Cardiorespiratory Effects of Automatic Tube Compensation during Airway Pressure Release Ventilation in Patients with Acute Lung Injury

Background: Spontaneous breaths during airway pressure release ventilation (APRV) have to overcome the resistance of the artificial airway. Automatic tube compensation provides ventilatory assistance by increasing airway pressure during inspiration and lowering airway pressure during expiration, thereby compensating for resistance of the artificial airway. The authors studied if APRV with automatic tube compensation reduces the inspiratory effort without compromising cardiovascular function, end-expiratory lung volume, and gas exchange in patients with acute lung injury.

Methods: Fourteen patients with acute lung injury were breathing spontaneously during APRV with or without automatic tube compensation in random order. Airway pressure, esophageal and abdominal pressure, and gas flow were continuously measured, and tracheal pressure was estimated. Trans-diaphragmatic pressure time product was calculated. End-expiratory lung volume was determined by nitrogen washout. The validity of the tracheal pressure calculation was investigated in seven healthy ventilated pigs.

Results: Automatic tube compensation during APRV increased airway pressure amplitude from 7.7 +/- 1.9 to 11.3 +/- 3.1 cm H2O (mean +/- SD;P < 0.05) while decreasing trans-diaphragmatic pressure time product from 45 +/- 27 to 27 +/- 15 cm H2O [middle dot] s-1 [middle dot] min-1 (P < 0.05), whereas tracheal pressure am-plitude remained essentially unchanged (10.3 +/- 3.5 vs. 10.1 +/- 3.5 cm H2O). Minute ventilation increased from 10.4 +/- 1.6 to 11.4 +/- 1.5 l/min (P < 0.001), decreasing arterial carbon dioxide tension from 52 +/- 9 to 47 +/- 6 mmHg (P < 0.05) without affecting arterial blood oxygenation or cardiovascular function. End-expiratory lung volume increased from 2,806 +/- 991 to 3,009 +/- 994 ml (P < 0.05). Analysis of tracheal pressure-time curves indicated nonideal regulation of the dynamic pressure support during automatic tube compensation as provided by a standard ventilator.     

Spontaneous breathing improves lung aeration in oleic acid-induced lung injury

BACKGROUND: Experimental and clinical studies have shown reduction in intrapulmonary shunt with improved oxygenation by spontaneous breathing with airway pressure release ventilation (APRV) in acute lung injury. The mechanisms of these findings are not clear. The authors hypothesized that spontaneous breathing results in better aeration of lung tissue and that improvement in oxygenation can be explained by these changes. This hypothesis was studied in a porcine model of oleic acid-induced lung injury. METHODS: Two hours after induction of lung injury, 24 pigs were randomly assigned to APRV with or without spontaneous breathing at a positive end-expiratory pressure of 5 cm H(2)O. Hemodynamics, spirometry, and end-expiratory lung volume by nitrogen washout were measured at baseline, after 2 h of lung injury, and after 2 and 4 h of mechanical ventilation in the specific mode. Finally, spiral computed tomography of the chest was performed at end-expiratory lung volume in 22 pigs. RESULTS: Arterial carbon dioxide tension and mean and end-inspiratory airway pressures were comparable between settings. Four hours of APRV with spontaneous breathing resulted in improved oxygenation compared with APRV without spontaneous breathing (arterial oxygen tension, 144 +/- 65 vs. 91 +/- 50 mmHg, P < 0.01 for interaction time x mode), higher end-expiratory lung volume (786 +/- 320 vs. 384 +/- 148 ml, P < 0.001), and better aeration. End-expiratory lung volume and venous admixture were both correlated with the amount of lung reaeration (r(2) = 0.62 and r(2) = 0.61, respectively). CONCLUSIONS: The results support the hypothesis that spontaneous breathing during APRV improves oxygenation mainly by recruitment of nonaerated lung and improved aeration of the lungs.     

Spontaneous Breathing Improves Lung Aeration in Oleic Acid-induced Lung Injury

Background: Experimental and clinical studies have shown reduction in intrapulmonary shunt with improved oxygenation by spontaneous breathing with airway pressure release ventilation (APRV) in acute lung injury. The mechanisms of these findings are not clear. The authors hypothesized that spontaneous breathing results in better aeration of lung tissue and that improvement in oxygenation can be explained by these changes. This hypothesis was studied in a porcine model of oleic acid-induced lung injury.

Methods: Two hours after induction of lung injury, 24 pigs were randomly assigned to APRV with or without spontaneous breathing at a positive end-expiratory pressure of 5 cm H2O. Hemodynamics, spirometry, and end-expiratory lung volume by nitrogen washout were measured at baseline, after 2 h of lung injury, and after 2 and 4 h of mechanical ventilation in the specific mode. Finally, spiral computed tomography of the chest was performed at end-expiratory lung volume in 22 pigs.

