations in epidemiology, ventilation management and outcome of patients receiving intraoperative ventilation during general anesthesia- posthoc analysis of an observational study in 29 countries

BACKGROUND

The aim of this analysis is to determine geo-economic variations in epidemiology, ventilator settings and outcome in patients receiving general anesthesia for surgery.

METHODS

Posthoc analysis of a worldwide study in 29 countries. Lower and upper middle-income countries (LMIC and UMIC), and high-income countries (HIC) were compared. The coprimary endpoint was the risk for and incidence of postoperative pulmonary complications (PPC); secondary endpoints were intraoperative ventilator settings, intraoperative complications, hospital stay and mortality.

RESULTS

Of 9864 patients, 4% originated from LMIC, 11% from UMIC and 85% from HIC. The ARISCAT score was 17.5 [15.0-26.0] in LMIC, 16.0 [3.0-27.0] in UMIC and 15.0 [3.0-26.0] in HIC (P = .003). The incidence of PPC was 9.0% in LMIC, 3.2% in UMIC and 2.5% in HIC (P < .001). Median tidal volume in ml kg^- 1 predicted bodyweight (PBW) was 8.6 [7.7-9.7] in LMIC, 8.4 [7.6-9.5] in UMIC and 8.1 [7.2-9.1] in HIC (P < .001). Median positive end-expiratory pressure in cmH₂O was 3.3 [2.0-5.0]) in LMIC, 4.0 [3.0-5.0] in UMIC and 5.0 [3.0-5.0] in HIC (P < .001). Median driving pressure in cmH₂O was 14.0 [11.5-18.0] in LMIC, 13.5 [11.0-16.0] in UMIC and 12.0 [10.0-15.0] in HIC (P < .001). Median fraction of inspired oxygen in % was 75 [50-80] in LMIC, 50 [50-63] in UMIC and 53 [45-70] in HIC (P < .001). Intraoperative complications occurred in 25.9% in LMIC, in 18.7% in UMIC and in 37.1% in HIC (P < .001). Hospital mortality was 0.0% in LMIC, 1.3% in UMIC and 0.6% in HIC (P = .009).

CONCLUSION

The risk for and incidence of PPC is higher in LMIC than in UMIC and HIC. Ventilation management could be improved in LMIC and UMIC.

TRIAL REGISTRATION

Clinicaltrials.gov , identifier: NCT01601223.

Sex difference and intra-operative tidal volume: Insights from the LAS VEGAS study

BACKGROUND

One key element of lung-protective ventilation is the use of a low tidal volume (VT). A sex difference in use of low tidal volume ventilation (LTVV) has been described in critically ill ICU patients.

OBJECTIVES

The aim of this study was to determine whether a sex difference in use of LTVV also exists in operating room patients, and if present what factors drive this difference.

DESIGN, PATIENTS AND SETTING

This is a posthoc analysis of LAS VEGAS, a 1-week worldwide observational study in adults requiring intra-operative ventilation during general anaesthesia for surgery in 146 hospitals in 29 countries.

MAIN OUTCOME MEASURES

Women and men were compared with respect to use of LTVV, defined as VT of 8 ml kg-1 or less predicted bodyweight (PBW). A VT was deemed 'default' if the set VT was a round number. A mediation analysis assessed which factors may explain the sex difference in use of LTVV during intra-operative ventilation.

RESULTS

This analysis includes 9864 patients, of whom 5425 (55%) were women. A default VT was often set, both in women and men; mode VT was 500 ml. Median [IQR] VT was higher in women than in men (8.6 [7.7 to 9.6] vs. 7.6 [6.8 to 8.4] ml kg-1 PBW, P < 0.001). Compared with men, women were twice as likely not to receive LTVV [68.8 vs. 36.0%; relative risk ratio 2.1 (95% CI 1.9 to 2.1), P < 0.001]. In the mediation analysis, patients' height and actual body weight (ABW) explained 81 and 18% of the sex difference in use of LTVV, respectively; it was not explained by the use of a default VT.

CONCLUSION

In this worldwide cohort of patients receiving intra-operative ventilation during general anaesthesia for surgery, women received a higher VT than men during intra-operative ventilation. The risk for a female not to receive LTVV during surgery was double that of males. Height and ABW were the two mediators of the sex difference in use of LTVV.

TRIAL REGISTRATION

The study was registered at Clinicaltrials.gov, NCT01601223.

The effects of preoperative moderate to severe anaemia on length of hospital stay: A propensity score-matched analysis in non-cardiac surgery patients

BACKGROUND

Anaemia is frequently recorded during preoperative screening and has been suggested to affect outcomes after surgery negatively.

OBJECTIVES

The objectives were to assess the frequency of moderate to severe anaemia and its association with length of hospital stay.

DESIGN

Post hoc analysis of the international observational prospective 'Local ASsessment of VEntilatory management during General Anaesthesia for Surgery' (LAS VEGAS) study.

PATIENTS AND SETTING

The current analysis included adult patients requiring general anaesthesia for non-cardiac surgery. Preoperative anaemia was defined as a haemoglobin concentration of 11 g dl-1 or lower, thus including moderate and severe anaemia according to World Health Organisation criteria.

MAIN OUTCOME MEASURES

The primary outcome was length of hospital stay. Secondary outcomes included hospital mortality, intra-operative adverse events and postoperative pulmonary complications (PPCs).

RESULTS

Haemoglobin concentrations were available for 8264 of 9864 patients. Preoperative moderate to severe anaemia was present in 7.7% of patients. Multivariable analysis showed that preoperative moderate to severe anaemia was associated with an increased length of hospital stay with a mean difference of 1.3 ((95% CI 0.8 to 1.8) days; P < .001). In the propensity-matched analysis, this association remained present, median 4.0 [IQR 1.0 to 5.0] vs. 2.0 [IQR 0.0 to 5.0] days, P = .001. Multivariable analysis showed an increased in-hospital mortality (OR 2.9 (95% CI 1.1 to 7.5); P  = .029), and higher incidences of intra-operative hypotension (36.3 vs. 25.3%; P < .001) and PPCs (17.1 vs. 10.5%; P = .001) in moderately to severely anaemic patients. However, this was not confirmed in the propensity score-matched analysis.

CONCLUSIONS

In this international cohort of non-cardiac surgical patients, preoperative moderate to severe anaemia was associated with a longer duration of hospital stay but not increased intra-operative complications, PPCs or in-hospital mortality.

TRIAL REGISTRATION

The LAS VEGAS study was registered at Clinicaltrials.gov, NCT01601223.

