比例辅助通气对撤机阶段慢性阻塞性肺疾病合并呼吸衰竭患者通气参数的影响

目的观察比例辅助通气（PAV）与压力支持通气（PSV）对撤机阶段慢性阻塞性肺疾病（COPD）合并呼吸衰竭患者通气参数的影响。方法COPD合并呼吸衰竭患者15例,所有患者均为气管插管并接受机械通气支持1周以上者,在治疗过程中病情稳定准备撤机。随机选用PAV和不同水平的PSV[PSV水平为10cmH2O（PS10）和PSV水平为15cmH2O（PS15）,1cmH20=0．098kPa]辅助通气60min,应用PAV前采用最小平方拟合法（LSF）测定患者的呼吸系统弹性阻力（Ers）和气道阻力（Rrs）,设置容量辅助（VA）和流量辅助（FA）,辅助比例为80％。观察患者在不同通气条件下通气参数及动脉血气分析的变化。结果与低水平PSV（PS10）时相比,高水平PSV（PS15）与PAV时的潮气量显著增加[（443±12）ml与（532±34）ml、（464±23）ml,P〈0．05];PAV时的呼吸频率与气道峰压稍高于PS10时。但差异无统计学意义。PAV支持后,患者的气道闭合压由PS10时的（5．70±0．25）cmH2O降至（4．53±0．25）cmH2O（P〈0．05）,气道压力及吸气触发压力时间乘积也显著降低[由（0．42±0．04）cmH2O降至（0．32±0．03）cmH2O,P〈0．05];而氧合指数与动脉血二氧化碳分压均得到明显改善,与PS15时相近。PAV时的浅快呼吸指数较PS10时无明显改变。结论PAV通过采用正反馈调节机制,成比例地提供同步辅助,显著减少COPD呼吸衰竭患者的自主吸气做功,改善人机同步性。     

Comparison of proportional assist ventilation and pressure support ventilation in chronic respiratory failure due to neuromuscular and chest wall deformity

BACKGROUND: The physiological and symptomatic effects of proportional assist ventilation (PAV) and pressure support ventilation (PSV) were compared in stable awake patients with neuromuscular and chest wall deformity (NMCWD). METHODS: Oxygen saturation (SaO(2)), transcutaneous carbon dioxide (TcCO(2)), minute ventilation (VE), tidal volume (VT), respiratory rate (RR), and diaphragm electromyography (EMGdi) were measured in 15 patients during both modes. Subjective effort of breathing and synchrony with the ventilator were assessed using visual analogue scales. RESULTS: Three of 15 patients failed to trigger the ventilator in either mode and were excluded. In the 12 remaining patients there were similar improvements in SaO(2), TcCO(2), VE, VT, and RR during both modes. The mean (SD) percentage fall in EMGdi was greater during PSV (-80.5 (10.7)%) than during PAV (-41.3 (35.2)%; p= 0.01). Effort of breathing (p=0.004) and synchrony with the ventilator (p=0.004) were enhanced more with PSV than with PAV. CONCLUSION: Both PSV and PAV produced similar improvements in physiological parameters. However, greater diaphragm unloading was observed with PSV than with PAV, associated with greater symptomatic benefit. These findings suggest that tolerance to PAV may be compromised in patients with NMCWD.     

Pressure support ventilation decreases inspiratory work of breathing during general anesthesia and spontaneous ventilation.

Spontaneous ventilation may offer advantages over controlled mechanical ventilation (CMV), but increase in work of breathing may diminish its usefulness. During general anesthesia, respiratory depression and increased work of breathing often preclude spontaneous ventilation, and patients then receive CMV. We compared the inspiratory work of breathing of anesthetized patients who breathed with pressure support ventilation (PSV) with that associated with a demand gas flow and a standard anesthesia circle system. We studied nine consenting patients who underwent general inhaled anesthesia with or without regional supplementation. An anesthesia/ventilator system (Siemens 900D, Solna, Sweden) provided PSV (5 cm H2O) or demand gas flow during spontaneous inspiration. Gas flow during demand breathing and PSV was initiated when inspiration produced a 2-cm H2O reduction in airway pressure. An anesthesia machine (Dr?ger Narkomed 3, Telford, Pa.) provided a gas flow rate of 6 L/min through a standard semiclosed circle system. Airway pressure, airway gas flow rate, and esophageal pressure were continuously transduced, and data or signals were conveyed to a computer. Tidal volume and respiratory rate were computed from the flow curve. The inspiratory work of breathing was calculated as the integral of the area subserved by a plot of esophageal pressure and tidal volume during inspiration. Heart rate and mean arterial blood pressure were recorded, and arterial blood was sampled for gas tension and pH analysis. No differences were found in pHa, Paco2, Pao2, tidal volume, respiratory rate, heart rate, or mean arterial blood pressure among the three modes of ventilation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effectiveness of pressure support ventilation for mechanical ventilatory support in patients with status asthmaticus

