Does alveolar recruitment occur with positive end-expiratory pressure in adult respiratory distress syndrome patients?

We studied the effects of positive end-expiratory pressure (PEEP) (2 to 14 cm H₂O) on alveolar recruitment (Vrec), static respiratory compliance, and end-expiratory lung volume (EELV) in nine sedated, paralyzed, mechanically ventilated adult respiratory distress syndrome patients. Positive end-expiratory pressure was applied in increasing and decreasing steps of 2 cm H₂O. Flow, tidal volume, and airway pressure were measured. We used the rapid airway occlusion technique to determine static end-inspiratory elastic recoil pressure of the respiratory system (Pst, rs) and intrinsic PEEP (PEEPi). The changes in EELV were measured with respiratory inductive plethysmography. Alveolar recruitment was estimated as the difference in lung volume between PEEP and zero end-expiratory pressure (ZEEP) for the same end-inspiratory Pst,rs (20 cm H₂O). We found that (1) Vrec with PEEP up to 14 cm H₂O was in general rather small and was absent in two patients; (2) all patients exhibited PEEN at ZEEP (5.6 ± 1.0 cm H₂O) and little change in EELV and Vrec was achieved until the external PEEP exceeded PEEPi; (3) if end-inspiratory Pst, rs is high at ZEEP, there is little or no alveolar recruitment with PEEP; and (4) Vrec and EELV were slightly higher during stepwise deflation than stepwise inflation with PEEP, except at ZEEP where EELV did not change after inflation-deflation runs with PEEP.     

Effect of PEEP on work of breathing in mechanically ventilated COPD patients

Objective: To study the effects of PEEP on the inspiratory work done per breath on the respiratory system (W_I,rs) in patients with chronic obstructive pulmonary disease (COPD).¶Design: Physiological study.¶Setting: Fourteen-bed Medical ICU of a 1000-bed teaching tertiary hospital.¶Patients and participants: Ten patients with COPD intubated and mechanically ventilated for acute respiratory failure.¶Interventions: PEEP of 0 (ZEEP), ¶5, 10, and 15 cm H₂O were applied randomly and measurements done at the end of a 15–20 min period.¶Measurements and results: Using the rapid airway occlusion technique during constant flow inflation, we partitioned W_I,rs into its static and dynamic components. On ZEEP, the mean ± SD values of W_I,rs amounted to 15.1 ± 5.7 cm H₂O × l. With increasing PEEP, W_I,rs was significantly reduced to 12.6 ± 5.7, 11.1 ± 4.1, and 10.4 ± 2.8 cm H₂O × l at PEEP of 5, 10, and 15 cm H₂O, respectively (P < 0.05). This reduction was entirely due to the decline of the work due to intrinsic PEEP (PEEPi) and was abolished when the applied PEEP counterbalanced PEEPi. The other components of W_I,rs were not affected by PEEP. By increasing PEEP up to the level of PEEPi on ZEEP, no further increase in end-expiratory lung volume was observed.¶Conclusions: In COPD patients the application of PEEP levels close to PEEPi can substantially reduce W_I,rs without promoting further dynamic pulmonary hyperinflation.     

Effects of positive end-expiratory pressure on gas exchange and expiratory flow limitation in adult respiratory distress syndrome

