Hemodynamic effects of positive end-expiratory pressure during partial liquid ventilation in newborn lambs

BACKGROUND/PURPOSE: The aim of this study was to compare the effect of positive end-expiratory pressure (PEEP) application on hemodynamics, lung mechanics, and oxygenation in the intact newborn lung during conventional ventilation (CV) and partial liquid ventilation (PLV) at functional residual capacity (FRC). CV or PLV modes of ventilation do not affect hemodynamics nor the optimum PEEP for oxygenation. METHODS: Seven newborn lambs (1 to 3 days old) were instrumented to measure pulmonary hemodynamics and airway mechanics. Each lamb was used as their own control to compare different modes of ventilation (CV followed by PLV) under graded variations of PEEP (4, 8, 12, and 16 cm H(2)O) on the influence on pulmonary blood flow and pulmonary vascular resistance. RESULTS: There was a significant drop in pulmonary blood flow (PBF) from baseline (PEEP of 4 cm H(2)O on CV, 1,229 +/- 377 mL/min) in both modes of ventilation on a PEEP of 16 cm H(2)O (CV, 750 +/- 318 mL/min v PLV, 926 +/- 396 mL/min, respectively; P <.05). Peak inspiratory pressure (PIP) was higher on PLV at PEEP states of 4 cm H(2)O (16.5 +/- 1.3 cm H(2)O to 10.6 +/- 2.1 cm H(2)O; P <.05) and 8 cm H(2)O (18.8 +/- 2.2 cm H(2)O to 15.1 +/- 2.6 cm H(2)O; P <.05) when compared with CV. Conversely, PIP required to maintain the pCO(2) was lower on PLV at PEEP states of 12 (22.5 +/- 3.6 cm H(2)O to 24.2 +/- 3.8 cm H(2)O; P <.05) and 16 cm H(2)O (27.0 +/- 1.6 cm H(2)O to 34.0 +/- 5.9 cm H(2)O; P <.05). CONCLUSIONS: Hemodynamically, CO is impaired at a PEEP above 12 cm H(2)O in intact lungs. PFC at FRC does provide an advantage in lung mechanics more than 10 to 12 cm H(2)O of PEEP by decreasing the amount PIP needed to achieve the similar levels of gas exchange and minute ventilation, implying a reduced risk for barotrauma with chronic ventilation. Thus, selection of the appropriate level of PEEP appears to be important if PLV is to be utilized at FRC. The best strategy for PLV, including the selection of PEEP, remains to be determined.     

Nasal ventilation in acute exacerbations of chronic obstructive pulmonary disease: effect of ventilator mode on arterial blood gas tensions.

BACKGROUND--There are no controlled trials of the use of different modes of nasal intermittent positive pressure ventilation (NIPPV) in patients with exacerbations of chronic obstructive pulmonary disease (COPD). This study describes the effect on blood gas tensions of four different modes of nasal ventilation. METHODS--Twelve patients with acute exacerbations of COPD were studied (mean (SD) FEV1 0.59 (0.13) l, PaO2 (air) 5.10 (1.12) kPa, PaCO2 9.28 (1.97) kPa, pH 7.32 (0.03)). Each patient underwent four one-hour periods of nasal ventilation in randomised order: (a) inspiratory pressure support 18 cm H2O; (b) pressure support 18 cm H2O+positive end expiratory pressure (PEEP) 6 cm H2O (IPAP+EPAP); (c) continuous positive airway pressure (CPAP) 8 cm H2O; and (d) volume cycled NIPPV. Arterial blood samples were obtained before each period of ventilation and at one hour. RESULTS--Pressure support, CPAP, and volume cycled NIPPV all produced significant improvements in PaO2; there was no difference between these three modes. The change in PaO2 with IPAP+EPAP did not reach statistical significance. None of the modes produced significant changes in mean PaCO2; patients with higher baseline levels tended to show a rise in PaCO2 whereas those with lower baseline levels tended to show a fall. CONCLUSIONS--Although PaO2 improved in all patients there are differences in efficacy between the modes, while the changes in PaCO2 were variable. The addition of EPAP conferred no advantage in terms of blood gas tensions.     