The Association of Intraoperative driving pressure with postoperative pulmonary complications in open versus closed abdominal surgery patients - a posthoc propensity score-weighted cohort analysis of the LAS VEGAS study

BACKGROUND

It is uncertain whether the association of the intraoperative driving pressure (ΔP) with postoperative pulmonary complications (PPCs) depends on the surgical approach during abdominal surgery. Our primary objective was to determine and compare the association of time-weighted average ΔP (ΔP_TW) with PPCs. We also tested the association of ΔP_TW with intraoperative adverse events.

METHODS

Posthoc retrospective propensity score-weighted cohort analysis of patients undergoing open or closed abdominal surgery in the 'Local ASsessment of Ventilatory management during General Anaesthesia for Surgery' (LAS VEGAS) study, that included patients in 146 hospitals across 29 countries. The primary endpoint was a composite of PPCs. The secondary endpoint was a composite of intraoperative adverse events.

RESULTS

The analysis included 1128 and 906 patients undergoing open or closed abdominal surgery, respectively. The PPC rate was 5%. ΔP was lower in open abdominal surgery patients, but ΔP_TW was not different between groups. The association of ΔP_TW with PPCs was significant in both groups and had a higher risk ratio in closed compared to open abdominal surgery patients (1.11 [95%CI 1.10 to 1.20], P <  0.001 versus 1.05 [95%CI 1.05 to 1.05], P <  0.001; risk difference 0.05 [95%CI 0.04 to 0.06], P <  0.001). The association of ΔP_TW with intraoperative adverse events was also significant in both groups but had higher odds ratio in closed compared to open abdominal surgery patients (1.13 [95%CI 1.12- to 1.14], P <  0.001 versus 1.07 [95%CI 1.05 to 1.10], P <  0.001; risk difference 0.05 [95%CI 0.030.07], P <  0.001).

CONCLUSIONS

ΔP is associated with PPC and intraoperative adverse events in abdominal surgery, both in open and closed abdominal surgery.

TRIAL REGISTRATION

LAS VEGAS was registered at clinicaltrials.gov (trial identifier NCT01601223 ).

Effect of Intraoperative High Positive End-Expiratory Pressure (PEEP) With Recruitment Maneuvers vs Low PEEP on Postoperative Pulmonary Complications in Obese Patients: A Randomized Clinical Trial

IMPORTANCE

An intraoperative higher level of positive end-expiratory positive pressure (PEEP) with alveolar recruitment maneuvers improves respiratory function in obese patients undergoing surgery, but the effect on clinical outcomes is uncertain.

OBJECTIVE

To determine whether a higher level of PEEP with alveolar recruitment maneuvers decreases postoperative pulmonary complications in obese patients undergoing surgery compared with a lower level of PEEP.

DESIGN, SETTING, AND PARTICIPANTS

Randomized clinical trial of 2013 adults with body mass indices of 35 or greater and substantial risk for postoperative pulmonary complications who were undergoing noncardiac, nonneurological surgery under general anesthesia. The trial was conducted at 77 sites in 23 countries from July 2014-February 2018; final follow-up: May 2018.

INTERVENTIONS

Patients were randomized to the high level of PEEP group (n = 989), consisting of a PEEP level of 12 cm H2O with alveolar recruitment maneuvers (a stepwise increase of tidal volume and eventually PEEP) or to the low level of PEEP group (n = 987), consisting of a PEEP level of 4 cm H2O. All patients received volume-controlled ventilation with a tidal volume of 7 mL/kg of predicted body weight.

MAIN OUTCOMES AND MEASURES

The primary outcome was a composite of pulmonary complications within the first 5 postoperative days, including respiratory failure, acute respiratory distress syndrome, bronchospasm, new pulmonary infiltrates, pulmonary infection, aspiration pneumonitis, pleural effusion, atelectasis, cardiopulmonary edema, and pneumothorax. Among the 9 prespecified secondary outcomes, 3 were intraoperative complications, including hypoxemia (oxygen desaturation with Spo2 ≤92% for >1 minute).

RESULTS

Among 2013 adults who were randomized, 1976 (98.2%) completed the trial (mean age, 48.8 years; 1381 [69.9%] women; 1778 [90.1%] underwent abdominal operations). In the intention-to-treat analysis, the primary outcome occurred in 211 of 989 patients (21.3%) in the high level of PEEP group compared with 233 of 987 patients (23.6%) in the low level of PEEP group (difference, -2.3% [95% CI, -5.9% to 1.4%]; risk ratio, 0.93 [95% CI, 0.83 to 1.04]; P = .23). Among the 9 prespecified secondary outcomes, 6 were not significantly different between the high and low level of PEEP groups, and 3 were significantly different, including fewer patients with hypoxemia (5.0% in the high level of PEEP group vs 13.6% in the low level of PEEP group; difference, -8.6% [95% CI, -11.1% to 6.1%]; P < .001).

CONCLUSIONS AND RELEVANCE

Among obese patients undergoing surgery under general anesthesia, an intraoperative mechanical ventilation strategy with a higher level of PEEP and alveolar recruitment maneuvers, compared with a strategy with a lower level of PEEP, did not reduce postoperative pulmonary complications.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT02148692.

Tidal changes in PaO2 and their relationship to cyclical lung recruitment/derecruitment in a porcine lung injury model

BACKGROUND

Tidal recruitment/derecruitment (R/D) of collapsed regions in lung injury has been presumed to cause respiratory oscillations in the partial pressure of arterial oxygen (PaO₂). These phenomena have not yet been studied simultaneously. We examined the relationship between R/D and PaO₂ oscillations by contemporaneous measurement of lung-density changes and PaO₂.

METHODS

Five anaesthetised pigs were studied after surfactant depletion via a saline-lavage model of R/D. The animals were ventilated with a mean fraction of inspired O₂ (FiO₂) of 0.7 and a tidal volume of 10 ml kg^-1. Protocolised changes in pressure- and volume-controlled modes, inspiratory:expiratory ratio (I:E), and three types of breath-hold manoeuvres were undertaken. Lung collapse and PaO₂ were recorded using dynamic computed tomography (dCT) and a rapid PaO₂ sensor.

RESULTS

During tidal ventilation, the expiratory lung collapse increased when I:E <1 [mean (standard deviation) lung collapse=15.7 (8.7)%; P<0.05], but the amplitude of respiratory PaO₂ oscillations [2.2 (0.8) kPa] did not change during the respiratory cycle. The expected relationship between respiratory PaO₂ oscillation amplitude and R/D was therefore not clear. Lung collapse increased during breath-hold manoeuvres at end-expiration and end-inspiration (14% vs 0.9-2.1%; P<0.0001). The mean change in PaO₂ from beginning to end of breath-hold manoeuvres was significantly different with each type of breath-hold manoeuvre (P<0.0001).

CONCLUSIONS

This study in a porcine model of collapse-prone lungs did not demonstrate the expected association between PaO₂ oscillation amplitude and the degree of recruitment/derecruitment. The results suggest that changes in pulmonary ventilation are not the sole determinant of changes in PaO₂ during mechanical ventilation in lung injury.

