Laboratory and clinical evaluation of the impact Uni-Vent 750 portable ventilator

BACKGROUND: Transportation of critically ill, mechanically ventilated patients from intensive care units for diagnostic and therapeutic procedures has become common in the last decade. Maintenance of adequate oxygenation and ventilation during transport is essential. We evaluated the Impact Uni-Vent 750 portable ventilator in the laboratory and in the clinical arena to determine its usefulness during inhospital transport. MATERIALS & METHODS: In the laboratory, we determined the Uni-Vent 750's ability to assure tidal volume (VT) delivery in the face of decreasing compliance of a test lung and tested the alarm systems. Using a two-compartment lung model modified to simulate spontaneous breathing, we also evaluated the responsiveness of the demand valve. The clinical evaluation was accomplished by comparing arterial blood gas values and ventilator settings in the intensive care unit before transport to those during transport. RESULTS: As lung compliance was reduced from 0.1 to 0.02 mL/cm H2O [1.0 to 0.20 L/kPa], a slight, statistically insignificant decrease in delivered tidal volume was observed. All alarm systems operated according to manufacturer's specifications. The demand valve triggered appropriately with PEEP from 0 to 20 cm H2O [0 to 1.96 kPa]. Sensitivity settings less than -6 cm H2O [-0.59 kPa] sometimes resulted in inability to trigger the demand valve. During patient transport, arterial blood gas values and ventilator settings were comparable to those observed in the ICU. Because an FIO2 of 1.0 was used during transport, mean (SD) PaO2 was significantly greater 89 (26) vs 341 (78) [11.8 (3.5) vs 45.3 (10.4) kPa]. CONCLUSIONS: The Uni-Vent 750 is a reliable transport ventilator, capable of maintaining adequate oxygenation and ventilation in a majority of mechanically ventilated patients. The Uni-Vent 750's ability to (1) provide CMV, AMV, and SIMV; (2) provide low and high pressure alarms; and (3) provide PEEP compensation is unique among portable ventilators.     

呼气末正压对急性呼吸窘迫综合征肺复张容积及氧合影响的临床研究

目的　评价呼气末正压 (PEEP)对急性呼吸窘迫综合征 (ARDS)肺复张容积的影响 ,探讨ARDS患者 PEEP的选择方法。方法　以 11例血流动力学稳定、接受机械通气的 ARDS患者为研究对象 ,采用压力容积曲线法分别测定 PEEP为 5、10、15 cm H2 O(1cm H2 O=0 .0 98k Pa)时的肺复张容积 ,观察患者动脉血气、肺机械力学和血流动力学变化。结果　 PEEP分别 5、10和 15 cm H2 O时肺复张容积分别为 (4 0 .2±15 .3) ml、 (12 3.8± 4 3.1) ml和 (178.9± 4 3.5 ) m l,随着 PEEP水平的增加 ,肺复张容积亦明显增加 (P均 <0 .0 5 )。动脉氧合指数也随着 PEEP水平增加而增加 ,且其变化与肺复张容积呈正相关 (r=0 .4 83,P<0 .0 1)。不同 PEEP条件下 ,患者的肺静态顺应性无明显变化 (P>0 .0 5 )。将患者按有无低位转折点 (L IP)分为有 L IP组与无 L IP组 ,两组患者的肺复张容积都随着 PEEP水平的增加而增加 ,其中 PEEP15 cm H2 O时 L IP组患者的肺复张容积大于无 L IP组 (P<0 .0 5 )。结论　 PEEP水平越高 ,肺复张容积越大 ,肺复张容积增加与动脉氧合指数的变化呈正相关     

The effect of positive end-expiratory pressure during partial liquid ventilation in acute lung injury in piglets.