OBJECTIVE: To assess the effects of different positive end-expiratory pressure (PEEP) levels (0, 5, 10, and 15 cm H2O) on tidal expiratory flow limitation (FL), regional intrinsic positive end-expiratory pressure (PEEPi) inhomogeneity, alveolar recruited volume (Vrec), respiratory mechanics, and arterial blood gases in mechanically ventilated patients with acute respiratory distress syndrome (ARDS). DESIGN: Prospective clinical study. SETTING: Multidisciplinary intensive care unit of a university hospital. PATIENTS: Thirteen sedated, mechanically ventilated patients during the first 2 days of ARDS. INTERVENTIONS: Detection of tidal FL and evaluation of total dynamic PEEP (PEEPt,dyn), total static PEEP (PEEPt,st), respiratory mechanics, and Vrec from pressure, flow, and volume traces provided by the ventilator. The average (+/-sd) tidal volume was 7.1 +/- 1.5 mL/kg, the total cycle duration was 2.9 +/- 0.45 secs, and the duty cycle was 0.35 +/- 0.05. MEASUREMENTS: Tidal FL was assessed using the negative expiratory pressure technique. Regional PEEPi inhomogeneity was assessed as the ratio of PEEPt,dyn to PEEPt,st (PEEPi inequality index), and Vrec was quantified as the difference in lung volume at the same airway pressure between quasi-static inflation volume-pressure curves on zero end-expiratory pressure (ZEEP) and PEEP. RESULTS: On ZEEP, seven patients exhibited FL amounting to 31 +/- 8% of tidal volume. They had higher PEEPt,st and PEEPi,st ( p<.001) and lower PEEPi inequality index ( p<.001) than the six nonflow-limited (NFL) patients. Two FL patients became NFL with PEEP of 5 cm H2O and five with PEEP of 10 cm H2O. In both groups, PaO2 increased progressively with PEEP. In the FL group, there was a significant correlation of PaO2 to PEEPi inequality index ( p=.002). For a given PEEP, Vrec was greater in NFL than FL patients, and a significant correlation of Pao to Vrec ( p<.001) was found only in the NFL group. CONCLUSIONS: We conclude that on ZEEP, tidal FL is common in ARDS patients and is associated with greater regional PEEPi inhomogeneity than in NFL patients. With PEEP of 10 cm H2O, flow limitation with concurrent cyclic dynamic airway compression and re-expansion and the risk of "low lung volume injury" were absent in all patients. In FL patients, PEEP induced a significant increase in PaO2, mainly because of the reduction of regional PEEPi inequality, whereas in the NFL group, arterial oxygenation was improved satisfactorily because of alveolar recruitment.     

Bronchodilator delivery by metered-dose inhaler in mechanically ventilated COPD patients: influence of tidal volume

Objective: The delivery of bronchodilator drugs with metered-dose inhaler (MDI) and a spacer in mechanically ventilated patients has become a widespread practice. However, the various ventilator settings that influence the efficacy of MDI are not well established. The tidal volume (V_T) during drug delivery has been suggested as one of the factors that might increase the effectiveness of this therapy. To test this, the effect of two different V_T on the bronchodilation induced by β ₂-agonists administered with MDI and a spacer in a group of mechanically ventilated patients with chronic obstructive pulmonary disease (COPD) was examined.¶Methods: Nine patients with COPD, mechanically ventilated on volume-controlled mode, were prospectively randomised to receive six puffs of salbutamol (S, 100 μg/puff) either with a V_T of 8 ml/kg (normal V_T, 582 ± 85) or with a V_T of 12 ml/kg (high V_T, 912 ± 137). With both modes inspiratory flow was identical. S was administered with an MDI adapted to the inspiratory limb of the ventilator circuit using an aerosol cloud enhancer spacer. After a ¶6-h washout, patients were crossed-over to receive S by the alternative mode of administration. Static and dynamic airway pressures, minimum (Rint) and maximum (Rrs) inspiratory resistance, the difference between Rrs and Rint (ΔR), static end-inspiratory respiratory system compliance (Cst,rs), intrinsic positive end-expiratory pressure (PEEPi) and heart rate (HR) were measured before and at 15, 30 and 60 min after S.¶Results: S caused a significant decrease in dynamic and static airway pressures, PEEPi, Rint and Rrs. These changes were not influenced by V_T and were evident at 15, 30 and 60 min after S. With normal and high V_T, Cst,rs, ΔR and HR did not change after S.¶Conclusions: We conclude that S delivered with an MDI and a spacer device induces significant bronchodilation in mechanically ventilated patients with COPD, the magnitude of which is not affected by at least a 50 % increase in V_T. These results do not support the V_T manipulations when bronchodilators are administered in adequate doses during controlled mechanical ventilation.     

Regional distribution of gas and tissue in acute respiratory distress syndrome. III. Consequences for the effects of positive end-expiratory pressure