Potentially modifiable respiratory variables contributing to outcome in ICU patients without ARDS: a secondary analysis of PRoVENT

BACKGROUND

The majority of critically ill patients do not suffer from acute respiratory distress syndrome (ARDS). To improve the treatment of these patients, we aimed to identify potentially modifiable factors associated with outcome of these patients.

METHODS

The PRoVENT was an international, multicenter, prospective cohort study of consecutive patients under invasive mechanical ventilatory support. A predefined secondary analysis was to examine factors associated with mortality. The primary endpoint was all-cause in-hospital mortality.

RESULTS

935 Patients were included. In-hospital mortality was 21%. Compared to patients who died, patients who survived had a lower risk of ARDS according to the 'Lung Injury Prediction Score' and received lower maximum airway pressure (P_max), driving pressure (ΔP), positive end-expiratory pressure, and FiO₂ levels. Tidal volume size was similar between the groups. Higher P_max was a potentially modifiable ventilatory variable associated with in-hospital mortality in multivariable analyses. ΔP was not independently associated with in-hospital mortality, but reliable values for ΔP were available for 343 patients only. Non-modifiable factors associated with in-hospital mortality were older age, presence of immunosuppression, higher non-pulmonary sequential organ failure assessment scores, lower pulse oximetry readings, higher heart rates, and functional dependence.

CONCLUSIONS

Higher P_max was independently associated with higher in-hospital mortality in mechanically ventilated critically ill patients under mechanical ventilatory support for reasons other than ARDS. Trial Registration ClinicalTrials.gov (NCT01868321).

Specific anesthesia-induced lung volume changes from induction to emergence: a pilot study

BACKGROUND

Studies aimed at maintaining intraoperative lung volume to reduce post-operative pulmonary complications have been inconclusive because they mixed up the effect of general anesthesia and the surgical procedure. Our aims were to study: (1) lung volume during the entire course of anesthesia without the confounding effects of surgical procedures; (2) the combination of three interventions to maintain lung volume; and (3) the emergence phase with focus on the restored activation of the respiratory muscles.

METHODS

Eighteen ASA I-II patients undergoing ENT surgery under general anesthesia without muscle relaxants were randomized to an intervention group, receiving lung recruitment maneuver (LRM) after induction, 7 cmH₂ O positive end-expiratory pressure (PEEP) during anesthesia and continuous positive airway pressure (CPAP) during emergence with 0.4 inspired oxygen fraction (FiO₂ ) or a control group, ventilated without LRM, with 0 cmH₂ O PEEP, and 1.0 FiO₂ during emergence without CPAP application. End-expiratory lung volume (EELV) was continuously estimated by opto-electronic plethysmography. Inspiratory and expiratory ribcage muscles electromyography was measured in a subset of seven patients.

RESULTS

End-expiratory lung volume decreased after induction in both groups. It remained low in the control group and further decreased at emergence, because of active expiratory muscle contraction. In the intervention group, EELV increased after LRM and remained high after extubation.

CONCLUSION

A combined intervention consisting of LRM, PEEP and CPAP during emergence may effectively maintain EELV during anesthesia and even after extubation. An unexpected finding was that the activation of the expiratory muscles may contribute to EELV reduction during the emergence phase.

Atelectasis is inversely proportional to transpulmonary pressure during weaning from ventilator support in a large animal model

BACKGROUND

In mechanically ventilated, lung injured, patients without spontaneous breathing effort, atelectasis with shunt and desaturation may appear suddenly when ventilator pressures are decreased. It is not known how such a formation of atelectasis is related to transpulmonary pressure (P_L ) during weaning from mechanical ventilation when the spontaneous breathing effort is increased. If the relation between P_L and atelectasis were known, monitoring of P_L might help to avoid formation of atelectasis and cyclic collapse during weaning. The main purpose of this study was to determine the relation between P_L and atelectasis in an experimental model representing weaning from mechanical ventilation.

METHODS

Dynamic transverse computed tomography scans were acquired in ten anaesthetized, surfactant-depleted pigs with preserved spontaneous breathing, as ventilator support was lowered by sequentially reducing inspiratory pressure and positive end expiratory pressure in steps. The volumes of gas and atelectasis in the lungs were correlated with P_L obtained using oesophageal pressure recordings. Work of breathing (WOB) was assessed from Campbell diagrams.

RESULTS

Gradual decrease in P_L in both end-expiration and end-inspiration caused a proportional increase in atelectasis and decrease in the gas content (linear mixed model with an autoregressive correlation matrix; P < 0.001) as the WOB increased. However, cyclic alveolar collapse during tidal ventilation did not increase significantly.

CONCLUSION

We found a proportional correlation between atelectasis and P_L during the 'weaning process' in experimental mild lung injury. If confirmed in the clinical setting, a gradual tapering of ventilator support can be recommended for weaning without risk of sudden formation of atelectasis.

Respiratory oscillations in alveolar oxygen tension measured in arterial blood

Arterial oxygen partial pressure can increase during inspiration and decrease during expiration in the presence of a variable shunt fraction, such as with cyclical atelectasis, but it is generally presumed to remain constant within a respiratory cycle in the healthy lung. We measured arterial oxygen partial pressure continuously with a fast intra-vascular sensor in the carotid artery of anaesthetized, mechanically ventilated pigs, without lung injury. Here we demonstrate that arterial oxygen partial pressure shows respiratory oscillations in the uninjured pig lung, in the absence of cyclical atelectasis (as determined with dynamic computed tomography), with oscillation amplitudes that exceeded 50 mmHg, depending on the conditions of mechanical ventilation. These arterial oxygen partial pressure respiratory oscillations can be modelled from a single alveolar compartment and a constant oxygen uptake, without the requirement for an increased shunt fraction during expiration. Our results are likely to contribute to the interpretation of arterial oxygen respiratory oscillations observed during mechanical ventilation in the acute respiratory distress syndrome.

Epidemiological characteristics, practice of ventilation, and clinical outcome in patients at risk of acute respiratory distress syndrome in intensive care units from 16 countries (PRoVENT): an international, multicentre, prospective study

BACKGROUND

Scant information exists about the epidemiological characteristics and outcome of patients in the intensive care unit (ICU) at risk of acute respiratory distress syndrome (ARDS) and how ventilation is managed in these individuals. We aimed to establish the epidemiological characteristics of patients at risk of ARDS, describe ventilation management in this population, and assess outcomes compared with people at no risk of ARDS.