OBJECTIVES: To investigate the effects of positive end-expiratory pressure (PEEP) application during partial liquid ventilation (PLV) on gas exchange, lung mechanics, and hemodynamics in acute lung injury. DESIGN: Prospective, randomized, experimental study. SETTING: University research laboratory. SUBJECTS: Six piglets weighing 7 to 12 kg. INTERVENTIONS: After induction of anesthesia, tracheostomy, and controlled mechanical ventilation, animals were instrumented with two central venous catheters, a pulmonary artery catheter and two arterial catheters, and an ultrasonic flow probe around the pulmonary artery. Acute lung injury was induced by the infusion of oleic acid (0.08 mL/kg) and repeated lung lavage procedures with 0.9% sodium chloride (20 mL/kg). The protocol consisted of four different PEEP levels (0, 5, 10, and 15 cm H2O) randomly applied during PLV. The oxygenated and warmed perfluorocarbon liquid (30 mL/kg) was instilled into the trachea over 5 mins without changing the ventilator settings. MEASUREMENTS AND MAIN RESULTS: Airway pressures, tidal volumes, dynamic and static pulmonary compliance, mean and expiratory airway resistances, and arterial blood gases were measured. In addition, dynamic pressure/volume loops were recorded. Hemodynamic monitoring included right atrial, mean pulmonary artery, pulmonary capillary wedge, and mean systemic arterial pressures and continuous flow recording at the pulmonary artery. The infusion of oleic acid combined with two to five lung lavage procedures induced a significant reduction in PaO2/FI(O2) from 485 +/- 28 torr (64 +/- 3.6 kPa) to 68 +/- 3.2 torr (9.0 +/- 0.4 kPa) (p < .01) and in static pulmonary compliance from 1.3 +/- 0.06 to 0.67 +/- 0.04 mL/cm H2O/kg (p < .01). During PLV, PaO2/FI(O2) increased significantly from 68 +/- 3.2 torr (8.9 +/- 0.4 kPa) to >200 torr (>26 kPa) (p < .01). The highest PaO2 values were observed during PLV with PEEP of 15 cm H2O. Deadspace ventilation was lower during PLV when PEEP levels of 10 to 15 cm H2O were applied. There were no differences in hemodynamic data during PLV with PEEP levels up to 10 cm H2O. However, PEEP levels of 15 cm H2O resulted in a significant decrease in cardiac output. Dynamic pressure/volume loops showed early inspiratory pressure spikes during PLV with PEEP levels of 0 and 5 cm H2O. CONCLUSIONS: Partial liquid ventilation is a useful technique to improve oxygenation in severe acute lung injury. The application of PEEP during PLV further improves oxygenation and lung mechanics. PEEP levels of 10 cm H2O seem to be optimal to improve oxygenation and lung mechanics.     

俯卧位通气联合呼气末正压治疗急性呼吸窘迫综合征家猪的疗效研究

目的 探讨俯卧位通气联合呼气末正压(PEEP)治疗急性呼吸窘迫综合征(ARDS)的疗效及其机制.方法 12头家猪静脉注射油酸建立ARDS模型,分为仰卧位组和俯卧位组,均给予0(ZEEP)、10(PEEP10)、20 cm H2O(PEEP20,1 cm H2O=0.098 kPa)PEEP的机械通气15 min,监测家猪血流动力学、肺气体交换和呼吸力学指标;处死动物观察肺组织病理学变化.结果 俯卧位组ZEEP、PEEP10时氧合指数(PaO2/FiO2)明显优于仰卧位组[ZEEP:(234.00±72.55)mm Hg比(106.58±34.93)mm Hg,PEEP10:(342.97±60.15) mm Hg比(246.80±83.69)mm Hg,1 mm Hg=0.133 kPa,P均<0.05];PEEP20时两组PaO2/FiO2差异无统计学意义(P>0.05).PEEP10时两组肺复张容积(RV)差异无统计学意义(P>0.05);但PEEP20时俯卧位组RV显著高于仰卧位组[(378.55±101.80)ml比(302.95±34.31)ml,P<0.05].两组间心率(HR)、平均动脉压(MAP)、心排血指数(CI)、呼吸系统顺应性(Cst)及动脉血二氧化碳分压(PaCO2)差异均无统计学意义(P均>0.05);仰卧位组背侧肺组织的肺损伤总评分明显高于俯卧位组[(12.00±1.69)分比(6.03±1.56)分,P<0.05].结论 俯卧位通气联合合适的PEEP可改善ARDS家猪氧合,并且不影响血流动力学和呼吸力学,肺组织损伤的重新分布可能是其机制之一.     

Selective tracheal gas insufflation during partial liquid ventilation improves lung function in an animal model of unilateral acute lung injury.