Sevoflurane anaesthesia for one-lung ventilation with PEEP to the dependent lung in sheep: effects on right ventricular function and oxygenation

This study was undertaken to examine the effect of sevoflurane on right ventricular junction, the safety of sevoflurane for onelung ventilation and the effects of PEEP (positive end-expiratory pressure) to the dependent lung in this model using 12 openchest sheep. Haemodynamic variables, including cardiac output, mean arterial blood pressure, right ventricular pressure and pulmonary arterial pressure, and right ventricular segment shortening (sonomicrometry) were measured. First, animals received 2.0, 3.0 or 4.0% sevoflurane for 20 min each, respectively, during two-lung ventilation to measure the dose-dependent haemodynamic effects of sevoflurane. Then one-lung ventilation was performed with a randomized sequence of 0 (ZEEP), 5 and 10 cm H₂O PEEP to the dependent lung under 2.0% sevoflurane anaesthesia after one-hour stabilization. A decrease in systolic segment shortening along with increases in both the end-diastolic and end-systolic lengths of the right ventricle were observed at 3.0 and 4.0% sevoflurane, while global right ventricular function remained substantially unchanged during twolung ventilation. During one-lung ventilation the PaO₂ was greater with 5 cm H₂O PEEP 198 mmHg (± 25 SEM) than with ZEEP 138 mmHg (± 22) or with 10 cm H₂O PEEP 153 mmHg (± 23) (P < 0.05). No differences in haemodynamic variables or segment shortening between ZEEP and PEEPs during one-lung ventilation were observed. We conclude that although sevoflurane causes a dose-dependent depression of right ventricular function, sevoflurane anaesthesia can be safely applied to one-lung ventilation, and that 5 cm H₂O PEEP to the dependent lung can improve arterial oxygenation without causing changes in right ventricular function.     

"Best" PEEP during one-lung ventilation

Eight patients were studied under general anaesthesia for elective pulmonary lobectomy to see if intrinsic positive end-expired pressure (PEEPi) would appear or increase in the dependent lung during one-lung ventilation (OLV) or if application of external PEEP equal to individually measured PEEPi would produce better arterial oxygenation, haemodynamic state and oxygen delivery than either zero PEEP (ZEEP) or an external PEEP 5 cm H2O greater than PEEPi. Patients were non-obese, without obstructive airways disease, aged 53-76 yr and ASA < III. They received standardized anaesthesia with fentanyl, 50% nitrous oxide in oxygen and isoflurane; monitoring included radial and fibreoptic pulmonary arterial catheters and intermittent positive pressure ventilation with a tidal volume of 8 ml kg-1, 16 bpm, and an I:E ratio of 1:2. PEEPi was measured during two-lung ventilation (TLV) and OLV, using rapid airway occlusion at end-expiration. There was no PEEPi during TLV, but 2-6 mm Hg of PEEPi appeared during OLV. Applying external PEEP equal to individually measured PEEPi reduced venous admixture and increased PaO2 without a decrease in cardiac index (thus increasing oxygen delivery) compared with ZEEP, but the improvement in pulmonary gas exchange was lost and an additional penalty of reduced cardiac output was imposed when external PEEP was increased to 5 mm Hg above PEEPi.      

