Positive end-expiratory pressure

The immediate haemodynamic effects of the addition of a positive end-expiratory pressure (PEEP) of 5 cmH2O has been studied in eleven patients undergoing artificial ventilation for respiratory failure. Mean cardiac output fell from 6.0 to 5.5 litres/min. This was due to a similar decrease in stroke volume. Individual patients showed greater, though short-lived, changes. There was also a statistically significant increase in central venous pressure (from 8-9 cmH2O) and peripheral resistance (from 1280 to 1380 dyn sec cm-5) associated with the application of PEEP. Overall changes in heart rate and mean arterial blood pressure were insignificant. Arterial oxygen tension increased in the majority of patients but the mean figure was unchanged. Mean oxygen delivery to the tissues fell from 830 to 780 ml/min.     

Positive end-expiratory pressure : adjustment in acute lung injury

Treatment of patients suffering from acute lung injury is a challenge for the treating physician. In recent years ventilation of patients with acute hypoxic lung injury has changed fundamentally. Besides the use of low tidal volumes, the most beneficial setting of positive end-expiratory pressure (PEEP) has been in the focus of researchers. The findings allow adaption of treatment to milder forms of acute lung injury and severe forms. Additionally computed tomography techniques to assess the pulmonary situation and recruitment potential as well as bed-side techniques to adjust PEEP on the ward have been modified and improved. This review gives an outline of recent developments in PEEP adjustment for patients suffering from acute hypoxic and hypercapnic lung injury and explains the fundamental pathophysiology necessary as a basis for correct treatment.     

Positive end-expiratory pressure ventilation increases extravascular lung water due to a decrease in lung lymph flow

Positive end-expiratory pressure (PEEP) is used to improve gas exchange, increase functional residual capacity, recruit air spaces, and decrease pulmonary shunt in patients suffering from respiratory failure. The effect of PEEP on extravascular lung water (EVLW), however, is still not fully understood. This study was designed as a prospective laboratory experiment to evaluate the effects of PEEP on EVLW and pulmonary lymph flow (QL) under physiologic conditions. Twelve adult sheep were operatively prepared to measure haemodynamics of the systemic and pulmonary circulation, and to assess EVLW In addition, the lung lymphatic duct was cannulated and a tracheostomy performed. The animals were then mechanically ventilated in the awake-state without end-expiratory pressure (PEEP 0). After a two-hour baseline period, PEEP was increased to 10 cmH2O for the duration of two hours, and then reduced back to 0 cmH2O. Cardiopulmonary variables, QL, and arterial blood gases were recorded intermittently; EVLW was determined two hours after each change in PEEP. The increase in PEEP resulted in a decrease in QL (7 +/- 1 vs 5 +/- 1 ml/h) and an increase in EVLW (498 +/- 40 vs 630 +/- 58 ml; P<0.05 each) without affecting cardiac output. As PEEP was decreased back to baseline, QL increased significantly (5 +/- 1 vs 10 +/- 2 ml/h), whereas EVLW returned back to baseline. This study suggests that institution of PEEP produces a reversible increase in EVLW that is linked to a decrease in QL.     

Experimental pulmonary edema: the effect of positive end-expiratory pressure on lung water.

The effect of 10 cm of positive end-expiratory pressure (PEEP) on lung water was studied during pulmonary edema induced in dogs by inflating a Foley balloon placed in the left atrium. Colloid oncotic pressure (COP) was measured directly. Intrapleural pressure (IPP) was measured after surgical closure of the chest. Transmural left atrial (LA) pressure (LA minus IPP) minus COP was considered to be the net force driving water out of the capillaries. LA pressure was elevated so that transmural LA pressure minus COP averaged +7.5 mm Hg. Water accumulation was expressed as the ratio of wet to dry weight. The control ratio of wet to dry lung weight was 4.30 +/- 0.10 (+/- SE). After 2 hours of standardized pulmonary edema and ventilation without PEEP, wet-to-dry lung weight was 5.63 +/- 0.24. In animals ventilated with 10 cm of PEEP through 2 hours of pulmonary edema the ratio was 5.36 +/- 0.14. Animals ventilated with 10 cm of PEEP showed a significant increase in functional residual capacity and decreased intrapulmonary shunt. Ten centimeters of PEEP, however, had no statistically significant effect on water accumulation during experimental pulmonary edema.     

Effect of positive end-expiratory pressure on lung water in pulmonary edema caused by increased membrane permeability

Pulmonary edema caused by increased membrane permeability was created in dogs by alloxan and infusion of saline solution. Pulmonary extravascular water volume was measured gravimetrically using the supernatant hemoglobin concentration to estimate red cell mass in the calculation of residual pulmonary blood volume. Three groups were studied for two hours: a control group, a group given alloxan and mechanical ventilation without positive end-expiratory pressure (PEEP), and a group given alloxan and mechanical ventilation with 10 cm H2O of PEEP. After two hours, alloxan caused moderately severe pulmonary edema in the two experimental groups, but PEEP had no effect on the accumulation of pulmonary extravascular water volume. No sustained differences in pulmonary or systemic hemodynamics were present throughout two hours of pulmonary edema. The pulmonary shunt was increased in the group without PEEP but was similar in the control group and the group with PEEP. No significant changes in alveolar dead space were noted among the three groups.     