OBJECTIVE: During unilateral lung injury, we hypothesized that we can improve global lung function by applying selective tracheal gas insufflation (TGI) and partial liquid ventilation (PLV) to the injured lung. DESIGN: Prospective, interventional animal study. SETTING: Animal laboratory in a university hospital. SUBJECTS: Adult mixed-breed dogs. INTERVENTIONS: In six anesthetized dogs, left saline lung lavage was performed until PaO(2)/FiO(2) fell below 100 torr (13.3 kPa). The dogs were then reintubated with a Univent single-lumen endotracheal tube, which incorporates an internal catheter to provide TGI. In a consecutive manner, we studied 1) the application of 10 cm H(2)O of positive end-expiratory pressure (PEEP); 2) instillation of 10 mL/kg of perflubron (Liquivent) to the left lung at a PEEP level of 10 cm H(2)O (PLV+PEEP 10 initial); 3) application of selective TGI (PLV+TGI) while maintaining end-expiratory lung volume (EELV) constant; 4) PLV+TGI at reduced tidal volume (VT); and 5) PLV+PEEP 10 final. MEASUREMENTS AND MAIN RESULTS: Application of PLV+PEEP 10 initial did not change gas exchange, lung mechanics, or hemodynamics. PLV+TGI improved PaO(2)/FiO(2) from 189 +/- 13 torr (25.2 +/- 1.7 kPa) to 383 +/- 44 torr (51.1 +/- 5.9 kPa) (p <.01) and decreased PaCO(2) from 55 +/- 5 torr (7.3 +/- 0.7 kPa) to 30 +/- 2 torr (4.0 +/- 0.3 kPa) (p <.01). During ventilation with PLV+TGI, reducing VT from 15 mL/kg to 3.5 mL/kg while keeping EELV constant decreased PaO(2)/FiO(2) to 288 +/- 49 torr (38.4 +/- 6.5 kPa) (not significant) and normalized PaCO(2). At this stage, end-inspiratory plateau pressure decreased from 19.2 +/- 0.7 cm H(2)O to 13.6 +/- 0.7 cm H(2)O (p <.01). At PLV+PEEP 10 final, measurements returned to those observed at previous baseline stage (PLV+PEEP 10 initial). CONCLUSIONS: During unilateral lung injury, PLV with a moderate PEEP did not improve oxygenation, TGI superimposed on PLV improved gas exchange, and combination of TGI and PLV allowed a 77% reduction in VT without any adverse effect on PaCO(2).     

Setting positive end-expiratory pressure during jet ventilation to replicate the mean airway pressure of oscillatory ventilation

BACKGROUND: High-frequency ventilation can be delivered with either oscillatory ventilation (HFOV) or jet ventilation (HFJV). Traditional clinician biases may limit the range of function of these important ventilation modes. We hypothesized that (1) the jet ventilator can be an accurate monitor of mean airway pressure (P (aw)) during HFOV, and (2) a mathematical relationship can be used to determine the positive end-expiratory pressure (PEEP) setting required for HFJV to reproduce the P (aw) of HFOV. METHODS: In phase 1 of our experiment, we used a differential pressure pneumotachometer and a jet adapter in-line between an oscillator circuit and a pediatric lung model to measure P (aw), PEEP, and peak inspiratory pressure (PIP). Thirty-six HFOV setting combinations were studied, in random order. We analyzed the correlation between the pneumotachometer and HFJV measurements. In phase 2 we used the jet as the monitoring device during each of the same 36 combinations of HFOV settings, and recorded P (aw), PIP, and DeltaP. Then, for each combination of settings, the jet ventilator was placed in-line with a conventional ventilator and was set at the same rate and PIP as was monitored during HFOV. To determine the appropriate PEEP setting, we calculated the P (aw) contributed by the PIP, respiratory rate, and inspiratory time set for HFJV, and subtracted this from the goal P (aw). This value was the PEEP predicted for HFJV to match the HFOV P (aw). RESULTS: The correlation coefficient between the pneumotachometer and HFJV measurements was r = 0.99 (mean difference 0.62 +/- 0.30 cm H(2)O, p < 0.001). The predicted and actual PEEP required were highly correlated (r = 0.99, p < 0.001). The mean difference in these values is not statistically significantly different from zero (mean difference 0.25 +/- 1.02 cm H(2)O, p > 0.15). CONCLUSIONS: HFJV is an accurate monitor during HFOV. These measurements can be used to calculate the predicted PEEP necessary to match P (aw) on the 2 ventilators. Replicating the P (aw) with adequate PEEP on HFJV may help simplify transitioning between ventilators when clinically indicated.     

Positive end expiratory pressure and critical oxygenation during transport in ventilated patients

Transportation of patients critically dependent on positive end expiratory pressure (PEEP) can be problematic, as a patient of ours with adult respiratory distress syndrome (ARDS) and bilateral broncho-pleural fistulae demonstrated. He required intermittent positive pressure ventilation (IPPV) (Siemens 900C) with 100% O₂ and PEEP of 2 kPa to maintain his arterial O₂ saturation (SaO₂)>90%. Severe hypoxemia (SaO₂<75%) occurred on change to a portable ventilator (Oxylog, Dräger) with a PEEP valve (Ambu 20) at its expiratory port, despite adjusting the valve to 2 kPa, continuing use of 100% O₂, and varying the ventilatory pattern. The problem appeared due to loss of PEEP because of gas leak from the lungs via his intercostal catheters. It was solved by introducing a continuous O₂ flow of 51/min into the circuit between the Oxylog non-rebreathing valve and endotracheal tube. We used a model lung to investigate the effect of a gas leak from the lungs or circuit on the performance of the Oxylog IPPV/PEEP system. Lung compliance and ventilatory pattern were adjusted so that tidal volume (V_T)=0.61, peak inspiratory Airway pressure (PIP)=5 kPa, PEEP=1.5 kPa, and respiratory rate=10/min. A small leak was introduced from the lung resulting in a decrease in PIP, V_T, and PEEP. Adjustment of ventilator minute volume to restore PIP to 5 kPa failed to restore PEEP, airway pressure continuing to fall throughout the expiratory pause. PEEP was restored by providing a compensatory flow of O₂ of 5l/min to the system between the Oxylog nonrebreathing valve and the lung. We conclude that significant loss of PEEP can occur in patients with gas leaks from the lung when ventilators, such as the Oxylog, are used that do not provide a compensatory flow of gas into the lung during expiration and the expiratory pause. If the patient is critically dependent on PEEP this loss will result in severe hypoxemia.     

