Addition Of Positive End-expiratory Pressure Research Articles

Effects of intraoperative positive end-expiratory pressure and fraction of inspired oxygen on postoperative oxygenation in dogs undergoing stifle surgery

ObjectiveTo compare the effects of fraction of inspired oxygen (FiO2) with the addition of positive end-expiratory pressure (PEEP) during anesthesia on arterial oxygenation in the first 4 postoperative hours in dogs. We hypothesized that compared with dogs breathing FiO2 ≥ 0.95 and no PEEP (ZEEP), the use of intraoperative PEEP would improve postoperative oxygenation, and that the use of PEEP combined with an FiO2 of 0.4 would further improve it. Study designProspective, randomized study. AnimalsA total of 30 dogs undergoing unilateral stifle surgery. MethodsUsing a standardized anesthetic protocol, dogs were assigned to either FiO2 ≥ 0.95 and ZEEP, FiO2 ≥ 0.95 and 5 cmH2O PEEP or FiO2 0.4 and 5 cmH2O PEEP. All dogs were mechanically ventilated with a tidal volume of 12 mL kg–1. Dogs breathed room air after recovery from anesthesia. Arterial blood gases were measured during surgical closure and 10, 120 and 240 minutes after extubation. Demographic characteristics were compared with Kruskal–Wallis tests. The effects of treatment and time on the PaO2, PaCO2, PaO2:FiO2 and shunt fraction (F-shunt) were assessed with mixed-effect models. ResultsThe PaO2 and F-shunt were lower during anesthesia for dogs breathing FiO2 0.4. No differences among groups were measured after extubation for any variable. Conclusions and clinical relevanceCompared with dogs ventilated with FiO2 ≥ 0.95 and ZEEP, application of 5 cmH2O PEEP did not improve intraoperative gas exchange. The combination of 5 cmH2O PEEP and FiO2 0.4 resulted in lower intraoperative F-shunt values. However, no benefits from those maneuvers on postoperative PaO2 and F-shunt were recorded after extubation, suggesting that alterations in pulmonary function imposed by anesthesia were reversed soon after extubation.

Positive end-expiratory pressure attenuates hemodynamic effects induced by an overload of inspiratory muscles in patients with COPD.

BackgroundInspiratory muscle training (IMT) using a Threshold® device is commonly used to improve the strength and endurance of inspiratory muscles. However, the effect of IMT, alone or with positive end-expiratory pressure (PEEP), on hemodynamic parameters in patients with chronic obstructive pulmonary disease (COPD) remains unknown.ObjectiveTo assess the effects of an overload of inspiratory muscles using IMT fixed at 30% of the maximal inspiratory pressure (MIP), and IMT associated with 5 cmH2O of PEEP (IMT + PEEP), on the echocardiographic parameters in healthy subjects and patients with COPD.MethodsTwenty patients with COPD (forced expiratory volume in 1 second 53.19±24.71 pred%) and 15 age-matched healthy volunteers were evaluated using spirometry, MIP, the COPD assessment test (CAT), and the modified Medical Research Council (mMRC) dyspnea scale. The E- (fast-filling phase) and A- (atrial contraction phase) waves were evaluated at the tricuspid and mitral valves during inspiration and expiration in the following sequence: at basal conditions, using IMT, and using IMT + PEEP.ResultsPatients with COPD had reduced MIPs versus the control group. Ten patients had CAT scores <10 and 12 patients had mMRC scores <2. E-wave values at the mitral valve were significantly decreased with IMT during the inspiratory phase in both groups. These effects were normalized with IMT + PEEP. During the expiratory phase, use of IMT + PEEP normalized the reduction in E-wave values in the COPD group. During inspiration at the tricuspid valve, reduction in E-wave values during IMT was normalized by IMT + PEEP in COPD group. During the expiratory phase, the value of the E-waves was significantly reduced with overload of the inspiratory muscles in both groups, and these effects were normalized with IMT + PEEP. A-waves did not change under any conditions.ConclusionAcute hemodynamic effects induced by overloading of the inspiratory muscles were attenuated and/or reversed by the addition of PEEP in COPD patients.

The influence of positive end-expiratory pressure on surgical field conditions during functional endoscopic sinus surgery.