Objective: To determine whether differences in lung morphology assessed by computed tomography (CT) affect the response to positive end-expiratory pressure (PEEP).¶Design: Prospective study over a 53-month period.¶Setting: Fourteen-bed surgical intensive care unit of a university hospital.¶Patients and participants: Seventy-one consecutive patients with early adult respiratory distress syndrome (ARDS).¶Measurements and results: Fast spiral thoracic CT was performed at zero end-expiratory pressure (ZEEP) and after implementation of PEEP 10 cmH₂O. Hemodynamic and respiratory parameters were measured in both conditions. PEEP-induced overdistension and alveolar recruitment were quantified by specifically designed software (Lungview). Overdistension occurred only in the upper lobes and was significantly correlated with the volume of lung, characterized by a CT attenuation ranging between –900 and –800 HU in ZEEP conditions. Cardiorespiratory effects of PEEP were similar in patients with primary and secondary ARDS. PEEP-induced alveolar recruitment of the lower lobes was significantly correlated with their lung volume (gas + tissue) at functional residual capacity. PEEP-induced alveolar recruitment was greater in the lower lobes with “inflammatory atelectasis” than in the lower lobes with “mechanical atelectasis.” Lung morphology as assessed by CT markedly influenced the effects of PEEP: in patients with diffuse CT attenuations PEEP induced a marked alveolar recruitment without overdistension, whereas in patients with lobar CT attenuations PEEP induced a mild alveolar recruitment associated with overdistension of previously aerated lung areas. These results can be explained by the uneven distribution of regional compliance characterizing patients with lobar CT attenuations (compliant upper lobes and stiff lower lobes) contrasting with a more even distribution of regional compliances observed in patients with diffuse CT attenuations.¶Conclusions: In patients with ARDS, the cardiorespiratory effects of PEEP are affected by lung morphology rather than by the cause of the lung injury (primary versus secondary ARDS). The regional distribution of the loss of aeration and the type of atelectasis –“mechanical” with a massive loss of lung volume, or “inflammatory” with a preservation of lung volume – characterizing the lower lobes are the main determinants of the cardiorespiratory effects of PEEP.     

Effects of prone position on alveolar recruitment and oxygenation in acute lung injury

Objective: To investigate the effects of prone position (PP) on alveolar recruitment and oxygenation in acute respiratory failure.¶Design: Prospective physiologic study.¶Setting: Medical ICU two in a university hospital.¶Patients: Twelve adult patients intubated and mechanically ventilated with medical primary acute lung injury/adult respiratory distress syndrome (ALI/ARDS) in whom PP was indicated.¶Measurements and results: We constructed the static inflation volume-pressure curves (V-P) of the respiratory system in the 12 patients and differentiated between lung and chest wall in ten of them. We determined the difference between end-expiratory lung volume on positive end-expiratory pressure (PEEP) and relaxation volume of the respiratory system on zero PEEP (ΔFRC). The recruited alveolar volume was computed as the ΔFRC times the ratio of static elastance of the respiratory system to the lung. These measurements together with arterial blood gases determination were made in supine position (SP1), after 1 h of PP and after 1 h of supine repositioning (SP2) at the same level of PEEP. The PaO₂/FIO₂ ratio improved from SP1 to PP (136 ± 17 vs 204 ± 24 mm Hg; p < 0.01). An PP-induced alveolar recruitment was found in five patients. The change in oxygenation correlated to the recruited volume. The static elastance of the chest wall decreased from 4.62 ± 0.99 cmH₂O/l in SP1 to 6.26 ± 0.54 cmH₂O/l in PP (p < 0.05) without any correlation to the change in oxygenation.¶Conclusions: Alveolar recruitment may be a mechanism of oxygenation improvement in some patients with acute hypoxemic respiratory failure. No correlation was found between change in oxygenation and chest wall elastic properties.     

Volume-dependent compliance in ARDS: proposal of a new diagnostic concept

Objective: Adaptation of ventilator settings to the individual's respiratory system mechanics requires information about the pressure-volume relationship and the change of compliance which is dependent on inflated volume. Unfortunately, established methods of obtaining this information are invasive and time-consuming, and, therefore, not well suited for clinical routine. We propose a new standardized diagnostic concept based on the recently developed slice method. This multiple linear regression method (MLR) determines volume-dependent respiratory system compliance (C_SLICE) within the tidal volume (V_T) during ongoing mechanical ventilation. The impact of a ventilator strategy, recommended by a consensus conference, on the course of compliance within V_T was investigated in patients with the acute respiratory distress syndrome (ARDS) or acute lung injury (ALI).¶Design: Prospective observational study.¶Setting: Intensive care unit of a university hospital.¶Patients: 14 ARDS patients, 2 patients with ALI.¶Interventions: None.¶Measurements and results: After measurement of flow and airway pressure and calculation of tracheal pressure, C_SLICE was determined. The resulting course of C_SLICE within V_T was estimated using a mathematical algorithm. C_SLICE data were compared to those obtained by standard MLR. We found decreasing C_SLICE mainly in the upper part of V_T in all patients. In 7 patients, we found an additional increasing C_SLICE mainly in the lower part of V_T.¶Conclusions: C_SLICE was not constant in patients with ARDS/ALI whose lungs were ventilated according to consensus conference recommendations. The proposed diagnostic concept may serve as a new tool to obtain a standardized estimation of respiratory system compliance within V_T non-invasively without interfering with ongoing mechanical ventilation.     