We compared the effects of pressure support ventilation (PSV) with those of assist control ventilation (ACV) on breathing patterns and blood gas exchange in six patients with status asthmaticus. Both PSV and ACV delivered adequate minute ventilation (PSV: 7.5 +/- 1.4 l/min/m2, ACV: 7.3 +/- 1.3 l/min/m2) to correct respiratory acidosis (pH = 7.33 +/- 0.12 during both PSV and ACV) and prevent hypoxia. Peak airway pressure during PSV was significantly lower with the same tidal volume than that during ACV (PSV: 30 +/- 10 cmH2O (2.9 +/- 1.0 kPa), ACV: 50 +/- 13 cmH2O (4.9 +/- 1.3 kPa)). The lower airway pressure during PSV was due to persistent inspiratory muscle activity. The oxygen cost of breathing estimated by oxygen consumption was equivalent in both modes. We conclude that PSV is effective in supplying tidal volumes adequate to improve hypercarbia at markedly lower airway pressures than ACV.     

Changes in occlusion pressure (P0.1) and breathing pattern during pressure support ventilation

BACKGROUND: The purpose of this study was to investigate changes in breathing pattern, neuromuscular drive (P0.1), and activity of the sternocleidomastoid muscles (SCM) during a gradual reduction in pressure support ventilation (PSV) in patients being weaned off controlled mechanical ventilation. METHODS: Eight non-COPD patients recovering from acute respiratory failure were included in this prospective interventional study. All patients were unable to tolerate discontinuation from mechanical ventilation. Each patient was evaluated during a period of spontaneous breathing and during PSV. Four successive levels of PSV were assessed in the following order: 20 cm H2O (PS20), 15 cm H2O (PS15), 10 cm H2O (PS10), and 5 cm H2O (PS5). RESULTS: When pressure support was reduced from PS20 to PS10 the respiratory rate (f) and the rapid shallow breathing index (f/VT) significantly increased and tidal volume (VT) significantly decreased. These parameters did not vary when pressure support was reduced from PS10 to PS5. Conversely, P0.1 varied negligibly between PS20 and PS15 but increased significantly at low PSV levels. P0.1 values were always greater than 2.9 cm H2O (4.1 (1.1) cm H2O) when SCM activity was present. When contraction of the SCM muscles reappeared the P0.1 was the only parameter that changed significantly. CONCLUSIONS: In postoperative septic patients the value of P0.1 seems to be more useful than breathing pattern parameters for setting the optimal level of pressure assistance during PSV.     

Short-term Cardiorespiratory Effects of Proportional Assist and Pressure-support Ventilation in Patients with Acute Lung Injury/Acute Respiratory Distress Syndrome

Background: Recent data indicate that assisted modes of mechanical ventilation improve pulmonary gas exchange in patients with acute lung injury (ALI)/acute respiratory distress syndrome (ARDS). Proportional assist ventilation (PAV) is a new mode of support that amplifies the ventilatory output of the patient effort and improves patient-ventilator synchrony. It is not known whether this mode may be used in patients with ALI/ARDS. The aim of this study was to compare the effects of PAV and pressure-support ventilation on breathing pattern, hemodynamics, and gas exchange in a homogenous group of patients with ALI/ARDS due to sepsis.

Methods: Twelve mechanically ventilated patients with ALI/ARDS (mean ratio of partial pressure of arterial oxygen to fractional concentration of oxygen 190 +/- 49 mmHg) were prospectively studied. Patients received pressure-support ventilation and PAV in random order for 30 min while maintaining mean airway pressure constant. With both modes, the level of applied positive end-expiratory pressure (7.1 +/- 2.1 cm H2O) was kept unchanged throughout. At the end of each study period, cardiorespiratory data were obtained, and dead space to tidal volume ratio was measured.