Functional endoscopic sinus surgery (FESS) is the mainstay of surgical treatment for sinonasal disease. This surgery carries certain risks. Most of these risks relate to the quality of the surgical field. Thus, mechanisms by which the surgical field can be improved are important to study. We sought to determine whether positive end-expiratory pressure (PEEP) had a deleterious effect on the quality of the surgical field in patients undergoing primary FESS. Forty-seven patients were randomized to a ventilation strategy using either 5 cm H2O of PEEP or zero added PEEP. The quality of the surgical field was measured every 15 minutes using a validated surgical scoring method. The addition of PEEP did not have any measurable effect on the surgical field scores after onset of surgery (odds ratio [OR] (95% confidence interval [CI]) = 1.06 (0.44-2.58), P = 0.895 for side 1; OR (95% CI) = 0.56 (0.16-1.93), P = 0.356 for side 2). The peak inspiratory pressure did have an effect on surgical grades. Every cm H2O of added pressure over 15 cm H2O total pressure contributing to increased odds of higher surgical field score. For each cm H2O increase in inspiratory pressure above 15cm H2O increased the surgical field score (OR [95% CI] 1.13 [1.04-1.22], P = 0.002). During FESS surgery if PEEP is added, it is important to keep the mean inspiratory pressure below 15cm H2O to avoid worsening surgical field conditions.

Understanding negative pressure pulmonary edema.

Understanding negative pressure pulmonary edema.

A structural model of perfusion and oxygenation in low-flow states

Background Recent investigations underscore the critical importance of ventilation strategies on resuscitation outcomes. In low perfusion states, such as cardiac arrest and traumatic shock, the rise in intrathoracic pressure that accompanies positive-pressure ventilation can significantly impede venous return and lead to a decrease in cardiac output. The optimal ventilation strategy in these “low-flow” states remains unclear. Objective To create a mathematical model of perfusion and oxygenation to predict the effects of PPV with both normotension and hypotension. Methods The lung pressure–volume relationship was modeled using a novel formula allowing manipulation of various lung characteristics. A separate formula was then derived to predict mean intrathoracic pressure (MITP) for specific minute ventilation values using the pressure–volume formula. The addition of positive end-expiratory pressure was also modeled. Finally, a formula was derived to model oxygen absorbance as a function of alveolar surface area and flow based on ventilation rate and MITP. Results Mathematical models of the lung pressure–volume relationship, MITP, and absorbance were successfully derived. Manipulation of total lung capacity, compliance, upper and lower inflection points, positive end-expiratory pressure, and minute ventilation allowed prediction of optimal ventilation rate and tidal volume for a normal lung and with various abnormal characteristics to simulate particular disease states, such as acute respiratory distress syndrome (ARDS). For a normal lung, ventilation rates of 4–6 breaths/min with higher tidal volumes (15–20 mL/kg) resulted in the lowest predicted MITP values (5 cm H 2O) and the highest absorbance. The input of lung parameters that would simulate ARDS resulted in optimal ventilation rates of 10–12 breaths/min with lower tidal volumes (8–10 mL/kg) and higher predicted MITP values (10–15 cm H 2O). Conclusions A mathematical model of ventilation was successfully derived allowing manipulation of multiple pulmonary physiological variables to predict MITP and potentially identify optimal ventilation strategies. This model suggests the use of lower ventilation rates and larger tidal volumes to minimize the hemodynamic effects of positive pressure ventilation in patients with hypoperfusion but normal lung characteristics.

Hemodynamic effects of positive end-expiratory pressure during abdominal hyperpression: A preliminary study in healthy volunteers

PurposeAn increase in abdominal pressure induces an increase in left ventricular afterload under clinical conditions. We tested the hypothesis that positive end-expiratory pressure (PEEP) could reverse the hemodynamic consequences of abdominal hyperpression by opposing the increase in left ventricular afterload. Materials and methodsEight healthy volunteers were investigated during 3 experimental conditions: (1) baseline, (2) increase in abdominal pressure by means of medical antishock trousers (MAST) inflation, and (3) addition of PEEP +10 cm H2O. Heart loading conditions and left ventricular systolic and diastolic function were assessed by transthoracic echocardiography. ResultsThe application of PEEP significantly reduced the prior increase in end-systolic wall stress: 45 ± 11 vs 55 ± 14 kdyn/cm2, P < .05. Medical antishock trousers inflation significantly altered the deceleration time of mitral E wave: 199 ± 23 vs 156 ± 38 milliseconds, P < .05. Left ventricular preload and global systolic performance were unaffected by MAST and PEEP applications. ConclusionsThe increase in left ventricular afterload induced by MAST inflation can be efficiently reduced by the use of a moderate PEEP. Potential clinical applications in the abdominal compartment syndrome or in the setting of laparoscopic surgery should be developed.