METHODS

PRoVENT (PRactice of VENTilation in critically ill patients without ARDS at onset of ventilation) is an international, multicentre, prospective study undertaken at 119 ICUs in 16 countries worldwide. All patients aged 18 years or older who were receiving mechanical ventilation in participating ICUs during a 1-week period between January, 2014, and January, 2015, were enrolled into the study. The Lung Injury Prediction Score (LIPS) was used to stratify risk of ARDS, with a score of 4 or higher defining those at risk of ARDS. The primary outcome was the proportion of patients at risk of ARDS. Secondary outcomes included ventilatory management (including tidal volume [V_T] expressed as mL/kg predicted bodyweight [PBW], and positive end-expiratory pressure [PEEP] expressed as cm H₂O), development of pulmonary complications, and clinical outcomes. The PRoVENT study is registered at ClinicalTrials.gov, NCT01868321. The study has been completed.

FINDINGS

Of 3023 patients screened for the study, 935 individuals fulfilled the inclusion criteria. Of these critically ill patients, 282 were at risk of ARDS (30%, 95% CI 27-33), representing 0·14 cases per ICU bed over a 1-week period. V_T was similar for patients at risk and not at risk of ARDS (median 7·6 mL/kg PBW [IQR 6·7-9·1] vs 7·9 mL/kg PBW [6·8-9·1]; p=0·346). PEEP was higher in patients at risk of ARDS compared with those not at risk (median 6·0 cm H₂O [IQR 5·0-8·0] vs 5·0 cm H₂O [5·0-7·0]; p<0·0001). The prevalence of ARDS in patients at risk of ARDS was higher than in individuals not at risk of ARDS (19/260 [7%] vs 17/556 [3%]; p=0·004). Compared with individuals not at risk of ARDS, patients at risk of ARDS had higher in-hospital mortality (86/543 [16%] vs 74/232 [32%]; p<0·0001), ICU mortality (62/533 [12%] vs 66/227 [29%]; p<0·0001), and 90-day mortality (109/653 [17%] vs 88/282 [31%]; p<0·0001). V_T did not differ between patients who did and did not develop ARDS (p=0·471 for those at risk of ARDS; p=0·323 for those not at risk).

INTERPRETATION

Around a third of patients receiving mechanical ventilation in the ICU were at risk of ARDS. Pulmonary complications occur frequently in patients at risk of ARDS and their clinical outcome is worse compared with those not at risk of ARDS. There is potential for improvement in the management of patients without ARDS. Further refinements are needed for prediction of ARDS.

FUNDING

None.

Open lung approach ventilation abolishes the negative effects of respiratory rate in experimental lung injury

BACKGROUND

We recently reported that a high respiratory rate was associated with less inflammation than a low respiratory rate, but caused more pulmonary edema in a model of ARDS when an ARDSNet ventilatory strategy was used. We hypothesized that an open lung approach (OLA) strategy would neutralize the independent effects of respiratory rate on lung inflammation and edema. This hypothesis was tested in an ARDS model using two clinically relevant respiratory rates during OLA strategy.

METHODS

Twelve piglets were subjected to an experimental model of ARDS and randomized into two groups: LRR (20 breaths/min) and HRR (40 breaths/min). They were mechanically ventilated for 6 h according to an OLA strategy. We assessed respiratory mechanics, hemodynamics, and extravascular lung water (EVLW). At the end of the experiment, wet/dry ratio, regional histology, and cytokines were evaluated.

RESULTS

After the ARDS model was established, Cdyn,rs decreased from 21 ± 3.3 to 9.0 ± 1.8 ml/cmH2 O (P < 0.0001). After the lung recruitment maneuver, Cdyn,rs increased to the pre-injury value. During OLA ventilation, no differences in respiratory mechanics, hemodynamics, or EVLW were observed between groups. Wet/dry ratio and histological scores were not different between groups. Cytokine quantification was similar and showed a homogeneous distribution throughout the lung in both groups.

CONCLUSION

Contrary to previous findings with the ARDSNet strategy, respiratory rate did not influence lung inflammatory response or pulmonary edema during OLA ventilation in experimental ARDS. This indicates that changing the respiratory rate when OLA ventilation is used will not exacerbate lung injury.

CT-Assessment of Dependent Lung Densities in Man during General Anaesthesia

Purpose:

We aimed to describe the frequency of atelectasis occurring during anaesthesia, to describe the size and pattern of the atelectasis, and to standardise the method of identifying the atelectasis and calculate its area.

Material and Methods:

Patients (n=109) scheduled for elective abdominal surgery were examined with CT of the thorax during anaesthesia.

Results:

In 95 patients (87%) dependent pulmonary densities were seen, interpreted as atelectasis. Two different types of atelectasis were found — Homogeneous (78%) and non-homogeneous (9%). Attenuation values in histograms of the lung and atelectasis were studied using 2 methods of calculating the atelectatic area.

Conclusion:

On the basis of the present findings, we defined atelectasis as pulmonary dependent densities with attenuation values of —100 to +100 HU.

Zero expiratory pressure and low oxygen concentration promote heterogeneity of regional ventilation and lung densities

Background

It is not well known what is the main mechanism causing lung heterogeneity in healthy lungs under mechanical ventilation. We aimed to investigate the mechanisms causing heterogeneity of regional ventilation and parenchymal densities in healthy lungs under anesthesia and mechanical ventilation.

Methods

In a small animal model, synchrotron imaging was used to measure lung aeration and regional‐specific ventilation (sV̇). Heterogeneity of ventilation was calculated as the coefficient of variation in sV̇ (CV_sV̇). The coefficient of variation in lung densities (CV_D) was calculated for all lung tissue, and within hyperinflated, normally and poorly aerated areas. Three conditions were studied: zero end‐expiratory pressure (ZEEP) and F_IO₂ 0.21; ZEEP and F_IO₂ 1.0; PEEP 12 cmH₂O and F_IO₂1.0 (Open Lung‐PEEP = OLP).

Results

The mean tissue density at OLP was lower than ZEEP‐1.0 and ZEEP‐0.21. There were larger subregions with low sV̇ and poor aeration at ZEEP‐0.21 than at OLP: 12.9 ± 9.0 vs. 0.6 ± 0.4% in the non‐dependent level, and 17.5 ± 8.2 vs. 0.4 ± 0.1% in the dependent one (P = 0.041). The CV_sV̇ of the total imaged lung at PEEP 12 cmH₂O was significantly lower than on ZEEP, regardless of F_IO₂, indicating more heterogeneity of ventilation during ZEEP (0.23 ± 0.03 vs. 0.54 ± 0.37, P = 0.049). CV_D changed over the different mechanical ventilation settings (P = 0.011); predominantly, CV_D increased during ZEEP. The spatial distribution of the CV_D calculated for the poorly aerated density category changed with the mechanical ventilation settings, increasing in the dependent level during ZEEP.