Variations in tidal volume with portable transport ventilators

BACKGROUND: As intra- and interhospital transportation of ventilator-dependent patients has become more commonplace, the number of portable transport ventilators has increased. Transport ventilators should be capable of delivering consistent tidal volume (VT) from breath to breath following changes in lung-thorax compliance and airways resistance. We sought to determine the effect of changes in compliance (C) and resistance (R) on the VT delivered by eight commercially available, time-cycled transport ventilators. METHODS & MATERIALS: Each ventilator (PneuPAC Model 2, Autovent 3000, MAX, Bird Transport Mini-TXP, IC-2A, P7, E100i, and Logic 07a) was connected to a calibrated pneumotachograph and a test lung set for normal adult C (C = 100 mL/cm H2O [1.02 L/kPa]) and R (R = 2 cm H2O.s.L-1 [0.2 kPa.s.L-1]), with VT at 1,000 mL. RESULTS: As C and R were manipulated, VT varied widely. Tidal volume decreased least with the P7 and most with the Bird transport ventilator. CONCLUSION: Decreases in VT with a transport ventilator predispose patients to hypoventilation, hypercapnia, and acidemia. Tidal volume often is not monitored continuously during transport, yet large decreases in VT must not be allowed when pulmonary mechanics are unstable. Internal pressure-limiting valves, venturi flow-generating devices, and compression volume in the breathing circuit are at least three factors that affect VT with transport ventilators.     

肺保护性通气策略在急性百草枯中毒致肺损伤中的临床应用

目的探讨肺保护性通气策略（LPVS）在急性百草枯中毒（APP）致肺损伤中的临床应用价值。方法将31例急性百草枯中毒致肺损伤且符合上呼吸机指征的患者随机分为LPVS组16例和传统机械通气组（对照组）15例。分别观察3～5d内（通气期间）两组的潮气量（VT）、呼气末正压（PEEP）、气道峰压（PIP）、平均气道压（MAP）等呼吸机参数;动脉血氧分压（PaO2）、动脉血二氧化碳分压（PaCO2）、氧合指数（PaO2／FiO2）等动脉血气指标;以及呼吸机相关性肺损伤（VALI）、X线胸片与患者的病死率。结果LPVS组MAP水平低于对照组,差异有统计学意义（P〈0．01）,LPVS组患者的PIP、PEEP较对照组（P均〈0．05）;LPVS组患者的PaCO2、PaO2／FiO2高于对照组,差异均有统计学意义（P均〈0．01）,两组患者的PaO2比较差异无统计学意义（P〉0．05）;LPVS组VALI的发生率低于对照组,X线胸片显示进展延缓病例数高于对照组（P均〈0．05）：两组患者的病死率无统计学差异（P〉0．05）。结论采取LPVS可以一定程度改善APP肺损伤患者的呼吸功能、降低VALI的发生率,对APP导致的肺损伤有积极的治疗作用。     

Extending inspiratory time in acute respiratory distress syndrome

OBJECTIVE: To assess the short-term effects of extending inspiratory time by lengthening end-inspiratory pause (EIP) without inducing a clinically significant increase in intrinsic positive end-expiratory pressure (PEEPi) in patients with acute respiratory distress syndrome (ARDS). DESIGN: Controlled, randomized, crossover study. SETTING: Two medical intensive care units of university hospitals. PATIENTS: Sixteen patients with early (< or =48 hrs) ARDS. INTERVENTION: We applied two durations of EIP (0.2 secs and extended) each for 1 hr while keeping all the following ventilatory parameters constant: FIO2, total PEEP (PEEPtot = applied PEEP + PEEPi), tidal volume, inspiratory flow, and respiratory rate. The duration of extended EIP was titrated to avoid an increase of PEEPi of > or =1 cm H2O. MEASUREMENTS AND MAIN RESULTS: Despite an increase in mean airway pressure (20.6 +/- 2.3 vs. 17.6 +/- 2.1 cm H2O, p < .01), extended EIP did not significantly improve PaO2 (93 +/- 21 vs. 86 +/-16 torr [12.40 +/- 2.80 vs. 11.46 +/- 2.13 kPa] with 0.2 secs EIP, NS). However, although the difference in PaO2 between the two EIP durations was <20 torr (<2.66 kPa) in 14 patients, two patients exhibited a >40 torr (>5.33 kPa) increase in PaO2 with extended EIP. Extended EIP decreased PaCO2 (62 +/- 13 vs. 67 +/- 13 torr [8.26 +/- 1.73 vs. 8.93 +/- 1.73 kPa] with 0.2 secs EIP, p < .01), which resulted in a higher pH (7.22 +/- 0.10 vs. 7.19 +/- 0.09 with 0.2 secs EIP, p < .01) and contributed to a slight increase in arterial hemoglobin saturation (94 +/- 3 vs. 93 +/- 3% with 0.2 EIP, p < .01). No significant difference in hemodynamics was observed. CONCLUSION: In patients with ARDS, extending EIP without inducing a clinically significant increase in PEEPi does not consistently improve arterial oxygenation but enhances CO2 elimination.     