Influence of positive end-expiratory pressure (PEEP) on histopathological and bacteriological aspects of pneumonia during low tidal volume mechanical ventilation

Objective Ventilatory strategies combining low tidal volume (V_T) with positive end-expiratory pressure (PEEP) are considered to be lung protective. The influence of the PEEP level was investigated on bacteriology and histology in a model of ventilator-associated pneumonia.Subjects Nineteen New Zealand rabbits.Interventions The animals were mechanically ventilated with a positive inspiratory pressure of 15 cmH₂O and received either a zero end-expiratory pressure (ZEEP, n=6), a 5 cmH₂O PEEP (n=5) or a 10 cmH₂O PEEP (n=4). An inoculum of Enterobacter aerogenes was then instilled intrabronchially. The non-ventilated pneumonia group (n=4) was composed of spontaneously breathing animals which received the same inoculum. Pneumonia was assessed 24 h later.Main results The lung bacterial burden was higher in mechanically ventilated animals compared with spontaneously breathing animals. All animals from the latter group had negative spleen cultures. The spleen bacterial concentration was found to be lower in the 5 cmH₂O PEEP group when compared to the ZEEP and 10 cmH₂O PEEP groups (3.1±1.5 vs 4.9±1.1 and 5.0±1.3 log₁₀ cfu/g, respectively; p<0.05). Lung weight and histological score values were lower in the spontaneously breathing animals as well as in the 5 cmH₂O PEEP group compared with the ZEEP and 10 cmH₂O groups.Conclusions Mechanical ventilation substantially increased the lung bacterial burden and worsened the histological aspects of pneumonia in this rabbit model. Variations in terms of lung injury and systemic spreading of infection were noted with respect to the ventilatory strategy.Presented in January 2004 at the Congress of the French Society of Critical Care Medicine.     

Respiratory mechanics in brain-damaged patients

Objective To assess respiratory mechanics on the 1st and 5th days of mechanical ventilation in a cohort of brain-damaged patients on positive end-expiratory pressure (PEEP) of 8 cmH₂O or zero PEEP (ZEEP).Design and setting Physiological study with randomized control trial design in a multidisciplinary intensive care unit of a university hospital.Patients and measurements Twenty-one consecutive mechanically ventilated patients with severe brain damage and no acute lung injury were randomly assigned to be ventilated with ZEEP (n = 10) or with 8 cmH₂O of PEEP (n = 11). Respiratory mechanics and arterial blood gases were assessed on days 1 and day 5 of mechanical ventilation.Results In the ZEEP group on day 1 static elastance and minimal resistance were above normal limits (18.9 ± 3.8 cmH₂O/l and 5.6 ± 2.2 cmH₂O/l per second, respectively); on day 5 static elastance and iso-CO₂ minimal resistance values were higher than on day 1 (21.2 ± 4.1 cmH₂O/l; 7.0 ± 1.9 cmH₂O/l per second, respectively). In the PEEP group these parameters did not change significantly. One of the ten patients on ZEEP developed acute lung injury. On day 5 there was a significant decrease in PaO₂/FIO₂ in both groups.Conclusions On day 1 of mechanical ventilation patients with brain damage exhibit abnormal respiratory mechanics. After 5 days of mechanical ventilation on ZEEP static elastance and minimal resistance increased significantly, perhaps reflecting “low lung volume” injury. Both could be prevented by administration of moderate levels of PEEP.This work was supported by the Thorax foundation.This article is discussed in the editorial available at:     

High positive end-expiratory pressure: only a dam against oedema formation?

Introduction

Healthy piglets ventilated with no positive end-expiratory pressure (PEEP) and with tidal volume (V_T) close to inspiratory capacity (IC) develop fatal pulmonary oedema within 36 h. In contrast, those ventilated with high PEEP and low V_T, resulting in the same volume of gas inflated (close to IC), do not. If the real threat to the blood-gas barrier is lung overinflation, then a similar damage will occur with the two settings. If PEEP only hydrostatically counteracts fluid filtration, then its removal will lead to oedema formation, thus revealing the deleterious effects of overinflation.

Methods

Following baseline lung computed tomography (CT), five healthy piglets were ventilated with high PEEP (volume of gas around 75% of IC) and low V_T (25% of IC) for 36 h. PEEP was then suddenly zeroed and low V_T was maintained for 18 h. Oedema was diagnosed if final lung weight (measured on a balance following autopsy) exceeded the initial one (CT).