Results: With both modes, none of the patients exhibited clinical signs of distress. With PAV, breathing frequency and cardiac index were slightly but significantly higher than the corresponding values with pressure-support ventilation (24.5 +/- 6.9 vs. 21.4 +/- 6.9 breaths/min and 4.4 +/- 1.6 vs. 4.1 +/- 1.3 l [middle dot] min-1 [middle dot] m-2, respectively). None of the other parameters differ significantly between modes.     

Short-term cardiorespiratory effects of proportional assist and pressure-support ventilation in patients with acute lung injury/acute respiratory distress syndrome

BACKGROUND: Recent data indicate that assisted modes of mechanical ventilation improve pulmonary gas exchange in patients with acute lung injury (ALI)/acute respiratory distress syndrome (ARDS). Proportional assist ventilation (PAV) is a new mode of support that amplifies the ventilatory output of the patient effort and improves patient-ventilator synchrony. It is not known whether this mode may be used in patients with ALI/ARDS. The aim of this study was to compare the effects of PAV and pressure-support ventilation on breathing pattern, hemodynamics, and gas exchange in a homogenous group of patients with ALI/ARDS due to sepsis. METHODS: Twelve mechanically ventilated patients with ALI/ARDS (mean ratio of partial pressure of arterial oxygen to fractional concentration of oxygen 190 +/- 49 mmHg) were prospectively studied. Patients received pressure-support ventilation and PAV in random order for 30 min while maintaining mean airway pressure constant. With both modes, the level of applied positive end-expiratory pressure (7.1 +/- 2.1 cm H2O) was kept unchanged throughout. At the end of each study period, cardiorespiratory data were obtained, and dead space to tidal volume ratio was measured. RESULTS: With both modes, none of the patients exhibited clinical signs of distress. With PAV, breathing frequency and cardiac index were slightly but significantly higher than the corresponding values with pressure-support ventilation (24.5 +/- 6.9 vs. 21.4 +/- 6.9 breaths/min and 4.4 +/- 1.6 vs. 4.1 +/- 1.3 l . min . m, respectively). None of the other parameters differ significantly between modes. CONCLUSIONS: In patients with ALI/ARDS due to sepsis, PAV and pressure-support ventilation both have clinically comparable short-term effects on gas exchange and hemodynamics.     

Pressure support ventilation versus continuous positive airway pressure with the laryngeal mask airway: a randomized crossover study of anesthetized adult patients

BACKGROUND: The authors tested the hypothesis that pressure support ventilation (PSV) provides more effective gas exchange than does unassisted ventilation with continuous positive airway pressure (CPAP) in anesthetized adult patients treated using the laryngeal mask airway. METHODS: Forty patients were randomized to two equal-sized crossover groups, and data were collected before surgery. In group 1, patients underwent CPAP, PSV, and CPAP in sequence. In group 2, patients underwent PSV, CPAP, and PSV in sequence. PSV comprised positive end expiratory pressure set at 5 cm H2O and inspiratory pressure support set at 5 cm H2O above positive end expiratory pressure. CPAP was set at 5 cm H2O. Each ventilatory mode was maintained for 10 min. The following data were recorded every minute for the last 5 min of each ventilatory mode and the average reading taken: end tidal carbon dioxide, oxygen saturation, expired tidal volume, leak fraction, respiratory rate, noninvasive mean arterial pressure, and heart rate. RESULTS: In both groups, PSV showed lower end tidal carbon dioxide (P < 0.001), higher oxygen saturation, (P < 0.001), and higher expired tidal volume (P < 0.001) compared with CPAP. In both groups, PSV had similar leak fraction, respiratory rate, mean arterial pressure, and heart rate compared with CPAP. In group 1, measurements for CPAP were similar before and after PSV. In group 2, measurements for PSV were similar before and after CPAP. CONCLUSION: The authors concluded that PSV provides more effective gas exchange than does unassisted ventilation with CPAP during LMA anesthesia while preserving leak fraction and hemodynamic homeostasis.     