Prone Positioning

Prone Positioning

Positive End-expiratory Pressure Redistributes Regional Blood Flow and Ventilation Differently in Supine and Prone Humans

Animal studies have demonstrated an interaction between posture and the effect of positive end-expiratory pressure (PEEP) on regional ventilation and lung blood flow. The aim of this study was to explore this interaction in humans. Regional lung blood flow and ventilation were compared between mechanical ventilation with and without PEEP in the supine and prone postures. Six normal subjects were studied in each posture. Regional lung blood flow was marked with In-labeled macroaggregates and ventilation with Technegas (Tc). Radiotracer distributions were mapped using quantitative single-photon emission computed tomography. In supine subjects, PEEP caused a similar redistribution of both ventilation and blood flow toward dependent (dorsal) lung regions, resulting in little change in the V/Q correlation. In contrast, in prone subjects, the redistribution toward dependent (ventral) regions was much greater for blood flow than for ventilation, causing increased V/Q mismatch. Without PEEP, the vertical ventilation-to-perfusion gradient was less in prone postures than in supine, but with PEEP, the gradient was similar. During mechanical ventilation of healthy volunteers, the addition of PEEP, 10 cm H2O, causes redistribution of both lung blood flow and ventilation, and the effect is different between the supine and prone postures. Our results suggest that the addition of PEEP in prone might be less beneficial than in supine and that optimal use of the prone posture requires reevaluation of the applied PEEP.

Effects of Positive End-Expiratory Pressure on Internal Jugular Vein Cross-Sectional Area in Anesthetized Adults

We tested whether positive end-expiratory pressure (PEEP) increases right internal jugular vein (RIJV) cross-sectional area (CSA) in 45 ASA physical status I and II adults. All patients received a standardized IV fluid bolus, induction of general anesthesia, tracheal intubation, and mechanical ventilation. We evaluated the CSA of the RIJV using ultrasound without PEEP (control) and with PEEP (10 cm H(2)O) in the supine, level position. Addition of PEEP increased RIJV CSA 0.42 + or - 0.41 cm(2) (mean + or - SD, median 0.34 cm(2), P < 0.001), which represented a 41% mean increase in CSA.

The Ability of Pleth Variability Index to Predict the Hemodynamic Effects of Positive End-Expiratory Pressure in Mechanically Ventilated Patients Under General Anesthesia

Pleth variability index (PVI) is a new algorithm allowing automated and continuous monitoring of respiratory variations in the pulse oximetry plethysmographic waveform amplitude. PVI can predict fluid responsiveness noninvasively in mechanically ventilated patients during general anesthesia. We hypothesized that PVI could predict the hemodynamic effects of 10 cm H2O positive end-expiratory pressure (PEEP). We studied 21 mechanically ventilated and sedated patients in the postoperative period after coronary artery bypass grafting. Patients were monitored with a pulmonary artery catheter and a pulse oximeter sensor attached to the index finger. Hemodynamic data (cardiac index [CI], PVI, pulse pressure variation, central venous pressure) were recorded at 3 successive tidal volumes (V(T)) (6, 8, and 10 mL/kg body weight) during zero end-expiratory pressure (ZEEP) and then after addition of a 10 cm H2O PEEP for each V(t). Hemodynamically unstable patients were defined as those with a >15% decrease in CI after the addition of PEEP. PEEP induced changes in CI and PVI for V(t) of 8 and 10 mL/kg. Hemodynamic instability occurred in 5 patients for a V(T) of 6 mL/kg, in 6 patients for a V(T) of 8 mL/kg, and in 9 patients for a V(T) of 10 mL/kg. For V(T) of 8 mL/kg, a PVI threshold value of 12% during ZEEP predicted hemodynamic instability with a sensitivity of 83% and a specificity of 80% (area under the receiver operating characteristic curve 0.806; P = 0.03). For V(T) of 10 mL/kg, a PVI threshold value of 13% during ZEEP predicted hemodynamic instability with a sensitivity of 78% and a specificity of 83% (area under the receiver operating characteristic curve 0.829; P = 0.01). PVI may be useful in automatically and noninvasively detecting the hemodynamic effects of PEEP when V(T) is >8 mL/kg in ventilated and sedated patients with acceptable sensitivity and specificity.