Positive end-expiratory pressure (PEEP)]

PEEP has become a widely used ventilatory technique. The beneficial effects of PEEP were first described in asphyctic neonates, and it was later used in the treatment of cardiogenic pulmonary edema. Since the 1970s PEEP has been well established for the treatment of ARDS; the technique is also used for scoring the degree of severity of ARDS. Two mechanisms have been identified to explain pulmonary function and gas exchange following PEEP therapy: increasing FRC and alveolar recruitment. Both factors result in improvement in the ventilation/perfusion ratio with a consequent decrease in the intrapulmonary right-to-left shunt fraction. PEEP should be used in cardiogenic pulmonary edema as well as in ARDS; there are few contraindications. To choose the individual level of PEEP, PEEP should be titrated in 3- to 5-cm increments and its effects on haemodynamic function, pulmonary gas exchange and respiratory mechanics taken into account. In this article the effects of PEEP, its use and abuse are reviewed from a practical point of view.     

Positive end-expiratory pressure and lung compliance: effect on delivered tidal volume

The effects of positive endexpiratory pressure (PEEP) and lung compliance (C_L) on delivered tidal volume (V_Tdel) and ventilator output were evaluated in the following anaesthesia machine/ ventilator systems: Narkomed III with a Model AV-E ventilator (III/AV-E system) and an Ohmeda Modulus II with either a 7810 anaesthesia ventilator (II/7810 system) or a Model 7000 anaesthesia ventilator (II/7000 system). With a standard circle anaesthesia breathing circuit connected to a test lung simulating C_L gas flow was measured and integrated over time at each combination of V_T settings (V_Tset), 500 ml or 1000 ml; C_L settings, 0.15 to 0.01 L · cm H₂O⁻¹ decreased incrementally; and PEEP settings, 0 to 30 cm H₂O increased in 5- cm H₂O increments. The integral of gas flow at the Y- piece of the breathing circuit was recorded as V_Tdel and at the output of the ventilator bellows as ventilator output. As C_L decreased to 0.01 L · cm H₂O^{− 1} and PEEP increased to 30 cm H₂O, at V_Tset of 500 ml and 1000 ml, respective V_Tdel, decreased linearly to 251 ± 6 ml and 542 ± 7 with the III/AV- E, 201 ± 5 and 439 ± 5, with the II/7810, and 181 ± 4 and 433 ± 7 ml with the II/7000 (P < 0.05 among the three systems). Loss in V_Tdel due to PEEP alone, which increased only slightly when V_Tset was increased, accounted for an increasingly greater percentage of V_Tset as it was decreased, which was less pronounced with low C_L. Effects of PEEP and C_L on ventilator output were similar to those on V_Tdel but of lesser magnitude. During PEEP, V_Tset must be increased to compensate for loss in V_Tdel and expired V_T must be monitored to prevent hypoventilation. Les effets de la pression expiratoire positive (PEEP) et de la compliance pulmonaire (C_L) sur le volume courant généré (V_Tdel) et le débit du ventilateur mécanique sont évalués sur les appareils d’anesthésie équipés d’un ventilateur: Narkomed III avec un ventilateur AV- E (système III/AV- E) et un appareil d’anesthésie Ohmeda Modulus II équipé d’un ventilateur d’anesthésie 7810 (système 11/7810 ou d’un ventilateur d’anes thésie 7000 (système 11/7000). Avec une circuit standard avec absorption branché sur un poumon artificiel simulant la C_L, le débit gazeux et mesuré et intégré par rapport au temps pour chacune des combinaisons de réglage du V_T (V_Tset), 500 ml ou 1000 ml; de réglage de la C_L de 0,15 à 0,01 L · cm H₂O^{− 1} diminuée par plateau; réglage du PEEP augmenté par plateau de 0 à 30 cm H₂O. Lorsque que la C_L à 0,01 L · cm H₂O^{− 1} et que le PEEP augmente à 30 cm H₂O, aux V_Tset de 500 ml et 1000 ml, le V_Tdel diminue linéairement à 251 ± 6 ml et 542 ± 7 ml avec le III/AV- E, à 201 ± 5 et 439 ± 5 avec le II/7810, et à 181 ± 4 et 433 ± 7 ml avec le II/7000 (P < 0,05 entre les trois systèmes.) La perte de V_Tdel due au PEEP seul, qui n’augmente que légèrement quand le V_Tset est augmenté, explique un plus grand pourcentage de V_Tset, quand celuici est diminué, ce qui est moins prononcé quand la C_L est basse. Les effets du PEEP et de la C_L, sur le débit du ventilateur sont les mêmes que ceux notés sur le V_Tdel mais de moindre importance. Pendant le PEEP, V_Tset doit être augmenté pour compenser pour la perte de V_Tdel. Le V_T expiré doit être monitoré pour prévenir l’hypoventilation.     