Pressure Support Ventilation versus Continuous Positive Airway Pressure with the Laryngeal Mask Airway: A Randomized Crossover Study of Anesthetized Adult Patients

Background: The authors tested the hypothesis that pressure support ventilation (PSV) provides more effective gas exchange than does unassisted ventilation with continuous positive airway pressure (CPAP) in anesthetized adult patients treated using the laryngeal mask airway.

Methods: Forty patients were randomized to two equal-sized crossover groups, and data were collected before surgery. In group 1, patients underwent CPAP, PSV, and CPAP in sequence. In group 2, patients underwent PSV, CPAP, and PSV in sequence. PSV comprised positive end expiratory pressure set at 5 cm H2O and inspiratory pressure support set at 5 cm H2O above positive end expiratory pressure. CPAP was set at 5 cm H2O. Each ventilatory mode was maintained for 10 min. The following data were recorded every minute for the last 5 min of each ventilatory mode and the average reading taken: end tidal carbon dioxide, oxygen saturation, expired tidal volume, leak fraction, respiratory rate, noninvasive mean arterial pressure, and heart rate.

Results: In both groups, PSV showed lower end tidal carbon dioxide (P < 0.001), higher oxygen saturation, (P < 0.001), and higher expired tidal volume (P < 0.001) compared with CPAP. In both groups, PSV had similar leak fraction, respiratory rate, mean arterial pressure, and heart rate compared with CPAP. In group 1, measurements for CPAP were similar before and after PSV. In group 2, measurements for PSV were similar before and after CPAP.     

Relationship between work of breathing provided by a ventilator and patients' inspiratory drive during pressure support ventilation; effects of inspiratory rise time

Inspiratory drive and work of breathing provided by a ventilator (WOBv) during pressure support ventilation (PSV) were examined in 15 patients. At PSV 10 and 15 cm H2O during CPAP 5 cm H2O, patients with low P0.1 (<4.2 cm H2O, n=9) showed WOBv 0.57 and 0.92 J/l, those with high P0.1 (>4.2 cm H2O, n=6) showed 0.31 and 0.62 J/l respectively. WOBv was smaller and pressure-time product of oesophageal pressure (PTP) was significantly larger in high P0.1 patients. Peak inspiratory flow for low P0.1 patients increased as PSV level increased but high P0.1 patients showed no significant change. In a lung model, effects of inspiratory rise time (IRT) and PSV were studied at high and low inspiratory drives by using ventilators with (Servo 300) and without (Mallinckrodt 7200a) adjustable IRT. With 7200a, PSV 10 cm H2O during low drive was compared with PSV 10 and 15 cm H20 during high drive. In Servo 300, PSV 10 cm H2O (IRT 0.6 and 0.0 sec) during low drive was compared with PSV 10 cm H20 (IRT 0.6 and 0.0 sec) and PSV 15 cm H2O (IRT 0.6 sec) during high drive. Raising PSV and shortening IRT both increased peak inspiratory flow. Initial inspiratory flow increased in inverse proportion to IRT, but higher PSV had a little effect. WOBv with high drive was less than with low drive. Higher PSV preserved WOBv by increasing tidal volume. Shortening IRT recruited WOBv by increasing initial inspiratory flow without changing airway pressure and tidal volume. Compared with higher PSV, shorter IRT reduced PTP more. In conclusion, WOBv decreased as inspiratory drive increased due to inability to increase inspiratory flow. Increasing initial inspiratory flow was more effective than raising PSV to preserve inspiratory assistance of PSV at high inspiratory drive.     

Pressure support compared with controlled mechanical ventilation in experimental lung injury