Conclusion

ZEEP together with low F_IO₂ promoted heterogeneity of ventilation and lung tissue densities, fostering a greater amount of airway closure and ventilation inhomogeneities in poorly aerated regions.

Glucocorticoid receptor expression and binding capacity in patients with burn injury

BACKGROUND

Burn injuries are associated with strong inflammation and risk of secondary sepsis which both may affect the function of the glucocorticoid receptor (GR). The aim of this study was to determine GR expression and binding capacity in leucocytes from patients admitted to a tertiary burn center.

METHODS

Blood was sampled from 13 patients on admission and days 7, 14 and 21, and once from 16 healthy subjects. Patients were grouped according to the extent of burn and to any sepsis on day 7. Expression and binding capacity of GR were determined as arbitrary units using flow cytometry.

RESULTS

GR expression and binding capacity were increased compared to healthy subjects in most circulating leucocyte subsets on admission irrespective of burn size. Patients with sepsis on day 7 displayed increased GR expression in T lymphocytes (51.8%, P < 0.01) compared to admission. There was a negative correlation between GR binding capacity in neutrophils and burn size after 14 days (P < 0.05).

CONCLUSIONS

GR expression and binding capacity are increased in most types of circulating leucocytes of severely burned patients on their admission to specialized burn care. If sepsis is present after 1 week, it is associated with higher GR expression in T lymphocytes and NK cells.

Preserved oxygenation in obese patients receiving protective ventilation during laparoscopic surgery: a randomized controlled study

BACKGROUND

Venous admixture from atelectasis and airway closure impedes oxygenation during general anaesthesia. We tested the hypothesis that continuous positive airway pressure (CPAP) during pre-oxygenation and reduced fraction of inspiratory oxygen (FIO2 ) during emergence from anaesthesia can improve oxygenation in patients with obesity undergoing laparoscopic surgery.

METHODS

In the intervention group (n = 20, median BMI 41.9), a CPAP of 10 cmH2 O was used during pre-oxygenation and induction of anaesthesia, but no CPAP was used in the control group (n = 20, median BMI 38.1). During anaesthesia, all patients were ventilated in volume-controlled mode with an FIO2 of 0.4 and a positive end-expiratory pressure (PEEP) of 10 cmH2 O. During emergence, before extubation, the control group was given an FIO2 of 1.0 and the intervention group was divided into two subgroups, which were given an FIO2 of 1.0 or 0.31. Oxygenation was assessed perioperatively by the estimated venous admixture (EVA).

RESULTS

The median EVA before pre-oxygenation was about 8% in both groups. During anaesthesia after intubation, the median EVA was 8.2% in the intervention vs. 13.2% in the control group (P = 0.048). After CO2 pneumoperitoneum, the median EVA was 8.4% in the intervention vs. 9.9% in the control group (P > 0.05). One hour post-operatively, oxygenation had deteriorated in patients given an FIO2 of 1.0 during emergence but not in patients given an FIO2 of 0.31.

CONCLUSIONS

A CPAP of 10 cmH2 O during pre-oxygenation and induction, followed by PEEP after intubation, seemed to preserve oxygenation during anaesthesia. Post-operative oxygenation depended on the FIO2 used during emergence.

High respiratory rate is associated with early reduction of lung edema clearance in an experimental model of ARDS

BACKGROUND

The independent impact of respiratory rate on ventilator-induced lung injury has not been fully elucidated. The aim of this study was to investigate the effects of two clinically relevant respiratory rates on early ventilator-induced lung injury evolution and lung edema during the protective ARDSNet strategy. We hypothesized that the use of a higher respiratory rate during a protective ARDSNet ventilation strategy increases lung inflammation and, in addition, lung edema associated to strain-induced activation of transforming growth factor beta (TGF-β) in the lung epithelium.

METHODS

Twelve healthy piglets were submitted to a two-hit lung injury model and randomized into two groups: LRR (20 breaths/min) and HRR (40 breaths/min). They were mechanically ventilated during 6 h according to the ARDSNet strategy. We assessed respiratory mechanics, hemodynamics, and extravascular lung water (EVLW). At the end of the experiment, the lungs were excised and wet/dry ratio, TGF-β pathway markers, regional histology, and cytokines were evaluated.

RESULTS

No differences in oxygenation, PaCO2 levels, systemic and pulmonary arterial pressures were observed during the study. Respiratory system compliance and mean airway pressure were lower in LRR group. A decrease in EVLW over time occurred only in the LRR group (P < 0.05). Wet/dry ratio was higher in the HRR group (P < 0.05), as well as TGF-β pathway activation. Histological findings suggestive of inflammation and inflammatory tissue cytokines were higher in LRR.

CONCLUSION

HRR was associated with more pulmonary edema and higher activation of the TGF-β pathway. In contrast with our hypothesis, HRR was associated with less lung inflammation.

Veno-venous extracorporeal CO2 removal for the treatment of severe respiratory acidosis: pathophysiological and technical considerations

Introduction

While non-invasive ventilation aimed at avoiding intubation has become the modality of choice to treat mild to moderate acute respiratory acidosis, many severely acidotic patients (pH <7.20) still need intubation. Extracorporeal veno-venous CO₂ removal (ECCO₂R) could prove to be an alternative. The present animal study tested in a systematic fashion technical requirements for successful ECCO₂R in terms of cannula size, blood and sweep gas flow.

Methods

ECCO₂R with a 0.98 m² surface oxygenator was performed in six acidotic (pH <7.20) pigs using either a 14.5 French (Fr) or a 19Fr catheter, with sweep gas flow rates of 8 and 16 L/minute, respectively. During each experiment the blood flow was incrementally increased to a maximum of 400 mL/minute (14.5Fr catheter) and 1000 mL/minute (19Fr catheter).

Results

Amelioration of severe respiratory acidosis was only feasible when blood flow rates of 750 to 1000 mL/minute (19Fr catheter) were used. Maximal CO₂-elimination was 146.1 ± 22.6 mL/minute, while pH increased from 7.13 ± 0.08 to 7.41 ± 0.07 (blood flow of 1000 mL/minute; sweep gas flow 16 L/minute). Accordingly, a sweep gas flow of 8 L/minute resulted in a maximal CO₂-elimination rate of 138.0 ± 16.9 mL/minute. The 14.5Fr catheter allowed a maximum CO₂ elimination rate of 77.9 mL/minute, which did not result in the normalization of pH.

Conclusions

Veno-venous ECCO₂R may serve as a treatment option for severe respiratory acidosis. In this porcine model, ECCO₂R was most effective when using blood flow rates ranging between 750 and 1000 mL/minute, while an increase in sweep gas flow from 8 to 16 L/minute had less impact on ECCO₂R in this setting.