急性呼吸窘迫综合征患者适应性支持通气的效果

目的 评价适应性支持通气（ASV）模式与间歇正压通气（IPPV）模式在急性呼吸窘迫综合征（ARDS）患者中的效果。方法 ARDS患者30例，年龄19—46岁，男18例，女12例，ASAⅢ或Ⅳ级。先应用IPPV模式，吸入氧浓度60％，PEEP为0，潮气量（VT）10ml／kg，吸呼比（I：E）1：2，维持8h后随机选择换用ASV或继续IPPV通气模式，通气时依次按0、5、10cm H2O增加PEEP，每一PEEP水平的通气时间为60min，在同样的分钟通气量的设置下，4h后更换另一种通气模式，仍按0,5、10cm H2O增加PEEP，每一PEEP水平的通气时间为60min。每个PEEP水平通气50min时，用Swan-Ganz导管、心电监测仪、呼吸机监测记录血液动力学、呼吸力学和氧代谢数据。结果 与IPPV模式比较，ASV模式下气道峰值压降低，肺动态顺应性（Cdyn）、动脉氧分压（PaO2）和氧供（DO2）增加（P〈0．05）。两种通气模式的血液动力学参数比较差异无统计学意义（P〉0．05）。结论 ASV模式比IPPV模式更有利于ARDS患者的通气治疗。     

Airway occlusion pressure to titrate positive end-expiratory pressure in patients with dynamic hyperinflation

BACKGROUND: Although the use of external positive end-expiratory pressure (PEEP) is recommended for patients with intrinsic PEEP, no simple method exists for bedside titration. We hypothesized that the occlusion pressure, measured from airway pressure during the phase of ventilator triggering (P0.1t), could help to indicate the effects of PEEP on the work of breathing (WOB). METHODS: Twenty patients under assisted ventilation with chronic obstructive pulmonary disease were studied with 0, 5, and 10 cm H2O of PEEP while ventilated with a fixed level of pressure support. RESULTS: PEEP 5 significantly reduced intrinsic PEEP (mean +/- SD, 5.2 +/- 2.4 cm H2O at PEEP 0 to 3.6 +/- 1.9 at PEEP 5; P < 0.001), WOB per min (12. 6 +/- 6.7 J/min to 9.1 +/- 5.9 J/min; P = 0.003), WOB per liter (1.2 +/- 0.4 J/l to 0.8 +/- 0.4 J/l; P < 0.001), pressure time product of the diaphragm (216 +/- 86 cm H2O. s-1. min-1 to 155 +/- 179 cm H2O. s-1. min-1; P = 0.001) and P0.1t (3.3 +/- 1.5 cm H2O to 2.3 +/- 1.4 cm H2O; P = 0.002). At PEEP 10, no further significant reduction in muscle effort nor in P0.1t (2.5 +/- 2.1 cm H2O) occurred, and transpulmonary pressure indicated an increase in end-expiratory lung volume. Significant correlations were found between WOB per min and P0.1t at the three levels of PEEP (P < 0.001), and between the changes in P0.1t versus the changes in WOB per min (P < 0.005), indicating that P0.1t and WOB changed in the same direction. A decrease in P0.1 with PEEP indicated a decrease in intrinsic PEEP with a specificity of 71% and a sensitivity of 88% and a decrease in WOB with a specificity of 86% and a sensitivity of 91%. CONCLUSION: These results show that P0.1t may help to assess the effects of PEEP in patients with intrinsic PEEP.     

Combined effects of inhaled nitric oxide and positive end-expiratory pressure during mechanical ventilation in acute respiratory distress syndrome

We studied the combined effects of inhaled nitric oxide (INO) and positive end expiratory pressure (PEEP) during mechanical ventilation in patients with acute respiratory distress syndrome (ARDS). Eleven patients received 0 and 4 parts per million of INO in random order for 30 min at PEEP levels of 0, 5, and 10 cm H2O. Respiratory and cardiovascular parameters were measured. The addition of INO and PEEP significantly improved arterial oxygenation (p < 0.005 and p < 0.0001, respectively). The combined effect of INO and PEEP on arterial oxygenation was remarkable during 10 cm H2O PEEP. There was synergistic effect on arterial oxygenation by combining INO and 10 cm H2O PEEP. The present study showed that the combination of INO and 10 cm H2O PEEP enhanced arterial oxygenation in patients with ARDS.     