It has been suggested that, in acute lung injury (ALI), spontaneous breathing activity may increase oxygenation because of an improvement of ventilation-perfusion distribution. Pressure support ventilation (PSV) is one of the assisted spontaneous breathing modes often used in critical care medicine. We sought to determine the prolonged effects of PSV on gas exchange in experimental ALI. We hypothesized that PSV may increase oxygenation because of an improvement in ventilation-perfusion distribution. Thus, ALI was induced in 20 pigs by using repetitive lung lavage. Thereafter, the animals were randomized to receive either PSV with a pressure level set to achieve a tidal volume >4 mL/kg and a respiratory rate <40 min(-1) (n = 10) or controlled mechanical ventilation (CMV) with a tidal volume of 10 mL/kg and a respiratory rate of 20 min(-1) (n = 10). Positive end-expiratory pressure was set at 10 cm H(2)O in both groups. Blood gas analyses and determination of ventilation-perfusion (.V(A)/.Q) distribution were performed at the onset of ALI and after 2, 4, 8, and 12 h. The main result was an improvement of oxygenation because of a decrease of pulmonary shunt and an increase of areas with normal .V(A)/.Q ratios during PSV (P < 0.005). However, during CMV, a more pronounced reduction of shunt was observed compared with PSV (P < 0.005). We conclude that, in this model of ALI, PSV improves gas exchange because of a reduction of .V(A)/.Q inequality. However, improvements in .V(A)/.Q distribution may be more effective with CMV than with PSV. IMPLICATIONS: Assisted spontaneous breathing may have beneficial effects on gas exchange in acute lung injury. We tested this hypothesis for pressure support ventilation in an animal model of acute lung injury. Our results demonstrate that pressure support does not necessarily provide better gas exchange than controlled mechanical ventilation.     

Pressure support ventilation versus continuous positive airway pressure ventilation with the ProSeal laryngeal mask airway: a randomized crossover study of anesthetized pediatric patients

Continuous positive airway pressure (CPAP) and pressure support ventilation (PSV) improve gas exchange in adults, but there are little published data regarding children. We compared the efficacy of PSV with CPAP in anesthetized children managed with the ProSeal laryngeal mask airway. Patients were randomized into two equal-sized crossover groups and data were collected before surgery. In Group 1, patients underwent CPAP, PSV, and CPAP in sequence. In Group 2, patients underwent PSV, CPAP, and PSV in sequence. PSV comprised positive end-expiratory pressure set at 3 cm H(2)O and inspiratory pressure support set at 10 cm H(2)O above positive end-expiratory pressure. CPAP was set at 3 cm H(2)O. Each ventilatory mode was maintained for 5 min. The following data were recorded at each ventilatory mode: ETco(2), Spo(2), expired tidal volume, peak airway pressure, work of breathing patient (WOB), delta esophageal pressure, pressure time product, respiratory drive, inspiratory time fraction, respiratory rate, noninvasive mean arterial blood pressure, and heart rate. In Group 1, measurements for CPAP were similar before and after PSV. In Group 2, measurements for PSV were similar before and after CPAP. When compared with CPAP, PSV had lower ETco(2) (46 +/- 6 versus 52 +/- 7 mm Hg; P < 0.001), slower respiratory rate (24 +/- 6 versus 30 +/- 6 min(-1); P < 0.001), lower WOB (0.54 +/- 0.54 versus 0.95 +/- 0.72 JL(-1); P < 0.05), lower pressure time product (94 +/- 88 versus 150 +/- 90 cm H(2)O s(-1)min(-1); P < 0.001), lower delta esophageal pressure (10.6 +/- 7.4 versus 14.1 +/- 8.9 cm H(2)O; P < 0.05), lower inspiratory time fraction (29% +/- 3% versus 34% +/- 5%; P < 0.001), and higher expired tidal volume (179 +/- 50 versus 129 +/- 44 mL; P < 0.001). There were no differences in Spo(2), respiratory drive, mean arterial blood pressure, and heart rate. We conclude that PSV improves gas exchange and reduces WOB during ProSeal laryngeal mask airway anesthesia compared with CPAP in ASA physical status I children aged 1-7 yr.     

Proportional assist ventilation combined with automatic tube compensation. A promising concept of augmented spontaneous breathing?

The combination of proportional assist ventilation (PAV) and automatic tube compensation (ATC) is a promising concept for partial ventilatory support. In contrast to conventional pressure support ventilation (PSV), PAV+ATC provides dynamic pressure support depending on the patient's initial inspiratory effort. PAV+ATC should selectively unload the respiratory muscles from the additional workload imposed by increased respiratory system resistance and elastance as well as by endotracheal tube resistance. Patients have the ability to modify the tidal volume in response to changes in ventilatory demand, thereby improving patient-ventilator interaction and breathing comfort when compared with PSV. However, since routine measurements of respiratory mechanics during augmented spontaneous breathing are currently unavailable but would be necessary for setting the support level as a function of respiratory system mechanics during PAV, this mode cannot yet be generally recommended for routine clinical use.     