Results: Arterial carbon dioxide tension and mean and end-inspiratory airway pressures were comparable between settings. Four hours of APRV with spontaneous breathing resulted in improved oxygenation compared with APRV without spontaneous breathing (arterial oxygen tension, 144 +/- 65 vs. 91 +/- 50 mmHg, P < 0.01 for interaction time x mode), higher end-expiratory lung volume (786 +/- 320 vs. 384 +/- 148 ml, P < 0.001), and better aeration. End-expiratory lung volume and venous admixture were both correlated with the amount of lung reaeration (r2 = 0.62 and r2 = 0.61, respectively).     

Ventilatory support by continuous positive airway pressure breathing improves gas exchange as compared with partial ventilatory support with airway pressure release ventilation

In acute lung injury, airway pressure release ventilation (APRV) with superimposed spontaneous breathing improves gas exchange compared with controlled mechanical ventilation. However, the release of airway pressure below the continuous positive airway pressure (CPAP) level may provoke lung collapse. Therefore, we compared gas exchange and hemodynamics using a crossover design in nine pigs with oleic acid-induced lung injury during CPAP breathing and APRV with a release pressure level of 0 and 5 cm H(2)O. At an identical minute ventilation (V(E) 8 L/min) spontaneous breathing averaged 55%, 67%, and 100% of V(E) during the two APRV modes and CPAP, respectively. Because of the concept of APRV, mean airway pressure was highest during CPAP and lowest during APRV with a release pressure of 0 cm H(2)O. Shunt was reduced to almost half during CPAP (6.6% of Q(t)) compared with both APRV-modes (13.0% of Q(t)). Cardiac output and oxygen consumption, in contrast, were similar during all three ventilatory settings. Thus, in our lung injury model, CPAP was superior to partial ventilatory support using APRV with and without positive end-expiratory pressure. This may be attributable to beneficial effects of spontaneous breathing on gas exchange as well as to rapid lung collapse during the phases of airway pressure release below the CPAP level. These findings may suggest that the amount of mechanical ventilatory support using the APRV mode should be kept at the necessary minimum. IMPLICATIONS: Oxygenation is better with continuous positive airway pressure breathing than with partial mechanical ventilatory support using airway pressure release ventilation. Therefore, mechanical ventilatory support achieved by a cyclic release of airway pressure during APRV should be kept at the minimum level that enables enough ventilatory support for patients to avoid respiratory muscle fatigue.     

Constant positive airway pressure reduces hypoventilation induced by inhalation anesthesia

STUDY OBJECTIVE: To discover if reducing respiratory system impedance would increase tidal volume and improve ventilation during inhalation anesthesia. DESIGN: Prospective, randomized cross-over study. SUBJECTS: Nine ASA physical status I and II adult female oncology patients undergoing breast operations with or without lymph node dissection and general anesthesia while breathing spontaneously. INTERVENTIONS AND MEASUREMENTS: Patients underwent alternating trials of constant positive airway pressure, with or without pressure support. Constant positive airway pressure and pressure support were titrated to maximize respiratory system compliance and equal inspiratory pressure gradient across tracheal tube, respectively. Variables reflecting cardiovascular function, pulmonary mechanics and lung gas exchange, and respired gases and isoflurane concentrations were measured. MAIN RESULTS: End-tidal concentration of isoflurane (1.3 +/- 0.2%), Fio(2) (0.43 +/- 0.09 ), and CO(2) elimination (209 +/- 42 mL min(-1)) was unchanged throughout study in patients aged 63 +/- 12 years, weighing 72 +/- 12 kg. Constant positive airway pressure (12 +/- 2 cm H(2)O) increased respiratory system compliance from 52 +/- 8 to 80 +/- 9 mL cm H(2)O(-1) (P < .001), tidal volume from 156 +/- 32 to 325 +/- 52 mL (P < .001), and minute ventilation from 4.37 +/- 0.86 to 6.18 +/- 0.92 L min(-1) (P < .001). Respiratory rate decreased from 29 +/- 7 to 19 +/- 2 min(-1) (P < .001), Paco(2) decreased from 54 +/- 8 to 44 +/- 6 mm Hg (P < .001), and Pao(2) increased from 137 +/- 37 to 160 +/- 64 mm Hg (P < .001). Pressure support (3.1 +/- 0.3 cm H(2)O) did not alter ventilation or gas exchange. CONCLUSION: We conclude that constant positive airway pressure titrated to optimal respiratory system compliance will increase efficiency of inspiratory muscles and improve ventilation. Constant positive airway pressure facilitates a pattern of breathing that minimizes some of the adverse pulmonary effects of inhalation anesthesia.     