Airway Occlusion Pressure to Titrate Positive End-expiratory Pressure in Patients with Dynamic Hyperinflation

Background: Although the use of external positive end-expiratory pressure (PEEP) is recommended for patients with intrinsic PEEP, no simple method exists for bedside titration. We hypothesized that the occlusion pressure, measured from airway pressure during the phase of ventilator triggering (P0.1t), could help to indicate the effects of PEEP on the work of breathing (WOB).

Methods: Twenty patients under assisted ventilation with chronic obstructive pulmonary disease were studied with 0, 5, and 10 cm H2O of PEEP while ventilated with a fixed level of pressure support.

Results: PEEP 5 significantly reduced intrinsic PEEP (mean +/- SD, 5.2 +/- 2.4 cm H2O at PEEP 0 to 3.6 +/- 1.9 at PEEP 5;P < 0.001), WOB per min (12.6 +/- 6.7 J/min to 9.1 +/- 5.9 J/min;P = 0.003), WOB per liter (1.2 +/- 0.4 J/l to 0.8 +/- 0.4 J/l;P < 0.001), pressure time product of the diaphragm (216 +/- 86 cm H2O [middle dot] s-1 [middle dot] min-1 to 155 +/- 179 cm H2O [middle dot] s-1 [middle dot] min-1;P = 0.001) and P0.1t (3.3 +/- 1.5 cm H2O to 2.3 +/- 1.4 cm H2O;P = 0.002). At PEEP 10, no further significant reduction in muscle effort nor in P0.1t (2.5 +/- 2.1 cm H2O) occurred, and transpulmonary pressure indicated an increase in end-expiratory lung volume. Significant correlations were found between WOB per min and P0.1t at the three levels of PEEP (P < 0.001), and between the changes in P0.1tversus the changes in WOB per min (P < 0.005), indicating that P0.1t and WOB changed in the same direction. A decrease in P0.1 with PEEP indicated a decrease in intrinsic PEEP with a specificity of 71% and a sensitivity of 88% and a decrease in WOB with a specificity of 86% and a sensitivity of 91%.     

慢性阻塞性肺病病人上腹部手术后鼻罩通气的应用

目的 探讨鼻罩通气在上腹部手术后的呼吸支持作用。方法 ５０例择其行上腹部手术病人,术前肺功能检查证实存在轻度中度阻塞性呼吸功能障碍,术后随机分成两组。（１）对照组：术后常规鼻导管吸氧;（２）鼻罩组：术后即开始以鼻罩行压力支持通气。观察两组病人术后通气功能及血气变化。结果 术后早期给予鼻罩通气,病人的氧合优于对照组,ＰａＣＯ２低于对照组,ＶＣ、ＦＥＶ１．０％及ＭＭＦ均明显高于对照组。结论 慢性阻塞肺     

Optimal positive end-expiratory pressure during pumpless extracorporeal lung membrane support

The aim of this study was to determine the optimal positive end-expiratory pressure (PEEP) required during extracorporeal lung membrane support (interventional lung assist [iLA]; Novalung GmbH, Hechingen, Germany). Twenty healthy pigs were initially (4 h) mechanically ventilated with a tidal volume (V(T)) of 10 mL/Kg, respiratory rate (RR) of 20 breaths/min, PEEP of 5 cm H(2)O, and fraction of inspired O(2) (FiO(2)) of 1.0. Thereafter, the iLAs were placed arteriovenously transfemorally and settings reduced to reach near static ventilation (V(T) < or = 2 mL/Kg, RR 4 breaths/min, PEEP of 5, FiO(2) 1.0). Then, animals were assigned to four study groups evaluating 5 cm H(2)O increasing levels of PEEP for 8 h. Gas exchanges with PEEP < or = 10 cm H(2)O were significantly worse than those with PEEP > 12 cm H(2)O, and this without hemodynamical imbalance. This study suggests that the iLA may provide adequate gas exchange during static ventilation only with PEEP levels > 10 cm H(2)O, and this without pulmonary or systemic hemodynamic imbalance.     