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.     

不同潮气量联合呼气末正压通气对急性肺损伤犬小肠的影响

目的 比较大、小潮气量(VT)机械通气(MV)对急性肺损伤(ALI)犬小肠组织的影响.方法 用静脉注射油酸法制备犬ALI模型,制模成功后随机分为两组,分别接受不同VT的MV,通气时间均为6 h.小VT MV组(LV组,n=6):VT 6 ml/kg,呼气末正压(PEEP)10 cm H2O(1 cm H2O=0.098 kPa);大VT MV组(HV组,n=6):VT 20 ml/kg,PEEP 10 cm H2O.通气6 h后放血处死动物,开腹取小肠组织,用苏木素-伊红(HE)染色,观察组织病理学改变;用原位末端缺刻标记法(TUNEL)观察小肠组织细胞凋亡情况.结果 机械通气6 h后HV组ALI犬小肠胀气明显;而LV组无此表现;LV组小肠损伤评分低于HV组[(3.17±0.75)分比(2.00±0.89)分],差异有统计学意义(P<0.01);但各组犬小肠组织细胞凋亡均罕见.结论 大VT通气可诱导小肠功能不全;小VT通气在一定程度上可避免出现小肠功能障碍.     

Effects of different continuous positive airway pressure devices and periodic hyperinflations on respiratory function

OBJECTIVE: To compare the effect on respiratory function of different continuous positive airway pressure systems and periodic hyperinflations in patients with respiratory failure. DESIGN: Prospective SETTING: Hospital intensive care unit. PATIENTS: Sixteen intubated patients (eight men and eight women, age 54 +/- 18 yrs, PaO2/FiO2 277 +/- 58 torr, positive end-expiratory pressure 6.2 +/- 2.0 cm H2O). INTERVENTIONS: We evaluated continuous flow positive airway pressure systems with high or low flow plus a reservoir bag equipped with spring-loaded mechanical or underwater seal positive end-expiratory pressure valve and a continuous positive airway pressure by a Servo 300 C ventilator with or without periodic hyperinflations (three assisted breaths per minute with constant inspiratory pressure of 30 cm H2O over positive end-expiratory pressure). MEASUREMENTS AND MAIN RESULTS: We measured the respiratory pattern, work of breathing, dyspnea sensation, end-expiratory lung volume, and gas exchange. We found the following: a) Work of breathing and gas exchange were comparable between continuous flow systems; b) the ventilator continuous positive airway pressure was not different compared with continuous flow systems; and c) continuous positive airway pressure with periodic hyperinflations reduced work of breathing (10.7 +/- 9.5 vs. 6.3 +/- 5.7 J/min, p <.05) and dyspnea sensation (1.6 +/- 1.2 vs. 1.1 +/- 0.8 cm, p <.05) increased end-expiratory lung volume (1.6 +/- 0.8 vs. 2.0 +/- 0.9 L, p <.05) and PaO2 (100 +/- 21 vs. 120 +/- 25 torr, p <.05) compared with ventilator continuous positive airway pressure. CONCLUSIONS: The continuous flow positive airway pressure systems tested are equally efficient; a ventilator can provide satisfactory continuous positive airway pressure; and the use of periodic hyperinflations during continuous positive airway pressure can improve respiratory function and reduce the work of breathing.     