Results

Animals were ventilated with PEEP 18 ± 1 cmH₂O (volume of gas 875 ± 178 ml, 89 ± 7% of IC) and V_T 213 ± 10 ml (22 ± 5% of IC) for the first 36 h, and with no PEEP and V_T 213 ± 10 ml for the last 18 h. On average, final lung weight was not higher, and actually it was even lower, than the initial one (284 ± 62 vs. 347 ± 36 g; P = 0.01).

Conclusions

High PEEP (and low V_T) do not merely impede fluid extravasation but rather preserve the integrity of the blood-gas barrier in healthy lungs.     

Elastic pressure-volume curves of the respiratory system reveal a high tendency to lung collapse in young pigs

Objective: To study pressure-volume (P/V) curves over a wide pressure and volume range in pigs.¶Design: Dynamic and static P/V curves (P_dyn/V and P_st/V) and compliance of the respiratory system were studied. The effects of recruitment, positive end-expiratory pressure (PEEP) and body position were analysed.¶Setting: Research animal laboratory.¶Materials: Seven anaesthetised, paralysed and ventilated healthy pigs of 21 kg.¶Measurements: P/V curves up to a pressure of about 40 cmH₂O were recorded with a computer-controlled ventilator. P_st/V curves were obtained with the static occlusion method and P_dyn/V curves during an insufflation at a low, constant flow rate.¶Results: P_dyn/V recording showed a complex pattern. During the insufflation compliance increased, fell, increased and fell again. A 2nd ¶P_dyn/V recording immediately following the 1st one was displaced towards higher volumes and showed only one maximum of compliance. The difference between the two curves reflected: (1) lung collapse during a period of 5 min of ventilation at zero end-expiratory pressure (ZEEP) following a recruitment manoeuvre, (2) recruitment during the measurement of the 1st P_dyn/V curve. These observations were similar in the supine and in the left lateral position. After ventilation at PEEP, 4 cmH₂O, the signs of collapse and recruitment were reduced. It was confirmed that PEEP offers a partial protection against collapse. P_st/V curves showed higher volumes and higher compliance values compared to P_dyn/V curves. This reflects the influence of viscoelastance on P_dyn/V curves.¶Conclusion: The study demonstrates a particularly strong tendency to lung collapse in pigs.     

Potentially harmful effects of inspiratory synchronization during pressure preset ventilation

Purpose

Pressure preset ventilation (PPV) modes with set inspiratory time can be classified according to their ability to synchronize pressure delivery with patient’s inspiratory efforts (i-synchronization). Non-i-synchronized (like airway pressure release ventilation, APRV), partially i-synchronized (like biphasic airway pressure), and fully i-synchronized modes (like assist-pressure control) can be distinguished. Under identical ventilatory settings across PPV modes, the degree of i-synchronization may affect tidal volume (V _T), transpulmonary pressure (P _TP), and their variability. We performed bench and clinical studies.

Methods

In the bench study, all the PPV modes of five ventilators were tested with an active lung simulator. Spontaneous efforts of ?10 cmH₂O at rates of 20 and 30 breaths/min were simulated. Ventilator settings were high pressure 30 cmH₂O, positive end-expiratory pressure (PEEP) 15 cmH₂O, frequency 15 breaths/min, and inspiratory to expiratory ratios (I:E) 1:3 and 3:1. In the clinical studies, data from eight intubated patients suffering from acute respiratory distress syndrome (ARDS) and ventilated with APRV were compared to the bench tests. In four additional ARDS patients, each of the PPV modes was compared.

Results

As the degree of i-synchronization among the different PPV modes increased, mean V _T and P _TP swings markedly increased while breathing variability decreased. This was consistent with clinical comparison in four ARDS patients. Observational results in eight ARDS patients show low V _T and a high variability with APRV.

Conclusion

Despite identical ventilator settings, the different PPV modes lead to substantial differences in V _T, P _TP, and breathing variability in the presence spontaneous efforts. Clinicians should be aware of the possible harmful effects of i-synchronization especially when high V _T is undesirable.     

Lower tidal volume strategy (≈3 ml/kg) combined with extracorporeal CO2 removal versus ‘conventional’ protective ventilation (6 ml/kg) in severe ARDS

Background

Acute respiratory distress syndrome is characterized by damage to the lung caused by various insults, including ventilation itself, and tidal hyperinflation can lead to ventilator induced lung injury (VILI). We investigated the effects of a low tidal volume (V _T) strategy (V _T ≈ 3 ml/kg/predicted body weight [PBW]) using pumpless extracorporeal lung assist in established ARDS.