The outcome of early pressure-controlled inverse ratio ventilation on patients with severe acute respiratory distress syndrome in surgical intensive care unit

BACKGROUND: Pressure-controlled inverse ratio ventilation (PC-IRV) was used in patients with acute respiratory distress syndrome (ARDS) after failed volume-cycled conventional ratio ventilation (VC-CRV). The aim of this study was to evaluate the outcome of early PC-IRV in severe ARDS. METHODS: Twenty patients with severe ARDS were switched from VC-CRV to PC-IRV if they failed to maintain SaO(2) >90% by the following criteria: peak inspiratory pressure (PIP) >35 cm H(2)O, FIO(2) = 60%, and positive end-expiratory pressure (PEEP) 10 cm H(2)O. RESULTS: The values of PIP, mean airway pressure, minute volumes, and lung injury score in VC-CRV were 43.9 +/- 8.0 cm H(2)O, 19.5 +/- 6.4 cm H(2)O, 11.0 +/- 2.1 L/min, and 2.8 +/- 0.2 respectively. In PC-IRV, the corresponding data were 31.8 +/- 5.1 cm H(2)O, 25.4 +/- 4.6 cm H(2)O, 8.3 +/- 0.9 L/min, and 2.5 +/- 0.4. All of these parameters were significantly different. Fifteen patients (75%) survived their intensive care unit stay. CONCLUSIONS: Early PC-IRV in severe ARDS improves oxygenation, facilitates tapering of high fraction of inspiratory oxygen, and decreases high PEEP or PIP, and then results in the improvement of the patient's outcome.     

Different ventilation strategies affect lung function but do not increase tumor necrosis factor-alpha and prostacyclin production in lavaged rat lungs in vivo

BACKGROUND: Using an in vivo animal model of surfactant deficiency, the authors compared the effect of different ventilation strategies on oxygenation and inflammatory mediator release from the lung parenchyma. METHODS: In adult rats that were mechanically ventilated with 100% oxygen, acute lung injury was induced by repeated lung lavage to obtain an arterial oxygen partial pressure < 85 mmHg (peak pressure/positive end-expiratory pressure [PEEP] = 26/6 cm H2O). Animals were then randomly assigned to receive either exogenous surfactant therapy, partial liquid ventilation, ventilation with high PEEP (16 cm H2O), ventilation with low PEEP (8 cm H2O), or ventilation with an increase in peak inspiratory pressure (to 32 cm H2O; PEEP = 6 cm H2O). Two groups of healthy nonlavaged rats were ventilated at a peak pressure/PEEP of 32/6 and 32/0 cm H2O, respectively. Blood gases were measured. Prostacyclin (PGI2) and tumor necrosis factor-alpha (TNF-alpha) concentrations in serum and bronchoalveolar lavage fluid (BALF) as well as protein concentration in BALF were determined after 90 and 240 min and compared with mechanically ventilated and spontaneously breathing controls. RESULTS: Surfactant, partial liquid ventilation, and high PEEP improved oxygenation and reduced BALF protein levels. Ventilation with high PEEP at high mean airway pressure levels increased BALF PGI2 levels, whereas there was no difference in BALF TNF-alpha levels between groups. Serum PGI2 and TNF-alpha levels did not increase as a result of mechanical ventilation when compared with those of spontaneously breathing controls. CONCLUSIONS: Although alveolar protein concentration and oxygenation markedly differed with different ventilation strategies in this model of acute lung injury, there were no indications of ventilation-induced systemic PGI2 and TNF-alpha release, nor of pulmonary TNF-alpha release. Mechanical ventilation at high mean airway pressure levels increased PGI2 levels in the bronchoalveolar lavage-accessible space.     

Continuous positive airway pressure versus positive end-expiratory pressure in respiratory distress syndrome.