Optimizing speech production in the ventilator‐assisted individual following cervical spinal cord injury: a preliminary investigation

Mechanical ventilation is commonly used during the acute management of cervical spinal cord injury, and is required on an ongoing basis in the majority of patients with injuries at or above C3. However, to date there have been limited systematic investigations of the options available to improve speech while ventilator-assisted post-cervical spinal cord injury. To provide preliminary evidence of any benefits gained through the addition of positive end expiratory pressure (PEEP) and/or a tracheostomy speech valve to the condition of leak speech. Speech production in the three conditions was compared in two ventilator-assisted participants using a series of instrumental and perceptual speech measures. The addition of PEEP or the use of a speech valve resulted in speech that was superior to leak speech for both participants; however, individual variation was present. Leak speech alone or with the addition of PEEP or a tracheostomy speech valve can facilitate functional communication for the ventilated patient, though PEEP and valve speech were found to be superior in the current study. These findings will be of assistance for clinicians counselling the growing population of patients who may require tracheostomy positive pressure ventilation long-term regarding communication options.

Positive end-expiratory pressure alters the severity and spatial heterogeneity of ventilator-induced lung injury: An argument for cyclical airway collapse

PurposeVentilator-induced lung injury (VILI) is a recognized complication of mechanical ventilation. Although the specific mechanism by which mechanical ventilation causes lung injury remains an active area of study, both alveolar overdistension and cyclical airway collapse and recruitment have been suggested as contributing causes. We hypothesized that mechanical ventilation in the absence of positive end-expiratory pressure (PEEP) causes VILI to be more severe and regionally variable as compared with PEEP = 8 cm H2O. Materials and MethodsTo test this hypothesis, anesthetized, supine rabbits were mechanically ventilated with an end-inspiratory pressure of 28 cm H2O and either 0 or 8 cm H2O PEEP for 4 hours. Regional lung injury was determined by histologic scoring. ResultsIn the absence of PEEP, lung injury was regionally variable and greatest in the dorsal-caudal lung. This regional injury heterogeneity was abolished by the addition of PEEP = 8 cm H2O. ConclusionsThese results suggest that VILI is regionally heterogeneous and spatially correlates with regions in which cyclical airway collapse and recruitment is most likely to occur.

Beach Chair Position: Is It Really Feasible in Laparoscopic Procedures?

In the November 2007 issue of Anesthesiology, Valenza et al. 1described how they used the application of positive end-expiratory pressure and the beach chair position to counteract the respiratory derangements in morbidly obese patients undergoing laparoscopic gastric surgery. The authors state that although both the beach chair position and the addition of positive end-expiratory pressure each similarly improved lung volumes, oxygenation, and respiratory mechanics at baseline, it was only the combination of both positive end-expiratory pressure and the beach chair position that improved oxygenation during pneumoperitoneum. Because it is often the practice of surgeons to ask their anesthesiology colleagues to place the patients in some degree of Trendelenburg to “facilitate” the exposure during laparoscopic surgeries, we are forced to question how feasible it would be to use the beach chair position in any laparoscopic procedure, let alone in morbidly obese patients. Moreover, in certain types of laparoscopic surgeries (prostatectomies, and some gynecologic surgeries), extreme Trendelenburg is requested, thus making the use of beach chair position even more impractical. A number of variables contribute to the respiratory derangements in all anesthetized patients; notable among them are the general anesthetic, paralytics, pneumoperitoneum, and positioning. The authors describe performing recruitment maneuvers (three consecutive inspiratory holds of 5 s at 45 cm H2O airway pressure) in both positions, with and without pneumoperitoneum, to improve respiratory derangements. Although it is not always possible to consistently maintain improved oxygenation for a prolonged period of time after performing the recruitment maneuvers, it is quite more practical to perform these maneuvers as opposed to attempting to use the beach chair position when performing laparoscopic procedures.*University of Texas MD Anderson Cancer Center, Houston, Texas. jsheir@mdanderson.org

Refining Ventilatory Treatment for Acute Lung Injury and Acute Respiratory Distress Syndrome