Hemodynamic effects of pressure support ventilation in cardiac surgery patients

Hemodynamic consequences of pressure support ventilation (PSV) were compared with intermittent mandatory ventilation (IMV) in 20 patients following aortocoronary bypass. On the morning following surgery, all patients were weaned by IMV to a rate of eight breaths per minute, tidal volume of 12 ml/kg and inspired oxygen concentration of 40 per cent. With patients awake and able to breath spontaneously, PSV was begun at 20 cm of water. In patients with static lung compliance, less than 0.06 l/cm H2O, 30 cm H2O of PSV was used. Subsequently, all patients were weaned to PSV 10 cm of water, continuous positive airway pressure (CPAP) at 5 cm water and extubated. Hemodynamic data including oxygen transport were obtained at each level of PSV and at IMV prior to weaning. Analysis using ANOVA showed comparable hemodynamic and oxygen transport parameters for PSV of 30 cm H2O in comparison with IMV. PSV at levels of 20 and 10 cm H2O produced statistically significant increases in heart rate, mean arterial pressure, central venous pressure, and pulmonary capillary wedge pressure. Cardiac output was stable, and these increases were not clinically significant. In awake patients following cardiac surgery, PSV up to 30 cm H2O can be safely applied without hemodynamic embarrassment in patients with good left ventricular ejection fractions.     

Sigh improves gas exchange and lung volume in patients with acute respiratory distress syndrome undergoing pressure support ventilation

BACKGROUND: The aim of our study was to assess the effect of periodic hyperinflations (sighs) during pressure support ventilation (PSV) on lung volume, gas exchange, and respiratory pattern in patients with early acute respiratory distress syndrome (ARDS). METHODS: Thirteen patients undergoing PSV were enrolled. The study comprised 3 steps: baseline 1, sigh, and baseline 2, of 1 h each. During baseline 1 and baseline 2, patients underwent PSV. Sighs were administered once per minute by adding to baseline PSV a 3- to 5-s continuous positive airway pressure (CPAP) period, set at a level 20% higher than the peak airway pressure of the PSV breaths or at least 35 cm H2O. Mean airway pressure was kept constant by reducing the positive end-expiratory pressure (PEEP) during the sigh period as required. At the end of each study period, arterial blood gas tensions, air flow and pressures traces, end-expiratory lung volume (EELV), compliance of respiratory system (Crs), and ventilatory parameters were recorded. RESULTS: Pao2 improved (P < 0.001) from baseline 1 (91.4 +/- 27.4 mmHg) to sigh (133 +/- 42.5 mmHg), without changes of Paco2. EELV increased (P < 0.01) from baseline 1 (1,242 +/- 507 ml) to sigh (1,377 +/- 484 ml). Crs improved (P < 0.01) from baseline 1 (40.2 +/- 12.5 ml/cm H2O) to sigh (45.1 +/- 15.3 ml/cm H2O). Tidal volume of pressure-supported breaths and the airway occlusion pressure (P0.1) decreased (P < 0.01) during the sigh period. There were no significant differences between baselines 1 and 2 for all parameters. CONCLUSIONS: The addition of 1 sigh per minute during PSV in patients with early ARDS improved gas exchange and lung volume and decreased the respiratory drive.     

Breathing patterns during curare-induced muscle weakness

This study examines the pattern of breathing used by normal subjects to compensate for an acute decrease in muscle strength. A continuous infusion of curare was used to reduce peak inspiratory pressure in six normal subjects from normal control levels to -45 cm H2O (moderate weakness) and to -70 cm H2O (mild weakness). Before administration of curare, inspiratory pressure exceeded -120 cm H2O. A canopy-computer-spirometer system was used for noninvasive spirometry and measurements of gas exchange. Partial curarization to a mild level of muscle weakness did not produce significant changes in the respiratory functions studied. With a moderate level of muscle weakness, there were significant increases in tidal volume from 166 to 186 ml/m2 and in inspiratory time from 1.51 to 1.71 sec (P less than 0.05). Minute ventilation and inspiratory flow did not change. However, when given 3% CO2, both normal and partially curarized subjects increased minute ventilation, from 2.3 to 5.7 L/min/m2 and from 2.5 to 6.7 L/min/m2, respectively. The increases in both conditions were secondary to increases in tidal volume. There was also a small increase in respiratory frequency from 15.4 to 18 breaths/min, P less than 0.01 in the partially curarized group given 3% CO2. Because minute ventilation was preserved while vital capacity decreased, it is proposed that respiration is maintained in the presence of muscle weakness associated with curare by diaphragmatic function which remains relatively unaffected by curarization.     