The hemodynamic and respiratory effects of spontaneous ventilation with continuous positive airway pressure (CPAP) and mechanical ventilation with positive end-expiratory pressure (PEEP) were compared in nine patients who had adult respiratory distress syndrome. These patients were capable of maintaining spontaneous ventilation (tidal volume above 300 ml. and PaCO2 below 45 torr). Arterial and mixed venous blood gases, cardiac output, oxygen delivery and consumption, pulmonary artery pressure, and pulmonary wedge pressure were measured in 11 instances, with each patient on 5 or 10 cm. H2O CPAP or PEEP, and in nine instances, with each patient on the ventilator but without PEEP (O PEEP). During CPAP, when compared to PEEP at the same level of end-expiratory pressure, mean PaO2 increased significantly (p less than 0.05) and mean physiological shunt decreased (p less than 0.05). In nine of 11 instances, cardiac output was higher on CPAP than on a corresponding level of PEEP. Thus CPAP was more effective than the same amount of PEEP in improving arterial oxygenation by the lung without adversely affecting cardiac output.     

Experimental pulmonary edema. The effect of unilateral PEEP on the accumulation of lung water.

The effect of PEEP in retarding the development of pulmonary edema produced by elevation of left atrial (LA) pressure to 25cm water was studied in dogs, Using two synchronized volume respirators connected to a double lumen endotracheal tube, 10 or 25cm H2O PEEP was applied to one lung while the contralateral lung was ventilated with an equal tidal volume without PEEP. LA pressure was then elevated by inflating a Foley catheter ballon in the LA until the desired LA pressure was reached. After three hours of pulmonary edema with ventilation of one lung with PEEP lung water was quantitated by wet-to-dry weights. There were no differences in wet to dry weights of PEEP and nonPEEP lungs at either 10 or 25cm H2O level. Additional blood flow studies showed that 25cm H2O of PEEP reduced blood flow to the PEEP lung by 25% and to the nonPEEP lung by 7%. This study shows that PEEP will not mechanically retard the accumulation of lung water due to increased pulmonary capillary hydrostatic pressure.     

Positive pressure versus pressure support ventilation at different levels of PEEP using the ProSeal laryngeal mask airway

We compared positive pressure ventilation with pressure support ventilation at different levels of positive end expiratory pressure (PEEP) using the ProSeal laryngeal mask airway (PLMA). Forty-two anaesthetized adults (ASA 1-2, aged 19 to 63 years) underwent positive pressure ventilation and then pressure support ventilation each with PEEP set at 0, 5 and 10 cmH2O in random order. Pressure support ventilation was with the inspired tidal volume (VTInsp) set at 7 ml/kg and the respiratory rate adjusted to maintain the end-tidal CO2 (ETCO2) at 40 mmHg. Pressure support ventilation was with pressure support set at 5 cmH2O above PEEP and initiated when inspiration produced a 2 cmH2O reduction in airway pressure. Tidal volumes were similar during positive pressure and pressure support ventilation with PEEP, but were higher for the former without PEEP Respiratory rate and peak inspiratory flow rate were higher during pressure support than positive pressure ventilation (all P < 0.001). Peak airway pressure (Ppaw), mean airway pressure (Mpaw), peak expiratory flow rate, and expired airway resistance were lower during pressure support than positive pressure ventilation (all P < 0.001). With PEEP set at 10 cmH2O, ETCO2 was lower for pressure support than positive pressure ventilation. During positive pressure ventilation, there was an increase in Ppaw, Mpaw and dynamic compliance (Cdyn) with increasing levels of PEEP (all P < 0.01). During pressure support ventilation, there was an increase in inspired and expired tidal volume, Ppaw, peak inspiratory and expiratory flow rates and Cdyn, and a reduction in ETCO2, work of breathing, and expired airway resistance with increasing levels of PEEP (all P < 0.01). There were no differences in SpO2, non-invasive mean arterial pressure, heart rate or leak fraction. We conclude that pressure support ventilation provides equally effective gas exchange as positive pressure ventilation during PLMA anaesthesia with or without PEEP at the tested settings. During pressure support, PEEP increases ventilation and reduces work on breathing without increasing leak fraction.     