High surface tension pulmonary edema

Dogs were anesthetized with pentobarbital and placed on a piston ventilator with room air. Ten animals received an endobronchial lavage of normal saline (3 mg/kg). Ten other animals received an endobronchial lavage of the same volume of a nonionic detergent, Tween 20, 5% in saline. Detergent lavage was shown by Wilhelmy balance to increase surface tension of lung extracts. Saline lavage did not alter the surface tension of lung extracts. No significant differences between the groups were noted in cardiac output, left ventricular and diastolic pressure, mean pulmonary artery pressure, or colloid oncotic pressure. Static compliance and arterial PO2 were decreased following detergent lavage. Animals were sacrificed 2 hr after lavage and pulmonary extravascular water volume (PEWV) was measured gravimetrically. Saline-lavaged lungs with normal surface tension had a PEWV of 4.3 ml/g dry lung. Tween-lavaged lungs with increased surface tension had a PEWV of 5.3 ml/g dry lung (P less than 0.005). When the estimated volume of residual lavage solution remaining in the lung parenchyma was subtracted from the total wet lung wt, the corrected PEWV was 3.62 +/- 0.12 ml/g dry lung for saline-lavaged lung and 4.76 +/- 0.19 ml/g dry lung for Tween-lavaged lung. PEWV for 11 control animals ventilated 2 hr without lavage was 3.61 +/- 0.13 ml/g dry lung. It is concluded that, experimentally, high alveolar surface tension can induce pulmonary edema even when pulmonary microvascular hydrostatic and colloid oncotic pressures are normal.     

Positive end-expiratory pressure applied to the dependent lung during one-lung ventilation improves oxygenation and respiratory mechanics in patients with high FEV1

BACKGROUND AND OBJECTIVE: The aim of this study was to test the efficacy of positive end-expiratory pressure (PEEP) to the dependent lung during one-lung ventilation, taking into consideration underlying lung function in order to select responders to PEEP. METHODS: Forty-six patients undergoing open-chest thoracic surgical procedures were studied in an operating room of a university hospital. Patients were randomized to receive zero end-expiratory pressure (ZEEP) or 10 cmH2O of PEEP to the dependent lung during one-lung ventilation in lateral decubitus. The patients were stratified according to preoperative forced expiratory volume in 1 s (FEV1) as an indicator of lung function (below or above 72%). Oxygenation was measured in the supine position, in the lateral decubitus with an open chest, and after 20 min of ZEEP or PEEP. The respiratory system pressure-volume curve of the dependent hemithorax was measured in supine and open-chest lateral decubitus positions with a super-syringe. RESULTS: Application of 10 cmH2O of PEEP resulted in a significant increase in PaO2 (P < 0.05). This did not occur in ZEEP group, considered as a time matched control. PEEP improved oxygenation only in patients with high FEV1 (from 11.6+/-4.8 to 15.3+/-7.1 kPa, P < 0.05). There was no significant change in the low FEV1 group. Dependent hemithorax compliance decreased in lateral decubitus, more in patients with high FEV1 (P < 0.05). PEEP improved compliance to a greater extent in patients with high FEV1 (from 33.6+/-3.6 to 48.4+/-3.9 mLcmH2O(-1), P < 0.05). CONCLUSIONS: During one-lung ventilation in lateral decubitus, PEEP applied to the dependent lung significantly improves oxygenation and respiratory mechanics in patients with rather normal lungs as assessed by high FEV1.     

Beneficial effects of high positive end-expiratory pressure in lung respiratory mechanics during laparoscopic surgery