Positive end-expiratory pressure above lower inflection point minimizes influx of activated neutrophils into lung

OBJECTIVES: To compare the effects of low vs. high tidal volume (Vt) with three positive end-expiratory pressure (PEEP) strategies on activated neutrophil influx into the lung. DESIGN: Prospective, randomized controlled animal study. SETTING: Animal laboratory in a university hospital. SUBJECTS: Newborn piglets. INTERVENTIONS: Surfactant-depleted piglets were randomized in littermate pairs; to PEEP of either 0 (zero end-expiratory pressure [ZEEP]; n = 6), 8 cm H2O (PEEP 8; n = 5), or 1 cm H2O above the lower inflection point (LIP) (PEEP>LIP; n = 6). Within each pair piglets were randomized to a low VT (5-7 mL/kg) or high VT strategy (17-19 mL/kg). After 4 hrs of mechanical ventilation, 18-fluorodeoxyglucose (18FDG) was injected and positron emission tomography scanning was performed. MEASUREMENTS AND MAIN RESULTS: VT and PEEP changes on influx constants of 18FDG were assessed by analysis of variance. A within-litter comparison of Vt was nonsignificant (p = .50). A between-litter comparison, ordered in linear trend rank, from ZEEP, to PEEP 8, to PEEP>LIP, showed a strong effect of PEEP on influx constant (p = .019). CONCLUSIONS: PEEP set above the LIP on the inspiratory limb of the pressure-volume curve affords a stronger lung protection than VT strategy.     

Effects of helium-oxygen on intrinsic positive end-expiratory pressure in intubated and mechanically ventilated patients with severe chronic obstructive pulmonary disease

OBJECTIVE: To test the hypothesis that replacing 70:30 nitrogen: oxygen (Air-O2) with 70:30 helium:oxygen (He-O2) can decrease dynamic hyperinflation ("intrinsic" positive end-expiratory pressure) in mechanically ventilated patients with chronic obstructive pulmonary disease (COPD), and to document the consequences of such an effect on arterial blood gases and hemodynamics. DESIGN: Prospective, interventional study. SETTING: Medical intensive care unit, university tertiary care center. PATIENTS: Twenty-three intubated, sedated, paralyzed, and mechanically ventilated patients with COPD enrolled within 36 hrs after intubation. INTERVENTIONS: Measurements were taken at the following time points, all with the same ventilator settings: a) baseline; b) after 45 mins with He-O2; c) 45 mins after return to Air-O2. The results were then compared to those obtained in a test lung model using the same ventilator settings. MAIN RESULTS (MEAN + SD): Trapped lung volume and intrinsic positive end-expiratory pressure decreased during He-O2 ventilation (215+/-125 mL vs. 99+/-15 mL and 9+/-2.5 cm H2O vs. 5+/-2.7 cm H2O, respectively; p < .05). Likewise, peak and mean airway pressures declined with He-O2 (30+/-5 cm H2O vs. 25+/-6 cm H2O and 8+/-2 cm H2O vs. 7+/-2 cm H2O, respectively; p < .05). These parameters all rose to their baseline values on return to Air-O2 (p < .05 vs. values during He-O2). These results were in accordance with those obtained in the test lung model. There was no modification of arterial blood gases, heart rate, or mean systemic arterial blood pressure. In 12/23 patients, a pulmonary artery catheter was in place, allowing hemodynamic measurements and venous admixture calculations. Switching to He-O2 and back to Air-O2 had no effect on pulmonary artery pressures, right and left ventricular filling pressures, cardiac output, pulmonary and systemic vascular resistance, or venous admixture. CONCLUSION: In mechanically ventilated COPD patients with intrinsic positive end-expiratory pressure, the use of He-O2 can markedly reduce trapped lung volume, intrinsic positive end-expiratory pressure, and peak and mean airway pressures. No effect was noted on hemodynamics or arterial blood gases. He-O2 might prove beneficial in this setting to reduce the risk of barotrauma, as well as to improve hemodynamics and gas exchange in patients with very high levels of intrinsic positive end-expiratory pressure.     

Airway pressure release ventilation in a neonatal lamb model of acute lung injury