ACUTE RESPIRATORY DISTRESS SYNDROME (ARDS) IS the clinical manifestation of inflammatory lung edema originating from a variety of insults. Since its first description 40 years ago, the mainstays of management have been institution of mechanical ventilation to ventilate the incompliant lungs, inspired oxygen for hypoxemia, and when hypoxemia is severe, the addition of positive end-expiratory pressure (PEEP) to increase end-expiration lung volume, which facilitates O2 gas exchange. Early on, physicians recognized that the high intrathoracic pressures of mechanical ventilation caused parenchymal stress or rupture, known as barotrauma. However, it took several years to identify the local injury resulting from intratidal opening and closing of parts of the lung (atelectrauma) and the inflammatory reaction of the lung to nonphysiological stress (biotrauma). Subsequently computed tomographic scanning showed that the lung fraction open to gas exchange in ARDS is small, equivalent in size to that of a young child (baby lung model). This observation provided the anatomical basis for the concept of volutrauma, focused on the excessive strain within the baby lung induced by tidal ventilation. Taken together, these multiple potentially damaging factors are now called ventilatorinduced lung injury (VILI). In the last decade, prevention of VILI through gentle lung treatment, by adjusting either tidal volume or PEEP, has become the major goal of mechanical ventilatory support not just for ARDS but for the broader population of patients with acute lung injury (ALI). With regard to tidal volume, this line of reasoning and research was most conclusively supported by the National Heart, Lung, and Blood Institute ARDS Network trial demonstrating an improvement in survival for patients with ALI or ARDS who were ventilated with low tidal volumes (6 mL/kg of predicted body weight) compared with those ventilated with higher tidal volumes (12 mL/kg of predicted body weight). Although many argued that the tidal volume in the control group might have been higher than existing practice, the tidal volume in the interventional group marked a stark departure from usual care and has resulted in a dramatic change in the approach to tidal volume setting for ALI and ARDS. The optimal PEEP strategy, however, has remained unresolved. Evidence from animal studies suggested that higher PEEP (in the range of 10-15 cm H2O) could prevent VILI. Thus, many clinicians were surprised when the first large randomized clinical trial comparing higher levels of PEEP with lower levels of PEEP in patients with ALI and ARDS, the National Heart, Lung, and Blood Institute’s ARDS Network Assessment of Low Tidal Volume and Elevated EndExpiratory Lung Volume to Obviate Lung Injury (ALVEOLI) study, was stopped for futility. In this issue of the JAMA, 2 new large international randomized trials examining the effects of PEEP on outcome in patients with ALI and ARDS are presented. In the Lung Open Ventilation (LOV) trial, the level of PEEP administered, either lower or higher, was selected according to an oxygenation scale conceptually similar to the one used in the previous ALVEOLI study. In the Expiratory Pressure (Express) trial, PEEP selection was based on a more subtle and refined approach, using bedside assessment of lung mechanics instead of gas exchange. This method identified a minimal distention strategy (lower level of PEEP) and an increased recruitment strategy (higher level of PEEP). Despite the different criteria used for PEEP selection, the PEEP levels tested were similar in the 2 studies. In the LOV study, mean PEEP levels on day 1 were 15.6 cm H2O and 10.1 cm H2O, and the subsequent hospital mortality rates were 36.4% and 40.4%. In the Express study, mean PEEP levels on day 1 were 15.8 cm H2O and 8.4 cm H2O, and the

Abstract 45: A Mathematical Model of Ventilation, Perfusion, and Oxygenation in Low-Flow States

Background: Recent investigations underscore the critical importance of ventilation strategies on resuscitation outcomes. In low perfusion states, such as cardiac arrest and traumatic shock, the rise in intrathoracic pressure that accompanies positive-pressure ventilation (PPV) can significantly impede venous return and lead to a decrease in cardiac output. The optimal ventilation strategy in these “low-flow” states remains unclear. Objectives: To create a mathematical model of perfusion and oxygenation to predict the effects of PPV with both normotension and hypotension. Methods: The lung pressure-volume relationship was modeled using a novel formula allowing manipulation of various lung characteristics, including vital capacity, compliance, and the upper and lower inflection points. A separate formula was then derived to predict mean intrathoracic pressure for a given minute ventilation using the pressure-volume formula. The addition of positive end-expiratory pressure was also modeled. Finally, a formula was derived to model oxygen absorbance as a function of alveolar surface area and flow based on ventilation rate and mean intrathoracic pressure. Results: Mathematical models of the lung pressure-volume relationship, mean intrathoracic pressure, and absorbance were successfully derived. Manipulation of vital capacity, compliance, upper and lower inflection points, positive end-expiratory pressure, and minute ventilation allowed prediction of optimal ventilation rate and tidal volume for a normal and an ARDS lung. For a normal lung, optimal values for both mean intrathoracic pressure and absorption were achieved with a ventilation rate of 4 breaths/min. A decrease in the upper inflection point or increase in minute ventilation resulted in faster optimal ventilation rates, although none exceeded 14 breaths/min. Conclusions: A mathematical model of ventilation was successfully created allowing manipulation of multiple variables related to lung compliance and ventilation strategy. This model suggests the use of lower ventilation rates with larger tidal volumes to minimize the hemodynamic effects of PPV and maximize oxygen absorbance.