Methods

Seventy-nine patients were enrolled after a ‘stabilization period’ (24 h with optimized therapy and high PEEP). They were randomly assigned to receive a low V _T ventilation (≈3 ml/kg) combined with extracorporeal CO₂ elimination, or to a ARDSNet strategy (≈6 ml/kg) without the extracorporeal device. The primary outcome was the 28-days and 60-days ventilator-free days (VFD). Secondary outcome parameters were respiratory mechanics, gas exchange, analgesic/sedation use, complications and hospital mortality.

Results

Ventilation with very low V _T’s was easy to implement with extracorporeal CO₂-removal. VFD’s within 60 days were not different between the study group (33.2 ± 20) and the control group (29.2 ± 21, p = 0.469), but in more hypoxemic patients (PaO₂/FIO₂ ≤150) a post hoc analysis demonstrated significant improved VFD-60 in study patients (40.9 ± 12.8) compared to control (28.2 ± 16.4, p = 0.033). The mortality rate was low (16.5 %) and did not differ between groups.

Conclusions

The use of very low V _T combined with extracorporeal CO₂ removal has the potential to further reduce VILI compared with a ‘normal’ lung protective management. Whether this strategy will improve survival in ARDS patients remains to be determined (Clinical trials NCT 00538928).     

Infectious and inflammatory dissemination are affected by ventilation strategy in rats with unilateral pneumonia

Objective To evaluate the effect of V_T reduction and alveolar recruitment on systemic and contralateral dissemination of bacteria and inflammation during right-side pneumonia.Design Interventional animal study.Setting University hospital research laboratory.Subjects A total of 54 male Wistar rats.Interventions One day after right lung instillation of 1.4×10⁷ Pseudomonas aeruginosa, rats were left unventilated or ventilated for 2 h at low V_T (6 ml/kg) with different strategies of alveolar recruitment: no PEEP, 8 cm H₂O PEEP, 8 cm H₂O PEEP in a left lateral position, 3 cm H₂O PEEP with partial liquid ventilation, or high V_T (set such as end-inspiratory pressure was 30 cm H₂O) without PEEP (ZEEP). After ventilation the lungs, spleen and liver were cultivated for bacterial counts. Global bacterial dissemination was scored considering the percentage of positive spleen, liver and left lung cultures. TNF- was assayed in plasma before and after mechanical ventilation.Measurements and results All rats had right-side pneumonia with similar bacterial counts. All mechanical ventilation strategies, with the exception of low V_T-PEEP 8, promoted contralateral lung dissemination. Overall bacterial dissemination was less in non-ventilated controls (22%) and low V_T-PEEP 8 (22%) than in high V_T-ZEEP (67%), low V_T-PEEP 8 in left lateral position (59%) and low V_T-ZEEP (56%) (p<0.05). Partial liquid ventilation prevented systemic bacterial translocation, but at the expense of contralateral bacterial seeding. Plasma TNF- concentration increased significantly after mechanical ventilation with no PEEP at both high and low V_T.Conclusions Our results suggest that PEEP might reduce the risk of ventilation-induced bacterial and inflammatory mediator dissemination during pneumonia.Electronic Supplementary Material Supplementary material is available in the online version of this article at http://dx.doi.org/10.1007/s00134-003-2147-7This study was supported by a grant from Fondation pour la Recherche Médicale.     

Comparative evaluation of the haemodynamic effects of continuous negative external pressure (CNEP) and positive end-expiratory pressure (PEEP) in mechanically ventilated trauma patients

Objective To compare the haemodynamic effects of identical values of continuous negative external pressure (CNEP) and positive end-expiratory pressure (PEEP) in a group of mechanically ventilated patients. Setting General ICU, Vicenza Hospital, Italy. Patients 15 consecutive patients, admitted after road accident trauma. Methods We compared the haemodynamic effects of ZEEP, 10 cmH₂O of PEEP, and 10 cmH₂O CNEP, applied in random order, in 15 head trauma patients under going controlled mechanical ventilation; 9 had associated thoracic trauma, while 6 did not have lung involvement. CNEP was obtained with a “poncho”. Results We observed a significant increase in CI during CNEP, compared with both ZEEP and PEEP 10 cmH₂O. Accordingly the oxygen delivery index significantly increased during CNEP, compared with PEEP 10 cmH₂O. Conversely, decreased with CNEP, if compared with PEEP, both in patients with and without lung damage. Conclusion CNEP can significantly increase CI in mechanically ventilated patients in patients with and without associated lung damage.     