Lymphatic drainage between thorax and abdomen: please take good care of this well-performing machinery…

INTRODUCTION

Patients with sepsis often receive large amounts of fluids and the presence of capillary leak, trauma or bleeding results in ongoing fluid resuscitation. This increases interstitial and intestinal edema and finally leads to intra-abdominal hypertension (IAH), which in turn impedes lymphatic drainage. Patients with IAH often develop secondary respiratory failure needing mechanical ventilation with high intrathoracic pressure or PEEP that might further alter lymphatic drainage. This review will try to convince the reader of the importance of the lymphatics in septic patients with IAH.

METHODS

A Medline and PubMed literature search was performed using the terms "abdominal pressure", "lymphatic drainage" and "ascites formation". The references from these studies were searched for relevant articles that may have been missed in the primary search. These articles served as the basis for the recommendations below.

RESULTS

Induction of sepsis with lesion of the capillary alveolar barrier results in an increased water gradient between the capillaries and the interstitium in the lungs. The drainage flow to the thoracic duct is initially increased in order to protect the lung and maintain the pulmonary interstitium as dry as possible, however this results in increased intrathoracic pressure. Sepsis also increases the permeability of the capillaries in the splanchnic beds. In analogy to the lungs the lymphatic flow in the splanchnic areas increases together with the pressure inside as a physiological response in order to limit the increase in IAP. At a critical IAP level (around 20 cmH2O) the lymph flow starts to decrease and the splanchnic water content progressively increases. The lymph flow from the abdomen to the thorax is progressively decreased resulting in increased splanchnic water content and ascites formation. The presence of mechanical ventilation with high PEEP reduces the lymph drainage further which together with the increase in IAP decreases the lymphatic pressure gradient in the splanchnic regions, with a further increase in water content and IAP triggering a vicious cycle.

CONCLUSION

Although often overlooked the role of lymphatic flow is complex but very important to determine not only the fluid balance in the lung but also in the peripheral organs. Different pathologies and treatments can markedly influence the pathophysiology of the lymphatics with dramatic effects on endorgan function.

A practical approach to the theoretical models to calculate NO parameters of the respiratory system

Expired nitric oxide (NO) is used as a biomarker in different respiratory diseases. The recommended flow rate of 50 mL s⁻¹ (F(E)NO₀.₀₅) does not reveal from where in the lung NO production originated. Theoretical models of NO transfer from the respiratory system, linear or nonlinear approaches, have therefore been developed and applied. These models can estimate NO from distal lung (alveolar NO) and airways (bronchial flux). The aim of this study was to show the limitation in exhaled flow rate for the theoretical models of NO production in the respiratory system, linear and nonlinear models. Subjects (n = 32) exhaled at eight different flow rates between 10-350 mL s⁻¹ for the theoretical protocols. Additional subjects (n = 32) exhaled at tree flow rates (20, 100 and 350 mL s⁻¹) for the clinical protocol. When alveolar NO is calculated using high flow rates with the linear model, correction for axial back diffusion becomes negligible, -0.04 ppb and bronchial flux enhanced by 1.27. With Högman and Meriläinen algorithm (nonlinear model) the corrections factors can be understood to be embedded, and the flow rates to be used are ≤20, 100 and ≥350 mL s⁻¹. Applying these flow rates in a clinical setting any F(E)NO can be calculated necessitating fewer exhalations. Hence, measured F(E)NO₀.₀₅ 12.9 (7.2-18.7) ppb and calculated 12.9 (6.8-18.7) ppb. In conclusion, the only possibility to avoid inconsistencies between research groups is to use the measured NO values as such in modelling, and apply tight quality control to accuracies in both NO concentration and exhaled flow measurements.

Improved ventilation-perfusion matching by abdominal insufflation (pneumoperitoneum) with CO2 but not with air

BACKGROUND

Pneumoperitoneum (PP) by CO2-insufflation causes atelectasis however with maintained or even improved oxygenation. We studied the effect of abdominal insufflation by carbon dioxide (CO2) and air on gas exchange during PP.

METHODS

Twenty-seven anesthetized pigs were studied during PP with insufflations to 12 mmHg by either 1/CO2, 2/ air or 3/CO2 during intravenous nitroprusside infusion (SNP) (N.=9 in each group). In 3 pigs in each group, gamma camera technique (SPECT) was used to study ventilation and perfusion distributions, in another 6 pigs an inert-gas technique (MIGET) was used for assessing ventilation-perfusion matching (VA/Q). Measurements were made during anesthesia before and after 60 minutes of PP.

RESULTS

CO2-PP caused a shift of blood flow away from dependent, non-ventilated (atelectatic) to ventilated regions. Air-PP caused smaller, and SNP-PP even less shift of lung blood flow. Shunt decreased during CO2-PP (6 ± 1% compared to baseline 9 ± 2%, P<0.05), did not change during Air-PP (10 ± 2%) and increased during SNP-PP (16 ± 2%, P<0.05). PaO2 increased from baseline 35 ± 2 to 41 ± 3 kPa during CO2-PP and decreased to 32 ± 3 kPa during Air-PP and to 27 ± 3 kPa during SNP-PP (P<0.05 for all three comparisons). PaCO2 increased during CO2- and SNP-PP.

CONCLUSION

CO2-PP enhanced the shift of blood flow towards better ventilated areas of the lung compared to Air-PP and SNP blunted the effects seen with CO2-PP. SNP may thus have blunted and CO2 potentiated vasoconstriction, by hypoxic pulmonary vasoconstriction or another mechanism.

Optimizing positive end-expiratory pressure by oscillatory mechanics minimizes tidal recruitment and distension: an experimental study in a lavage model of lung injury

Introduction

It is well established that during mechanical ventilation of patients with acute respiratory distress syndrome cyclic recruitment/derecruitment and overdistension are potentially injurious for lung tissues. We evaluated whether the forced oscillation technique (FOT) could be used to guide the ventilator settings in order to minimize cyclic lung recruitment/derecruitment and cyclic mechanical stress in an experimental model of acute lung injury.

Methods

We studied six pigs in which lung injury was induced by bronchoalveolar lavage. The animals were ventilated with a tidal volume of 6 ml/kg. Forced oscillations at 5 Hz were superimposed on the ventilation waveform. Pressure and flow were measured at the tip and at the inlet of the endotracheal tube respectively. Respiratory system reactance (Xrs) was computed from the pressure and flow signals and expressed in terms of oscillatory elastance (E_X5). Positive end-expiratory pressure (PEEP) was increased from 0 to 24 cm H₂O in steps of 4 cm H₂O and subsequently decreased from 24 to 0 in steps of 2 cm H₂O. At each PEEP step CT scans and E_X5 were assessed at end-expiration and end-inspiration.