Ventilación mecánica en la bronquiolitis

Bronchiolitis is a prevalent viral disease in infants. Many of these infants require hospital admission and mechanical ventilation due to respiratory failure or apnea. The clinical and pathophysiological spectrum of this disease can range from two extremes, obstructive and restrictive disease, on which the indication for mechanical ventilation and the modality used should be based. Non-invasive ventilation is especially indicated in both obstructive and hypoxemic restrictive patterns and a pressure-controlled modality is recommended. In obstructive patterns, air trapping must be monitored, while in restrictive patterns the addition of positive end-expiratory pressure (PEEP) is indicated. High-frequency oscillatory ventilation is indicated in restrictive patterns with sever hypoxemia despite conventional ventilatory support or in cases of significant air leak syndromes. In all cases, a permissive hypercapnia strategy is recommended to prevent barotrauma. Sedation and muscle relaxation should be considered to facilitate adaptation to the ventilator and to try to limit the risks of air trapping, air leak, and barotrauma.

Positive end-expiratory pressure and response to inhaled nitric oxide: Changing nonresponders to responders

Background: Inhaled nitric oxide (INO) has been shown to improve oxygenation in two thirds of patients with acute respiratory distress syndrome (ARDS). Failure to respond to INO is multifactorial. We hypothesized that the addition of positive end expiratory pressure (PEEP) might modify the response to INO in patients who had previously failed to respond to INO. Methods: Patients with ARDS who failed to respond to INO at 1 ppm (Pa O 2 increase of <20%) were selected. Each patient underwent a PEEP trial using an improvement in static lung compliance as the end point. One hour after the new PEEP level was reached, hemodynamic and blood gas values were obtained. INO was then reinstituted at 1 ppm, and hemodynamic and blood gas variables were obtained 1 hour later. Results: Six of nine patients demonstrated an increase in Pa O 2/F IO 2 (161 ± 27 to 186 ± 29) with a mean increase in PEEP of 3.7 cm H 2O. Each patient responding to PEEP further improved Pa O 2/F IO 2 (186 ± 29 to 223 ± 36) with INO at 1 ppm. The three patients who failed to improve after the PEEP increase also failed to respond to a second trial of INO. There were no changes in cardiac output or systemic vascular resistance. Pulmonary artery pressures decreased slightly (39 ± 5 vs 38 ± 7 vs 35 ± 9 mm Hg). Pulmonary vascular resistance decreased significantly after reintroduction of INO (298 ± 131 vs 310 ± 122 vs 249 ± 105 dynes/sec/cm -5) in patients who responded positively. Conclusions: The response of ARDS patients to INO can be improved if optimum alveolar recruitment is achieved by the addition of PEEP. PEEP and INO have a synergistic effect on Pa O 2/F IO 2. Patients who fail to respond to INO may benefit from an optimum PEEP trial. (Surgery 2000;127:390-4.)

Effects of ventilation on the surfactant system in sepsis-induced lung injury.

The present study examined the effects of mechanical ventilation, with or without positive end-expiratory pressure (PEEP), on the alveolar surfactant system in an animal model of sepsis-induced lung injury. Septic animals ventilated without PEEP had a significant deterioration in oxygenation compared with preventilated values (arterial PO(2)/inspired O(2) fraction 316 +/- 16 vs. 151 +/- 14 Torr; P < 0.05). This was associated with a significantly lower percentage of the functional large aggregates (59 +/- 3 vs. 72 +/- 4%) along with a significantly reduced function (minimum surface tension 17.7 +/- 1.8 vs. 11.8 +/- 3.8 mN/m) compared with nonventilated septic animals (P < 0.05). Sham animals similarly ventilated without PEEP maintained oxygenation, percent large aggregates and surfactant function. With the addition of PEEP, the deterioration in oxygenation was not observed in the septic animals and was associated with no alterations in the surfactant system. We conclude that animals with sepsis-induced lung injury are more susceptible to the harmful effects of mechanical ventilation, specifically lung collapse and reopening, and that alterations in alveolar surfactant may contribute to the development of lung dysfunction.