Weaning from mechanical ventilation: a model for extubation

Objective: To develop a model able to determine the right time for extubation and to validate its performance.¶Design: A prospective clinical study.¶Setting: 14-bed medical intensive care unit in a university hospital.¶Patients: 101 patients (37 women/64 men) ventilated over more than 48 h (mean 10.4 ± 10.3 days) and considered ready to be weaned by the medical team (February 1996–February 1998).¶Methods: This study included two series: a development series with 53 patients and a validation series with 48 patients. Before extubation, a weaning test was performed measuring tidal volume (V_T), respiratory rate (f), f/V_T ratio, minute ventilation, vital capacity (VC) and maximum inspiratory and expiratory pressures (MIP and MEP). The success of extubation was assessed after 48 h. Receiver operating characteristic (ROC) curves allowed the analysis of the discriminating power of each parameter. Threshold values were determined using the Youden's index. To create the best predictive model, we performed a multiple logistic regression analysis. To assess the calibration and the discrimination of the model, the Hosmer-¶Lemeshow goodness-of-fit test and area under ROC curves (AUC) were adopted.¶Measurements and results: In a development series, 60 tests were carried out with 38 successful extubations and 22 extubation failures. The multivariate analysis found three significant variables: VC (threshold value = 635 ml), f/V_T ratio (threshold value = 88 breaths/min.l) and MEP (threshold value = 28 cmH₂O). The validation cohort included 59 tests (38 successes and 21 failures). The validation series shows a good discrimination (AUC = 0.855 ± 0.059) and calibration (goodness-of-fit test C: p = 0.224) of the model.¶Conclusion: VC together with the ¶f/V_T ratio and MEP offer accurate prediction of early extubation.     

Inhaled nitric oxide during partial liquid ventilation shifts pulmonary blood flow to the non-dependent lung regions

Objective: To elucidate the change in pulmonary blood flow brought about by nitric oxide (NO) inhalation during partial liquid ventilation (PLV).¶Design: Prospective, controlled study.¶Setting: A research laboratory at a university medical center.¶Subjects: Fourteen Japanese white rabbits (3.2 ± 0.05 kg body weight).¶Interventions: Animals were mechanically ventilated in the right decubitus position. Following saline lung lavage, PLV was started with perflubron (15 ml/kg). In the NO group (n = 7), PLV was supplemented by a 30-min challenge of NO inhalation (10 ppm) from 30 min after the initiation of PLV. In the control group (n = 7), PLV was continued for 60 min.¶Measurements and main results: For the pulmonary blood flow analysis, colored microspheres were administered from the right atrium at 30 min (T_PLV1) and 60 min (T_PLV2) after the initiation of PLV. The percentage of the left lung blood flow in the total pulmonary blood flow (%Q_L/Q_T) was significantly increased by NO inhalation in the NO group (p = 0.0164), while that in the control group was significantly decreased during the same period (p = 0.0107). PaO₂ in the NO group was significantly increased by NO inhalation (p = 0.0153), but not in the control group (p = 0.7911).¶Conclusion: Inhaled NO during PLV shifted the pulmonary blood flow to the non-dependent region and improved pulmonary gas exchange. This result suggested that inhaled NO took effect predominantly in the non-dependent region during PLV.     

Repeated generation of the pulmonary pressure-volume curve may lead to derecruitment in experimental lung injury

Objective Measurements from the pulmonary pressure-volume (PV) curve have been proposed to adjust ventilator settings. We investigated the effects of repeated construction of an inflation PV curve implemented in a standard ventilator on recruitment or derecruitment in acutely injured lungs.Design and setting Prospective experimental animal study in eight anesthetized and mechanically ventilated pigs.Interventions Acute lung injury was induced by lung lavage and animals were ventilated in volume controlled mode with PEEP 10 cmH₂O. The PV curve was constructed five times repeatedly by constant pressure rise, after which ventilation with the preset PEEP was resumed immediately. Studies of hemodynamics, lung mechanics, blood gases and computed tomography were carried out before and after maneuvers.Measurements and results Derecruitment was assessed as an increase in nonaerated lung volume (V_NON), and V_PEEP was the end-expiratory volume difference between PEEP and ZEEP. There was a significant decrease in PaO₂ from 90.4±33.3 to 70.9±36.3 mmHg and a rise in venous admixture from 47.8±12.7 to 59.1±16.6%. V_PEEP was reduced from 244 to 202 ml. A corresponding decrease in normally aerated lung volume was observed, while regression analysis revealed increase in V_NON depending on the amount of preexisting atelectasis.Conclusions Repeated generation of the PV curve with a readily available tool resulted in worsened oxygenation. Derecruitment of the lungs occurred with loss of PEEP at the start of the maneuver, which could not be recovered by a maximum inflation pressure of 40 cmH₂O. Repeated use of the investigated tool should be cautioned, and users should consider measures to preserve aerated lung volumes.This study was supported in part by an unrestricted research grant from Hamilton Medical, Rhäzüns, Switzerland     