Background: The effect of neuromuscular blockade (NMB) and positive end‐expiratory pressure (PEEP) on the elastic properties of the respiratory system during pneumoperitoneum (PnP) remains a controversial subject. The main objective of the present study was to evaluate the effects of NMB and PEEP on respiratory mechanics. Methods: We performed a dynamic analysis of respiratory mechanics in patients subjected to PnP. Twenty‐one patients underwent cholecystectomy videolaparoscopy and total intravenous anesthesia. The respiratory system resistance (R_RS), pulmonary elastance (E_P), chest wall elastance (E_CW), and respiratory system elastance (E_RS) were computed via the least squares fit technique using an equation describing the motion of the respiratory system, which uses primary signs such as airway pressure, tidal volume, air flow, and esophageal pressures. Measurements were taken after tracheal intubation, PnP, NMB, establishment of PEEP (10 cmH₂O), and PEEP withdrawal [zero end‐expiratory pressure (ZEEP)]. Results: PnP significantly increased E_RS by 27%; both E_P and E_CW increased 21.3 and 64.1%, respectively (P<0.001). NMB did not alter the respiratory mechanic properties. Setting PEEP reduced E_RS by 8.6% (P<0.05), with a reduction of 10.9% in E_P (P<0.01) and a significant decline of 15.7% in R_RS (P<0.05). These transitory changes in elastance disappeared after ZEEP. Conclusions: We concluded that the 10 cmH₂O of PEEP attenuates the effects of PnP in respiratory mechanics, lowering R_RS, E_P, and E_RS. These effects may be useful in the ventilatory approach for patients experiencing a non‐physiological increase in IAP owing to PnP in laparoscopic procedures.     

Low vs high positive end-expiratory pressure in the ventilatory management of acute lung injury

Positive end-expiratory pressure (PEEP) has become an essential component of the care of many critically ill patients who require ventilatory support. The application of PEEP is expected to improve lung mechanics and gas exchange as it recruits lung volume. In the last 3 decades, research of the effects of PEEP in animal models of lung injury and in patients with acute respiratory failure has produced a plethora of information. Support for the use of PEEP comes from historical comparisons and a few randomized controlled studies. Although the data from those animal studies and clinical trials could be seen as very convincing, there are insufficient data to propose an universal approach for the use of PEEP in patients with acute lung injury (ALI) and acute respiratory distress syndrome (ARDS). In this article I will review the basic mechanisms of PEEP and the current knowledge of the effects of PEEP on the evolution and outcome of ALI/ARDS.     

Positive end-expiratory pressure reduces pneumocephalus in spinal intradural tumor surgery

We tested the hypothesis that 5 cm H2O of positive end-expiratory pressure (PEEP) reduces the incidence of pneumocephalus in patients who undergo spinal intradural tumor surgery. Fifty-three ASA I to III patients who underwent thoracolumbar intradural tumor surgery between the years 2003 and 2006 were included in this study. All patients received propofol, fentanyl, and cisatracurium for induction of the anesthesia. Maintenance was provided by propofol infusion and, oxygen (50%) and air (50%). Group I (n=28) did not receive PEEP whereas group II (n=25) received PEEP as 5 cm H2O. Cranial computerized tomography was taken at 8 hours after the surgery and cases were evaluated for pneumocephalus using BAB Bs200ProP Image System software. Pneumocephalus areas between 0.03 and 4.24 cm2 were observed in 9 patients, 8 in group I and 1 patient in group II at the 8th postoperative hour, at various localizations. There were no neurologic findings in other patients except for 2 patients in group I who presented with headache and mental status change. Although the cerebrospinal fluid leakage is minimal, N2O is not used and the patients are well hydrated, pneumocephalus with neurologic deficits may occur in patients undergoing microsurgical spinal intradural tumor surgery in prone position. In our study, we showed that using 5 cm H2O PEEP perioperatively reduced the risk of pneumocephalus. However, more cases must be studied to support this hypothesis.     

Positive end-expiratory pressure improves arterial oxygenation during prolonged pneumoperitoneum

BACKGROUND: Laparoscopic surgery usually requires the use of a pneumoperitoneum by insufflating gas in the peritoneal space. The gas most commonly used for insufflation is carbon dioxide. Increased intra-abdominal pressure causes cephalad displacement of the diaphragm resulting in compressed lung areas, which leads to formation of atelectasis, especially during mechanical ventilation. The aim of this prospective study was to investigate the effect of prolonged intraperitoneal gas insufflation on arterial oxygenation and hemodynamics during mechanical ventilation with and without positive end-expiratory pressure (PEEP). METHODS: Twenty patients undergoing totally endoscopic robot-assisted radical prostatectomy were randomly allocated to one of two groups. In the PEEP group (n = 10) a constant PEEP of 5 cmH(2)O was used, whereas in the ZPEEP group (n = 10) no PEEP was used. RESULTS: Application of PEEP (5 cmH(2)O) resulted in significantly higher P(a)O(2) levels after 3 h (182 +/- 49 vs. 224 +/- 35 mmHg) and 4 h (179 +/- 48 vs. 229 +/- 29 mmHg) of pneumoperitoneum; after desufflation, P(a)O(2) values decreased significantly below preinsufflation values. While there were no significant differences in heart rate, central venous pressure (CVP) and mean arterial blood pressure (MAP) during pneumoperitoneum between both groups, baseline values in CVP and MAP differed significantly between both groups with higher levels in the ZPEEP group. CONCLUSION: The application of a constant positive airway pressure of 5 cmH(2)O preserves arterial oxygenation during prolonged pneumoperitoneum.