Sigh Improves Gas Exchange and Lung Volume in Patients with Acute Respiratory Distress Syndrome Undergoing Pressure Support Ventilation

Background: The aim of our study was to assess the effect of periodic hyperinflations (sighs) during pressure support ventilation (PSV) on lung volume, gas exchange, and respiratory pattern in patients with early acute respiratory distress syndrome (ARDS).

Methods: Thirteen patients undergoing PSV were enrolled. The study comprised 3 steps: baseline 1, sigh, and baseline 2, of 1 h each. During baseline 1 and baseline 2, patients underwent PSV. Sighs were administered once per minute by adding to baseline PSV a 3- to 5-s continuous positive airway pressure (CPAP) period, set at a level 20% higher than the peak airway pressure of the PSV breaths or at least 35 cm H2O. Mean airway pressure was kept constant by reducing the positive end-expiratory pressure (PEEP) during the sigh period as required. At the end of each study period, arterial blood gas tensions, air flow and pressures traces, end-expiratory lung volume (EELV), compliance of respiratory system (Crs), and ventilatory parameters were recorded.

Results: Pao2 improved (P < 0.001) from baseline 1 (91.4 +/- 27.4 mmHg) to sigh (133 +/- 42.5 mmHg), without changes of Paco2. EELV increased (P < 0.01) from baseline 1 (1,242 +/- 507 ml) to sigh (1,377 +/- 484 ml). Crs improved (P < 0.01) from baseline 1 (40.2 +/- 12.5 ml/cm H2O) to sigh (45.1 +/- 15.3 ml/cm H2O). Tidal volume of pressure-supported breaths and the airway occlusion pressure (P0.1) decreased (P < 0.01) during the sigh period. There were no significant differences between baselines 1 and 2 for all parameters.     

Tracheal pressure regulated volume assist ventilation in acute respiratory failure

PURPOSE: Proportional assist ventilation (PAV) uses volume assist (VAV) and flow assist ventilation (FAV) to reduce elastic and resistive effort, respectively. Proportional assist ventilation may be difficult to apply clinically, particularly due to FAV related considerations. It was hypothesized that regulating tracheal (Ptr) rather than airway opening pressure (Pao), to overcome endotracheal tube related resistive effort, during VAV would provide an effective alternative method of ventilation. We therefore compared the effects of Pao and Ptr regulated VAV on breathing pattern and inspiratory effort. METHODS: In seven intubated patients, flow, volume, Pao, Ptr, esophageal and transdiaphragmatic pressure were measured during VAV (0-80% respiratory system elastance) using Pao vs Ptr to regulate ventilator applied pressure. Breathing pattern and the pressure-time integral of the inspiratory muscles (integralP(mus) . dt) and diaphragm (integralP(di) . dt) were determined. RESULTS: Compared to spontaneous breathing, the respiratory rate to tidal volume ratio, or rapid shallow breathing index (RSBI), improved progressively with increasing VAV (130 +/- 64 vs 70 +/- 35, VAV 0 vs 80%; P < 0.05) while inspiratory effort fell (integralP(mus) . dt = 39.6 +/- 7.5 vs 28.5 +/- 7.2 cm H(2)O.sec.L(-1), integralP(di) . dt, = 35.4 +/- 7.8 vs 24.2 +/- 5.9 cm H(2)O.sec.L(-1), VAV 0 vs 80%; P < 0.05) due to a decrease in elastic related effort. At any given level of support, there was further reduction in RSBI, integralP(mus) . dt, and integralP(di) . dt (which averaged 23.6 +/- 2.7, 33.7 +/- 4.4, and 38.5 +/- 5.1%, respectively; P < 0.05) for Ptr compared to Pao regulated VAV due to a decrease in resistive effort. CONCLUSIONS: Tracheal pressure regulated VAV can be a simple and effective method of partial ventilatory support in acute respiratory failure. Further work will be needed to determine its efficacy and potential benefit relative to PAV and other modes of ventilation in routine clinical practice.     