Effect of positive end-expiratory pressure on splanchnic perfusion in acute lung injury

Objective: To evaluate the acute effects of an increased positive end-expiratory pressure (PEEP) on splanchnic tissue perfusion.¶Design: Clinical prospective study.¶Setting: Intensive care unit in a university clinic.¶Patients: Six patients with severe acute lung injury (ALI) requiring mechanical ventilation. All patients had bilateral infiltrates in chest X-ray, PaO₂/FiO₂ < 200 mmHg and stable hemodynamics without vasoactive drugs.¶Interventions: PEEP was increased by 5 cmH₂O from a clinically selected PEEP level (8/6–11 cmH₂O) up to (13/10–14 cmH₂O) followed by a return to baseline.¶Measurements and main results: Splanchnic blood flow was measured using primed continuous infusion of indocyanine green dye with hepatic venous sampling and systemic hemodynamics by routine monitoring. In addition, we estimated gastric mucosal-arterial PCO₂ difference and splanchnic lactate/pyruvate exchange. After a baseline measurement, PEEP was increased. After 60 min all measurements were repeated. PEEP was returned to the baseline level and a third measurement followed. PEEP had no effect on cardiac index (baseline I: 3.2/6.1–2.5 l/min/m²; PEEP: 3.3/5.7–¶2.3 l/min/m²; baseline II: 3.4/6.0–2.5 l/min/m²); neither did PEEP have any effect on splanchnic blood flow (baseline I: 0.91/1.39–0.62 l/min/m²; PEEP: 1.04/1.75–0.54 l/min/m²; baseline II:1.07/1.42–0.68 l/min/m², respectively) or perfusion (gastric mucosal-arterial PCO₂ difference baseline I: 2.1/12.8–0.6 kPa; PEEP: 1.7/14.5–0.7 kPa; baseline II: 1.7/8.8–0.1 kPa; lactate uptake baseline I: 0.5/1.1–0.3 mmol/min/m²; PEEP: 0.4/1.0–0.3 mmol/min/m²; baseline II: 0.5/0.9–0.3 mmol/min/m²; hepatic venous lactate/pyruvate baseline I: 9.7/10.6–5.7; PEEP: 9.7/14.2–6.4; baseline II: 8.4/12.4–7.3; respectively).¶Conclusion: PEEP by itself does not have a consistent effect on splanchnic blood flow and metabolism when cardiac index is stable and patients are ventilated within the linear part of the pv curve.     

Impact of positive end-expiratory pressure on the definition of acute respiratory distress syndrome

Objective We examined whether PEEP during the first hours of ARDS can induce such a change in oxygenation that could mask fulfillment of the AECC criteria of a PaO₂/FIO₂ 200 essential for ARDS diagnosis.Design and setting Observational, prospective cohort in two medical-surgical ICU in teaching hospitals.Patients 48 consecutive patients who met AECC criteria of ARDS on 0 PEEP (ZEEP) at the moment of diagnosis.Measurements and results PaO₂/FIO₂ and lung mechanics were recorded on admission (0 h) to the ICU on ZEEP, and after 6, 12, and 24 h on PEEP levels selected by attending physicians. Lung Injury Score (LIS) was calculated at 0 and 24 h. PaO₂/FIO₂ rose significantly from 121±45 on ZEEP at 0 h, to 234±85 on PEEP of 12.8±3.7 cmH₂O after 24 h. LIS did not change significantly (2.34±0.53 vs. 2.42±0.62). These variables behaved similarly in pulmonary and extrapulmonary ARDS, and in survivors and nonsurvivors. After 24 h only 18 patients (38%) still had a PaO₂/FIO₂ of 200 or lower. Their mortality was similar to that in the remaining patients (61% vs. 53%).Conclusions The use of PEEP improved oxygenation such that one-half of patients after 6 h, and most after 24 h did not fulfill AECC hypoxemia criteria of ARDS. However, LIS remained stable in the overall series. These results suggest that PEEP level should be taken into consideration for ARDS diagnosis.