OBJECTIVE: To determine if airway pressure release ventilation (APRV) is feasible in a neonatal animal model with acute lung injury. DESIGN: Nonrandomized, repeated, bracketed measures. SETTING: University research laboratory. SUBJECTS: Seven neonatal sheep (5.6 +/- 0.6 kg), less than 10 days of age. INTERVENTIONS: Acute lung injury was induced by oleic acid infusion and cardiorespiratory profiles were compared during spontaneous ventilation at ambient airway pressure, continuous positive airway pressure (CPAP), APRV, and conventional positive-pressure ventilation (PPV). MEASUREMENTS AND RESULTS: Oleic acid resulted in acute lung injury with stable cardiorespiratory status during the 3-hr study period. Mean airway pressure (Paw) was comparable for all three positive-pressure modes (CPAP 13.4 +/- 1.5, APRV 13.5 +/- 1.4, PPV 13.9 +/- 1.4 cm H2O, NS). After acute lung injury, CPAP increased arterial oxygenation compared with spontaneous ventilation (77.3 +/- 6.9 vs. 57.7 +/- 4.2 torr [10.3 +/- 0.9 vs. 7.7 +/- 0.6 kPa], p less than .05), and this increase was maintained during APRV (73.3 +/- 5.6 vs. 77.3 +/- 6.9 torr [9.8 +/- 0.7 vs. 10.3 +/- 0.9 kPa], NS). Alveolar ventilation was increased by APRV compared with CPAP (PaCO2 29 +/- 1 vs. 41 +/- 2 torr [3.9 +/- 0.1 vs. 5.4 +/- 0.3 kPa], p less than .05) without impairment of cardiovascular performance (cardiac output 1.18 +/- 0.16 vs. 1.20 +/- 0.17 L/min, NS). To achieve ventilation equivalent to APRV during PPV, peak Paw was greater (36.4 +/- 3.2 vs. 19.7 +/- 1.7 cm H2O, p less than .05) and cardiac output (0.94 +/- 0.11 vs. 1.18 +/- 0.16 L/min, p less than .05) and mean arterial pressure (91 +/- 7 vs. 96 +/- 6 mm Hg, p less than .05) were decreased during PPV compared with APRV. CONCLUSIONS: In this neonatal laboratory model of acute lung injury, APRV maintained oxygenation and augmented alveolar ventilation compared with CPAP. Compared with PPV, APRV provided similar ventilation and oxygenation, but at lower peak Paw than PPV, without compromising cardiovascular performance.     

Closed system endotracheal suctioning maintains lung volume during volume-controlled mechanical ventilation

OBJECTIVE: A closed suction system (CS) maintains connection with the mechanical ventilator during tracheal suctioning and is claimed to limit loss in lung volume and oxygenation. We compared changes in lung volume, oxygenation, airway pressure and hemodynamics during endotracheal suctioning performed with CS and with an open suction system (OS). DESIGN: Prospective, randomized study. SETTING: Intensive care unit in a university hospital. PATIENTS: We enrolled ten patients, volume-controlled (VC) ventilated with a Siemens Servo 900 ventilator (PaO2/FIO2 192 +/- 70, PEEP 10.7 +/- 3.9 cmH2O). INTERVENTIONS: We performed four consecutive tracheal suction maneuvers, two with CS and two with OS, at 20-min intervals. During the suction maneuvers continuous suction was applied for 20 s. MEASUREMENTS AND MAIN RESULTS: We measured end-expiratory lung volume changes (delta VL), tidal volume (VTrt), respiratory rate (RR) and minute volume (VErt) by respiratory inductive plethysmography; arterial oxygen saturation (SpO2), airway pressure and arterial pressure (PA). Loss in lung volume during OS (delta VL 1.2 +/- 0.7 l) was significantly higher than during CS (delta VL 0.14 +/- 0.1 l). During OS we observed a marked drop in SpO2, while during CS the change was only minor. During CS ventilation was not interrupted and we observed an immediate increase in RR (due to the activation of the ventilator's trigger), while VTrt decreased, VErt was maintained. CONCLUSIONS: Avoiding suction-related lung volume loss can be helpful in patients with an increased tendency to alveolar collapse; CS allows suctioning while avoiding dramatic drops in lung volumes and seems to be safe during the VC ventilation setting that we used.     

Heliox improves hemodynamics in mechanically ventilated patients with chronic obstructive pulmonary disease with systolic pressure variations

OBJECTIVE: To test the hypothesis that, compared with air-oxygen, heliox would improve cardiac performance in mechanically ventilated patients with severe chronic obstructive pulmonary disease and systolic pressure variations >15 mm Hg and to determine clinical variables associated with favorable hemodynamic responses to heliox. DESIGN: A prospective interventional study. SETTING: Medical and respiratory intensive care units at a university-affiliated tertiary medical center. PATIENTS: Twenty-five consecutive mechanically ventilated patients with severe chronic obstructive pulmonary disease and acute respiratory failure who had systolic pressure variations >15 mm Hg. INTERVENTIONS: Respiratory and hemodynamic measurements were taken at the following time with the same ventilator setting: a) baseline; b) after 30 mins with heliox; and c) 30 mins after return to air-oxygen. MEASUREMENTS AND MAIN RESULTS: Heliox ventilation decreased intrinsic positive end-expiratory pressure (air-oxygen vs. heliox [mean +/- sd] 13 +/- 4 cm H2O vs. 5 +/- 2 cm H2O, p < .05), trapped lung volume (air-oxygen vs. heliox 362 +/- 67 mL vs. 174 +/- 86 mL, p < .05), and respiratory changes in systolic pressure variations (DeltaPP) (air-oxygen vs. heliox 29 +/- 5% vs. 13 +/- 7%, p < .05). In the ten patients with pulmonary arterial catheters, heliox decreased mean pulmonary arterial pressure, right atrial pressure, and pulmonary arterial occlusion pressure and increased cardiac index. Preheliox DeltaPP correlated with the magnitude of reduction in intrinsic positive end-expiratory pressure during heliox ventilation. Age, preheliox Paco2, and ratio of forced expiratory volume at first second to forced vital capacity correlated inversely, whereas preheliox DeltaPP correlated positively with increases in cardiac index. CONCLUSIONS: Heliox may be a useful adjunct therapy in patients with severe chronic obstructive pulmonary disease during acute respiratory failure who have persistent intrinsic positive end-expiratory pressure-induced hemodynamic changes despite ventilator management.     