Augmentierte Spontanatmung

Impaired pulmonary gas exchange can result from lung parenchymal failure inducing oxygenation deficiency and fatigue of the respiratory muscles, which is characterized by hyercapnia or a combination of both mechanisms. Contractility of and coordination between the diaphragm and the thoracoabdominal respiratory muscles predominantly determine the efficiency of spontaneous breathing. Sepsis, cardiac failure, malnutrition or acute changes of the load conditions may induce fatigue of the respiratory muscles. Augmentation of spontaneous breathing is not only achieved by the application of different technical principles or devices; it also has to improve perfusion, metabolism, load conditions and contractility of the respiratory muscles. Intermittent mandatory ventilation (IMV) allows spontaneous breathing of the patient and augments alveolar ventilation by periodically applying positive airway pressure tidal volumes, which are generated by the respirator. Potential advantages include lower mean airway pressure (P_AW), as compared with controlled mechanical ventilation, and improved haemodynamics. Suboptimal IMV systems may impose increased work and oxygen cost of breathing, fatigue of the respiratory muscles and CO₂ retention. During pressure support ventilation (PSV), inspiratory alterations of P_AW or gas flow (trigger) are detected by the respirator, which delivers a gas flow to maintain P_AW at a fixed value (usually 5–20?cm H₂O) during inspiration. PSV may be combined with other modalities of respiratory therapy such as IMV or CPAP. Claimed advantages of PSV include decreased effort of breathing, reduced systemic and respiratory muscle consumption of oxygen, prophylaxis of diaphragmatic fatigue and an improved extubation rate after prolonged periods of mechanical ventilation. Minimum alveolar ventilation is not guaranteed during PSV; thus, close observation of the patient is mandatory to avoid serious respiratory complications. Continuous positive airway pressure breathing (CPAP) maintains P_AW above atmospheric pressure throughout the respiratory cycle, which may increase functional residual capacity and decrease the effort of breathing. CPAP has been conceptually designed for the augmentation of spontaneous breathing and requires the intact central and peripheral regulation of the respiratory system. Airway pressure release ventilation (APRV) improves alveolar ventilation by intermittent release of P_AW, which is kept above atmospheric pressure by means of a high-flow CPAP system. The opening of an expiratory valve for 1–2?s induces a decreased P_AW and lung volume, which increases rapidly to pre-exhalation values after closure of the valve due to the high gas flow within the circuit (90–100?l/min). APRV may improve haemodynamics and V_A/Q distribution as compared with conventional mechnical ventilation. Biphasic positive airway pressure (BIPAP) is characterized by the combination of spontaneous breathing and time-regulated, pressure-controlled mechanical ventilation. During the respiratory cycle the ventilator generates two alternating CPAP levels, which can be modified with regard to time and pressure. As with APRV, alveolar ventilation is maintained even if the spontaneous breathing efforts of the patient cease, which improves the safety of both modes of respiratory therapy. The contribution of spontaneous breathing to total minute ventilation may be important, since a decreased shunt and improved V_A/Q relationship have been observed in experimental non-cardiogenic lung oedema. These data give support to the concept that spontaneous breathing should be maintained and augmented in the setting of acute respiratory failure.     

Pressure-supported ventilation for posterior fossa operation

To maintain enough gas exchange while using spontaneous respiration as a monitor of the normal brainstem function, we tried pressure-supported ventilation (PSV) with a Servo 900C ventilator (Siemens Elema AB, Sweden) on 12 otherwise healthy patients during posterior fossa operation. Ventilation mode was switched from controlled to PSV after the dura was open uneventfully in all cases but one. With a trigger level of -1 to -2 cm H2O, spontaneous respiration was triggered to start the inspiration. With supporting inspiratory pressure of 4-20 cm H2O, PaCO2 was kept at 31.7-45.9 mm Hg. The ventilatory level could be monitored breath by breath by ventilatory frequency, tidal volume, minute volume, and end-tidal CO2 concentration shown on the ventilator system. Apnea was observed in two cases during surgical manipulation around the brainstem. It was indicated immediately by the ventilator's alarm for decreased expiratory minute volume, and no sign of brainstem dysfunction was observed postoperatively. PSV was useful in maintaining adequate ventilation whereas spontaneous respiration was used as an indicator of normal brainstem function. The alarm system of the ventilator was sensitive enough to detect the surgical invasion of the brainstem at a very early stage.