Relation of the Static Compliance Curve and Positive End-expiratory Pressure to Oxygenation during One-lung Ventilation

Background : Positive end-expiratory pressure (PEEP) is commonly applied to the ventilated lung to try to improve oxygenation during one-lung ventilation but is an unreliable therapy and occasionally causes arterial oxygen partial pressure (Pao2) to decrease further. The current study examined whether the effects of PEEP on oxygenation depend on the static compliance curve of the lung to which it is applied.

Methods : Forty-two adults undergoing thoracic surgery were studied during stable, open-chest, one-lung ventilation. Arterial blood gasses were measured during two-lung ventilation and one-lung ventilation before, during, and after the application of 5 cm H2O PEEP to the ventilated lung. The plateau end-expiratory pressure and static compliance curve of the ventilated lung were measured with and without applied PEEP, and the lower inflection point was determined from the compliance curve.

Results : Mean (+/- SD) Pao2 values, with a fraction of inspired oxygen of 1.0, were not different during one-lung ventilation before (192 +/- 91 mmHg), during (190 +/- 90), or after ( 205 +/- 79) the addition of 5 cm H2O PEEP. The mean plateau end-expiratory pressure increased from 4.2 to 6.8 cm H2O with the application of 5 cm H2O PEEP and decreased to 4.5 cm H2O when 5 cm H2O PEEP was removed. Six patients showed a clinically useful (> 20%) increase in Pao2 with 5 cm H2O PEEP, and nine patients had a greater than 20% decrease in Pao2. The change in Pao2 with the application of 5 cm H2O PEEP correlated in an inverse fashion with the change in the gradient between the end-expiratory pressure and the pressure at the lower inflection point (r = 0.76). The subgroup of patients with a Pao2 during two-lung ventilation that was less than the mean (365 mmHg) and an end-expiratory pressure during one-lung ventilation without applied PEEP less than the mean were more likely to have an increase in Pao2 when 5 cm H2O PEEP was applied.     

Effect of positive end-expiratory pressure on the incidence of venous air embolism and on the cardiovascular response to the sitting position during neurosurgery

We have studied prospectively the effect of 10 cm H2O of PEEP on the incidence of venous air embolism and on the cardiovascular response to change from the supine to the seated position in a large neurosurgical population. Patients were allocated randomly to receive either PEEP (10 cm H2O, n = 45) or conventional (control, n = 44) ventilation. Cardiovascular and respiratory variables were measured in the supine and sitting positions, and monitoring included precordial Doppler probe, pulmonary artery pressure and expiratory carbon dioxide concentration. Venous air embolism was assumed if changes in precordial Doppler sounds occurred, end-tidal carbon dioxide concentration decreased or air could be retrieved from a central venous multi-orifice catheter. The incidence of venous air embolism (26%) did not differ between patients undergoing conventional ventilation and those undergoing ventilation with 10 cm H2O of PEEP. Venous air embolism was always detected first by alterations in Doppler sounds. Cardiac output was significantly higher in patients undergoing conventional ventilation than in those undergoing ventilation with PEEP in the supine but not in the sitting position. Furthermore, pulmonary vascular resistance increased significantly only in the upright position in those undergoing ventilation with PEEP. The pulmonary artery wedge pressure to central venous pressure gradient did not attain negative values with PEEP or with upright positioning. We conclude that the use of PEEP during neurosurgical procedures performed in the sitting position should be abandoned as it does not decrease the incidence of venous air embolism but is associated with significant adverse cardiovascular effects.      