Effects of recruitment maneuvers in patients with acute lung injury and acute respiratory distress syndrome ventilated with high positive end-expiratory pressure

Objective: Positive end-expiratory pressure (PEEP) and recruitment maneuvers (RMs) may partially reverse atelectasis and reduce ventilation-associated lung injury. The purposes of this study were to assess a) magnitude and duration of RM effects on arterial oxygenation and on requirements for oxygenation support (Fio2/PEEP) in patients with acute lung injury and acute respiratory distress syndrome (ALI/ARDS) receiving ventilation with low tidal volumes and high levels of PEEP; and b) frequency of adverse respiratory and circulatory events attributable to RMs. Design: Prospective, randomized, crossover study. Setting: Thirty-four intensive care units at 19 hospitals. Patients: Seventy-two patients with early ALI/ARDS. Baseline PEEP and Fio2 were 13.8 +/- 3.0 cm H2O and 0.39 +/- 0.10, respectively (mean +/- sd). Interventions: We conducted RMs by applying continuous positive airway pressure of 35-40 cm H2O for 30 secs. We conducted sham RMs on alternate days. We monitored oxyhemoglobin saturation by pulse oximetry (SpO2), Fio2/PEEP, blood pressure, and heart rate for 8 hrs after RMs and sham RMs. We examined chest radiographs for barotrauma. Measurements and Main Results: Responses to RMs were variable. Greatest increments from baseline SpO2 within 10 mins after RMs were larger than after sham RMs (1.7 +/- 0.2 vs. 0.6 +/- 0.3 %, mean +/- SEM, p < .01). Systolic blood pressure decreased more +/- 1.1 mm Hg, p < .01). Changes in Fio2/PEEP requirements were not significantly different at any time after RMs vs. sham RMs. Barotrauma was apparent on first radiographs after one RM and one sham RM.Conclusions: In ALI/ARDS patients receiving mechanical ventilation with low tidal volumes and high PEEP, short-term effects of RMs as conducted in this study are variable. Beneficial effects on gas exchange in responders appear to be of brief duration. More information is needed to determine the role of recruitment maneuvers in the management of ALI/ARDS.     

A simple method for the measurement of intrinsic positive end-expiratory pressure during controlled and assisted modes of mechanical ventilation.

OBJECTIVE: To evaluate a new and simple method for the measurement of intrinsic positive end-expiratory pressure during controlled and assisted modes of mechanical ventilation. DESIGN: Prospective study. SETTING: Three university hospital medical ICUs. PATIENTS: A total of 13 intubated, mechanically ventilated patients with severe airway obstruction. INTERVENTIONS: Airway occlusions reproducibly timed to occur coincidently with end-expiration were obtained by: a) manipulation of a three-way manual valve placed in the inspiratory limb of the external ventilator circuit (manual valve method) and b) activation of the expiratory pause hold function of the mechanical ventilator (Siemens 900C). MEASUREMENTS AND MAIN RESULTS: Airway pressure, flow, and volume were recorded during controlled and assisted modes of mechanical ventilation. Intrinsic positive end-expiratory pressure was determined from the plateau in airway pressure, which was developed during end-expiratory occlusions. For controlled mechanical ventilation, intrinsic positive end-expiratory pressure averaged 11.42 +/- 0.77 (SEM) cm H2O with the manual valve method, compared with 11.38 +/- 0.70 cm H2O, using the ventilator expiratory pause hold function. There was close correlation between results over the wide range of intrinsic positive end-expiratory pressure observed, which varied from approximately 5 to 22 cm H2O (y = 1.08x - 0.92; r2 = .99). Values of intrinsic positive end-expiratory pressure were comparable for the two methods during assist-control ventilation, pressure support ventilation, and spontaneous breathing through the ventilator circuit. The manual valve method was also effective when tested with different mechanical ventilators using a mechanical lung model. CONCLUSIONS: The manual valve method can be used to determine intrinsic positive end-expiratory pressure during controlled and assisted modes of ventilatory support with current ventilators. The availability of such an approach should facilitate the routine monitoring of intrinsic positive end-expiratory pressure in mechanically ventilated patients, thereby aiding clinical decision-making and management in these critically ill individuals.