Results

During deflation the relationship between both end-expiratory and end-inspiratory E_X5 and PEEP was a U-shaped curve with minimum values at PEEP = 13.4 ± 1.0 cm H₂O (mean ± SD) and 13.0 ± 1.0 cm H₂O respectively. E_X5 was always higher at end-inspiration than at end-expiration, the difference between the average curves being minimal at 12 cm H₂O. At this PEEP level, CT did not show any substantial sign of intra-tidal recruitment/derecruitment or expiratory lung collapse.

Conclusions

Using FOT it was possible to measure E_X5 both at end-expiration and at end-inspiration. The optimal PEEP strategy based on end-expiratory E_X5 minimized intra-tidal recruitment/derecruitment as assessed by CT, and the concurrent attenuation of intra-tidal variations of E_X5 suggests that it may also minimize tidal mechanical stress.

Esophageal pressure: benefit and limitations

The recording of esophageal pressure (Pes) in supine position as a substitute for pleural pressure is difficult and fraught with potential errors. Pes is affected by the: 1) elastance and weight of the lung; 2) elastance and weight of the rib cage; 3) weight of the mediastinal organs; 4) elastance and weight of the diaphragm and abdomen; 5) elastance of the esophageal wall; and 6) elastance of the esophageal balloon (if filled with too much air). If the purpose is to measure lung compliance in the intensive care patient, reasonably useful information might be obtained by measuring airway pressure alone, considering chest wall compliance to be a weight that is forced away by the ventilation. Such weight requires a constant pressure for displacement. The transpulmonary pressure, whether calculated with Pes or by another measure of abdominal pressure, may guide in PEEP titration. It may also enable calculation of stresses applied to the lung and these may be more important in guiding an optimal ventilator setting than an optimum compliance or oxygenation of blood. Diaphragm function can be estimated by esophageal minus gastric pressure and with even more precision, when combined with diaphragm electromyography.

Non-toxic alveolar oxygen concentration without hypoxemia during apnoeic oxygenation: an experimental study

BACKGROUND

Oxygenation without tidal breathing, i.e. apnoeic oxygenation in combination with extracorporeal carbon dioxide removal, might be an option in the treatment of acute respiratory failure. However, ventilation with 100% O₂, which is potentially toxic, is considered a prerequisite to ensure acceptable oxygenation. We hypothesized that trapping nitrogen (N₂) in the lungs before the start of apnoeic oxygenation would keep the alveolar O₂ at a non-toxic level and still maintain normoxaemia. The aim was to test whether a predicted N₂ concentration would agree with a measured concentration at the end of an apnoeic period.

METHODS

Seven anaesthetized, muscle relaxed, endotracheally intubated pigs (22-27 kg) were ventilated in a randomized order with an inspired fraction of O₂ 0.6 and 0.8 at two positive end-expiratory pressure levels (5 cm and 10 cm H₂O) before being connected to continuous positive airway pressure using 100% O₂ for apnoeic oxygenation. N₂ was measured before the start of and at the end of the 10-min apnoeic period. The predicted N₂ concentration was calculated from the initial N₂ concentration, the end-expiratory lung volume, and the anatomical dead space.

RESULTS

The mean difference and standard deviation between measured and predicted N₂ concentration was -0.5 ± 2%, P = 0.587. No significant difference in the agreement between measured and predicted N₂ concentrations was seen in the four settings.

CONCLUSIONS

This study indicates that it is possible to predict and keep alveolar N₂ concentration at a desired level and, thus, alveolar O₂ concentration at a non-toxic level during apnoeic oxygenation.

Capnography reflects ventilation/perfusion distribution in a model of acute lung injury

BACKGROUND

Changes in the shape of the capnogram may reflect changes in lung physiology. We studied the effect of different ventilation/perfusion ratios (V/Q) induced by positive end-expiratory pressures (PEEP) and lung recruitment on phase III slope (S(III)) of volumetric capnograms.

METHODS

Seven lung-lavaged pigs received volume control ventilation at tidal volumes of 6 ml/kg. After a lung recruitment maneuver, open-lung PEEP (OL-PEEP) was defined at 2 cmH(2)O above the PEEP at the onset of lung collapse as identified by the maximum respiratory compliance during a decremental PEEP trial. Thereafter, six distinct PEEP levels either at OL-PEEP, 4 cmH(2)O above or below this level were applied in a random order, either with or without a prior lung recruitment maneuver. Ventilation-perfusion distribution (using multiple inert gas elimination technique), hemodynamics, blood gases and volumetric capnography data were recorded at the end of each condition (minute 40).

RESULTS

S (III) showed the lowest value whenever lung recruitment and OL-PEEP were jointly applied and was associated with the lowest dispersion of ventilation and perfusion (Disp(R-E)), the lowest ratio of alveolar dead space to alveolar tidal volume (VD(alv)/VT(alv)) and the lowest difference between arterial and end-tidal pCO(2) (Pa-ETCO(2)). Spearman's rank correlations between S(III) and Disp(R-E) showed a ρ=0.85 with 95% CI for ρ (Fisher's Z-transformation) of 0.74-0.91, P<0.0001.

CONCLUSION

In this experimental model of lung injury, changes in the phase III slope of the capnograms were directly correlated with the degree of ventilation/perfusion dispersion.

Effects of propofol anaesthesia on thoraco-abdominal volume variations during spontaneous breathing and mechanical ventilation

BACKGROUND

Anaesthesia based on inhalational agents has profound effects on chest wall configuration and breathing pattern. The effects of propofol are less well characterised. The aim of the current study was to evaluate the effects of propofol anaesthesia on chest wall motion during spontaneous breathing and positive pressure ventilation.

METHODS

We studied 16 subjects undergoing elective surgery requiring general anaesthesia. Chest wall volumes were continuously monitored by opto-electronic plethysmography during quiet breathing (QB) in the conscious state, induction of anaesthesia, spontaneous breathing during anaesthesia (SB), pressure support ventilation (PSV) and pressure control ventilation (PCV) after muscle paralysis.

RESULTS

The total chest wall volume decreased by 0.41 ± 0.08 l immediately after induction by equal reductions in the rib cage and abdominal volumes. An increase in the rib cage volume was then seen, resulting in total chest wall volumes 0.26 ± 0.09, 0.24 ± 0.10, 0.22 ± 0.10 l lower than baseline, during SB, PSV and PCV, respectively. During QB, rib cage volume displacement corresponded to 34.2 ± 5.3% of the tidal volume. During SB, PSV and PCV, this increased to 42.2 ± 4.9%, 48.2 ± 3.6% and 46.3 ± 3.2%, respectively, with a corresponding decrease in the abdominal contribution. Breathing was initiated by the rib cage muscles during SB.

CONCLUSION

Propofol anaesthesia decreases end-expiratory chest wall volume, with a more pronounced effect on the diaphragm than on the rib cage muscles, which initiate breathing after apnoea.