Mechanisms of impaired renal function with PEEP

Deterioration in renal function associated with positive end-expiratory pressure (PEEP) has been attributed to renal hypoperfusion from the fall in cardiac output and mean arterial blood pressure. Using a canine in vivo model, renal function was measured during control, zero end-expiratory pressure (ZEEP), and PEEP (5, 10, and 15 cm H2O) ventilatory cycles, while renal blood flow was maintained constant with a pump. High PEEP (15 cm H2O) led to a rise in renal vein pressure (RVP) and a fall in mean arterial pressure (MAP). PEEP resulted in no change in glomerular filtration rate (GFR) or solute exertion; however, free-water clearance (FWC) became less negative in the 15-cm H2O PEEP group. Intrarenal autoregulation maintains GFR during ventilation with PEEP when renal blood flow is constant, supporting the view that altered filtration and solute excretion clinically is secondary to changes in aortic pressure and renal perfusion.     

Pressure control inverse ratio ventilation in the treatment of adult respiratory distress syndrome in patients with blunt chest trauma.

The objective of this study was to evaluate the efficacy of pressure control inverse ratio ventilation (PCIRV) in improving oxygenation in trauma patients with adult respiratory distress syndrome (ARDS) and to assess the potential risks associated with this form of treatment. This was a cohort study assessing the trends in hemodynamic and ventilatory parameters after the initiation of PCIRV, conducted at a community Level I trauma center intensive care unit. The study comprised 15 trauma patients developing severe, progressive ARDS [two or more of the following criteria: positive end-expiratory pressure (PEEP) >10 cm H2O; arterial partial pressure of oxygen divided by fraction of inspired oxygen (PaO2:FiO2) ratio <150; and peak inspiratory pressure (PIP) >45 cm H2O]: ten due to blunt chest injuries, three due to sepsis, and two due to fat emboli syndrome. PCIRV was initiated. Main outcome measures were PIP, PEEP (total, auto), oxygen saturation, cardiac index, oxygen delivery, PaO2:FiO2 ratio, compliance, evidence of complications of PCIRV, and mortality. Within 24 hours of conversion to PCIRV, the patients stabilized and the mean PaO2:FiO2 ratio rose from 96.3+/-57.8 to 146.8+/-91.1 (P<0.05) and PIP fell from 47.9+/-13.8 to 38.8+/-8.4 cm H2O; auto-PEEP increased from 0.5+/-1.9 to 7.5+/-5.6 cm H2O (P<0.05); oxygen delivery index remained stable (563+/-152 to 497+/-175 mL/min/m2); three patients developed evidence of barotrauma, one patient developed critical illness polyneuropathy, and two patients died (13%). PCIRV is an effective salvage mode of ventilation in patients with severe ARDS, but it is not without complications. Auto-PEEP levels and cardiac index should be monitored to ensure tissue oxygen delivery is maintained.     

Splanchnic circulation and regional sympathetic outflow during peroperative PEEP ventilation in humans

The splanchnic organs represent a major target for sympathetic outflow and an important region for haemodynamic effects on cardiovascular homeostasis. We have studied regional haemodynamic and sympathetic changes in the splanchnic bed during standardized circulatory stress from positive end-expiratory pressure ventilation (PEEP). We investigated eight patients undergoing major upper abdominal surgery using a radiotracer method to measure plasma spillover of norepinephrine as an index of sympathetic nerve activity using arterial, portal and hepatic venous blood sampling. Mesenteric and hepatic perfusion were measured by ultrasound transit time flowmetry and blood-gas analyses. Steady state measurements were performed before and during PEEP ventilation at 10 cm H2O. Plasma spillover of norepinephrine in the mesenteric and hepatic organs represented mean 49 (SEM 8)% and 7 (2)%, respectively, of systemic norepinephrine spillover at baseline, and PEEP ventilation did not cause any significant changes. However, PEEP ventilation significantly decreased portal venous blood flow while hepatic blood flow was preserved by a compensatory increase in hepatic arterial blood flow. Mesenteric and hepatic oxygen delivery changed according to blood flow, and there were no changes in regional oxygen consumption. Thus PEEP ventilation altered mesenteric and hepatic perfusion, independent of any change in corresponding sympathetic nerve activity. Regulation of hepatic blood supply, not related to sympathetic activity, maintained liver oxygenation during PEEP ventilation despite a simultaneous decrease in mesenteric perfusion.