Prevention of venous air embolism in paediatric neurosurgical procedures performed in the sitting position by combined use of MAST suit and PEEP

We studied 60 children undergoing neurosurgical procedures in the sitting position. Routine monitoring included ECG, pulse oximetry, invasive arterial pressure, in particular mean arterial pressure (MAP), and right atrial pressure (RAP). Children were allocated to two groups. In group B lower body positive pressure and positive end-expiratory pressure (PEEP) were used for preventing venous air embolism (VAE). In this group, antishock trousers (MAST suit) were adjusted in supine children. After induction of anaesthesia, different positions were studied: supine and sitting before MAST suit inflation, sitting with MAST suit inflated up to a pressure of 40 mmHg in the lower compartments and 30 mmHg in the abdominal compartment, and finally a combination of lower body positive pressure and PEEP of 8-10 cm H2O. In group A no MAST suit or PEEP was used. Continuous monitoring of end-tidal carbon dioxide pressure throughout (PE'CO2) was used to detect VAE. In order to evaluate the transmission of pressures from the right atrium to the veins at the base of the skull, jugular bulb venous pressure (JBVP) was measured in 20 patients by retrograde catheterization. The incidence of VAE was compared in the two groups. On placing children into the sitting position, a significant decrease in RAP and JBVP was noted without significant changes in MAP in the two groups. Inflation of the MAST suit induced a dramatic increase in RAP and JBVP, reinforced by addition of PEEP. There was a strong positive relationship between RAP and JBVP. There were no deleterious side effects or differences between the two groups in peroperative blood product requirements or surgical general conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Imposed work and oxygen delivery during spontaneous breathing with adult disposable manual ventilators.

Manual ventilators (resuscitators) are used primarily to ventilate the lungs of patients lacking spontaneous ventilatory effort. However, in many settings patients are allowed to breathe through the manual ventilator. Although many aspects of manual ventilator function have been studied, very little has been reported on the use of manual ventilators during spontaneous breathing. The purpose of this study was to evaluate inspiratory and expiratory imposed work of breathing and oxygen delivery during spontaneous breathing through disposable manual ventilators. Simulated spontaneous breathing was established with a two-chambered test lung, with one chamber serving as the test chamber and the other as the driving chamber. Imposed work of breathing was evaluated with decelerating inspiratory flow at a rate of 20 breaths/min at tidal volume (VT) 0.25 1 and peak flow 40 l/min, at VT 0.5 l and peak flow 80 l/min, and VT 0.81 and peak flow 120 l/min. Flow (integrated to volume) and pressure were measured between the manual ventilator and test lung, and inspiratory and expiratory imposed work of breathing were calculated by integration of the volume-pressure curve. Oxygen concentration was measured with an oxygen analyzer placed between the manual ventilator and the test lung at 20 breaths/min, VT 0.5 l, and flow 45 l/min. An oxygen flow of 15 l/min was added to the device for all evaluations. Two of the manual ventilators had built-in positive end-expiratory pressure valves, and imposed work was evaluated at 10 cmH2O with these. There were significant differences in imposed work between inspiration and expiration (P < 0.001) and among the three levels of ventilatory demand (P < 0.001). For each ventilatory demand, there was a significant difference in work between manual ventilator brands for inspiratory work and expiratory work (P < 0.001). At a VT of 0.5 l and peak flow of 80 l/min, the pooled inspiratory imposed work for all manual ventilators was 0.44 +/- 0.12 J/l, and the pooled expiratory imposed work was 0.29 +/- 0.05 J/l. With 10 cmH2O positive end-expiratory pressure, the inspiratory imposed work was very high (> 1 J/l). Four of the devices were unable to deliver more than 0.85 oxygen concentration at the spontaneous ventilatory pattern evaluated. Adult disposable manual ventilators produce a substantial imposed work of spontaneous breathing, which is increased with the addition of positive end-expiratory pressure. With some manual ventilators, a high oxygen concentration may not be delivered during spontaneous breathing. We recommend that patients not be allowed to spontaneously breathe through disposable manual ventilators.