Oxygen concentration and characteristics of progressive atelectasis formation during anaesthesia

BACKGROUND

atelectasis is a common consequence of pre-oxygenation with 100% oxygen during induction of anaesthesia. Lowering the oxygen level during pre-oxygenation reduces atelectasis. Whether this effect is maintained during anaesthesia is unknown.

METHODS

during and after pre-oxygenation and induction of anaesthesia with 60%, 80% or 100% oxygen concentration, followed by anaesthesia with mechanical ventilation with 40% oxygen in nitrogen and positive end-expiratory pressure of 3 cmH(2) O, we used repeated computed tomography (CT) to investigate the early (0-14 min) vs. the later time course (14-45 min) of atelectasis formation.

RESULTS

in the early time course, atelectasis was studied awake, 4, 7 and 14 min after start of pre-oxygenation with 60%, 80% or 100% oxygen concentration. The differences in the area of atelectasis formation between awake and 7 min and between 7 and 14 min were significant, irrespective of oxygen concentration (P<0.05). During the late time course, studied after pre-oxygenation with 80% oxygen, the differences in the area of atelectasis formation between awake and 14 min, between 14 and 21 min, between 21 and 28 min and finally between 21 and 45 min were all significant (P<0.05).

CONCLUSION

formation of atelectasis after pre-oxygenation and induction of anaesthesia is oxygen and time dependent. The benefit of using 80% oxygen during induction of anaesthesia in order to reduce atelectasis diminished gradually with time.

Ventilation-perfusion distributions and gas exchange during carbon dioxide-pneumoperitoneum in a porcine model

BACKGROUND

Carbon dioxide (CO₂)-pneumoperitoneum (PP) of 12 mm Hg increases arterial oxygenation, but it also promotes collapse of dependent lung regions. This seeming paradox prompted the present animal study on the effects of PP on ventilation-perfusion distribution (V/Q) and gas exchange.

METHODS

Fourteen anaesthetized pigs were studied. In seven pigs, single photon emission computed tomography (SPECT) was used for spatial analysis of ventilation and perfusion distributions, and in another seven pigs, multiple inert gas elimination technique (MIGET) was used for detailed analysis of V/Q matching. SPECT/MIGET and central haemodynamics and pulmonary gas exchange were recorded during anaesthesia before and 60 min after induction of PP.

RESULTS

SPECT during PP showed no or only poorly ventilated regions in the dependent lung compared with the ventilation distribution during anaesthesia before PP. PP was accompanied by redistribution of blood flow away from the non- or poorly ventilated regions. V/Q analysis by MIGET showed decreased shunt from 9 (sd 2) to 7 (2)% after induction of PP (P<0.05). No regions of low V/Q were seen either before or during PP. Almost no regions of high V/Q developed during PP (1% of total ventilation). Pa(o₂) increased from 33 (1.2) to 35.7 (3.2) kPa (P<0.01) and arterial to end-tidal Pco₂ gradient (Pae'(co₂) increased from 0.3 (0.1) to 0.6 (0.2) kPa (P<0.05).

CONCLUSIONS

Perfusion was redistributed away from dorsal, collapsed lung regions when PP was established. This resulted in a better V/Q match. A possible mechanism is enhanced hypoxic pulmonary vasoconstriction.

Methodology investigation of expirograms for enabling contact free breath alcohol analysis

The present techniques for breath alcohol determination have usability limitations concerning practical use and the time and effort required for the test person. The rationale of the physiological assumptions in a recently demonstrated technique for breath analysis without a mouthpiece is investigated in this paper. Expirograms quantifying ethanol, carbon dioxide (CO(2)) and water (H(2)O) from 30 test subjects were analysed, with respect to the influence of individual variations in end-expiratory CO(2) and H(2)O concentrations, and possible benefits from simultaneous measurement of CO(2) or H(2)O. Both healthy subjects and patients suffering from pulmonary diseases performed breath tests with small and maximum volume expiration. The breath tests were recorded basically with a standard evidential instrument using infrared absorption spectroscopy, and equipped with a mouthpiece. Average concentrations were significantly higher for the maximum than for the small expirations. For the maximum expirations, the healthy subjects had a significantly higher end-expired PCO(2) of 4.4 ± 0.5 kPa (mean ± standard deviation) than the patients (3.9 ± 0.7 kPa). The corresponding values for H(2)O were 39 ± 1 and 38 ± 1 mg l(-1). The results indicate that the CO(2) variability is consistent with the requirements of accuracy for alcohol ignition interlocks. In addition, CO(2) as tracer gas is preferable to H(2)O due to its low concentration in ambient air. In instruments for evidential purposes H(2)O may be required as tracer gas for increased accuracy. Furthermore, the study provides support for early determination of breath alcohol concentration, indicating that determination after 2 s will introduce an additional random error of 0.02 mg l(-1) or less.

Development of atelectasis and arterial to end-tidal PCO2-difference in a porcine model of pneumoperitoneum

BACKGROUND

Intraperitoneal insufflation of carbon dioxide (CO2) may promote collapse of dependent lung regions. The present study was undertaken to study the effects of CO2-pneumoperitoneum (CO2-PP) on atelectasis formation, arterial oxygenation, and arterial to end-tidal PCO2-gradient (Pa-E'(CO2)).

METHODS

Fifteen anaesthetized pigs [mean body weight 28 (SD 2) kg] were studied. Spiral computed tomography (CT) scans were obtained for analysis of lung tissue density. In Group 1 (n=5) mechanical ventilation (V(T)=10 ml kg (-1), FI(O2)=0.5) was applied, in Group 2 (n=5) FI(O2) was increased for 30 min to 1.0 and in Group 3 (n=5) negative airway pressure was applied for 20 s in order to enhance development of atelectasis. Cardiopulmonary and CT data were obtained before, 10, and 90 min after induction of CO2-PP at an abdominal pressure of 12 mmHg.

RESULTS

Before CO2-PP, in Group 1 non-aerated tissue on CT scans was 1 (1)%, in Group 2 3 (2)% (P<0.05, compared with Group 1), and in Group 3 7 (3)% (P<0.05, compared with Group 1 and Group 2). CO2-PP significantly increased atelectasis in all groups. PaO2/FI(O2) fell and venous admixture ('shunt') increased in proportion to atelectasis during anaesthesia but CO2-PP had a varying effect on PaO2/FI(O2) and shunt. Thus, no correlation was seen between atelectasis and PaO2/FI(O2) or shunt when all data before and during CO2-PP were pooled. Pa-E'(CO2), on the other hand correlated strongly with the amount of atelectasis (r2=0.92).

CONCLUSIONS

Development of atelectasis during anaesthesia and PP may be estimated by an increased Pa-E'(CO2).