Breathing pattern and workload during automatic tube compensation,pressure support and T-piece trials in weaning patients

BACKGROUND AND OBJECTIVE: Automatic tube compensation has been designed as a new ventilatory mode to compensate for the non-linear resistance of the endotracheal tube. The study investigated the effects of automatic tube compensation compared with breathing through a T-piece or pressure support during a trial of spontaneous breathing used for weaning patients from mechanical ventilation of the lungs. METHODS: Twelve patients were studied who were ready for weaning after prolonged mechanical ventilation (10.2 +/- 8.4 days) due to acute respiratory failure. Patients with chronic obstructive pulmonary disease were excluded. Thirty minutes of automatic tube compensation were compared with 30 min periods of 7 cmH2O pressure support and T-piece breathing. Breathing patterns and workload indices were measured at the end of each study period. RESULTS: During T-piece breathing, the peak inspiratory flow rate (0.65 +/- 0.20 L s(-1)) and minute ventilation (8.9 +/- 2.7L min(-1)) were lower than during either pressure support (peak inspiratory flow rate 0.81 +/- 0.25 L s(-1) minute ventilation 10.2 +/- 2.3 L min(-1), respectively) or automatic tube compensation (peak inspiratory flow rate 0.75 +/- 0.26L s(-1); minute ventilation 10.8 +/- 2.7 L min(-1)). The pressure-time product as well as patients' work of breathing were comparable during automatic tube compensation (pressure-time product 214.5 +/- 104.6 cmH2O s(-1) min(-1), patient work of breathing 1.1 +/- 0.4 J L(-1)) and T-piece breathing (pressure-time product 208.3 +/- 121.6 cmH2O s(-1) min(-1), patient work of breathing 1.1 +/- 0.4 J L(-1)), whereas pressure support resulted in a significant decrease in workload indices (pressure-time product 121.2 +/- 64.1 cmH2O s(-1) min(-1), patient work of breathing 0.7 +/- 0.4 J L(-1)). CONCLUSIONS: In weaning from mechanical lung ventilation, patients' work of breathing during spontaneous breathing trials is clearly reduced by the application of pressure support 7 cmH2O, whereas the workload during automatic tube compensation corresponded closely to the values during trials of breathing through a T-piece.     

Effects of peak inspiratory flow on development of ventilator-induced lung injury in rabbits

BACKGROUND: A lung-protecting strategy is essential when ventilating acute lung injury/acute respiratory distress syndrome patients. Current emphasis is on limiting inspiratory pressure and volume. This study was designed to investigate the effect of peak inspiratory flow on lung injury. METHODS: Twenty-four rabbits were anesthetized, tracheostomized, ventilated with a Siemens Servo 300, and randomly assigned to three groups as follows: 1) the pressure regulated volume control group received pressure-regulated volume control mode with inspiratory time set at 20% of total cycle time, 2) the volume control with 20% inspiratory time group received volume-control mode with inspiratory time of 20% of total cycle time, and 3) the volume control with 50% inspiratory time group received volume-control mode with inspiratory time of 50% of total cycle time. Tidal volume was 30 ml/kg, respiratory rate was 20 breaths/min, and positive end-expiratory pressure was 0 cm H2O. After 6 h mechanical ventilation, the lungs were removed for histologic examination. RESULTS: When mechanical ventilation started, peak inspiratory flow was 28.8 +/- 1.4 l/min in the pressure regulated volume control group, 7.5 +/- 0.5 l/min in the volume control with 20% inspiratory time group, and 2.6 +/- 0.3 l/min in the volume control with 50% inspiratory time group. Plateau pressure did not differ significantly among the groups. Gradually during 6 h, Pao2 in the pressure regulated volume control group decreased from 688 +/- 39 to a significantly lower 304 +/- 199 mm Hg (P < 0.05) (mean +/- SD). The static compliance of the respiratory system for the pressure regulated volume control group also ended significantly lower after 6 h (P < 0.05). Wet to dry ratio for the pressure regulated volume control group was larger than for other groups (P < 0.05). Macroscopically and histologically, the lungs of the pressure regulated volume control group showed more injury than the other groups. CONCLUSION: When an injurious tidal volume is delivered, the deterioration in gas exchange and respiratory mechanics, and lung injury appear to be marked at a high peak inspiratory flow.     

Effects of Peak Inspiratory Flow on Development of Ventilator-induced Lung Injury in Rabbits

Background: A lung-protecting strategy is essential when ventilating acute lung injury/acute respiratory distress syndrome patients. Current emphasis is on limiting inspiratory pressure and volume. This study was designed to investigate the effect of peak inspiratory flow on lung injury.

Methods: Twenty-four rabbits were anesthetized, tracheostomized, ventilated with a Siemens Servo 300, and randomly assigned to three groups as follows: 1) the pressure regulated volume control group received pressure-regulated volume control mode with inspiratory time set at 20% of total cycle time, 2) the volume control with 20% inspiratory time group received volume-control mode with inspiratory time of 20% of total cycle time, and 3) the volume control with 50% inspiratory time group received volume-control mode with inspiratory time of 50% of total cycle time. Tidal volume was 30 ml/kg, respiratory rate was 20 breaths/min, and positive end-expiratory pressure was 0 cm H2O. After 6 h mechanical ventilation, the lungs were removed for histologic examination.

Results: When mechanical ventilation started, peak inspiratory flow was 28.8 +/- 1.4 l/min in the pressure regulated volume control group, 7.5 +/- 0.5 l/min in the volume control with 20% inspiratory time group, and 2.6 +/- 0.3 l/min in the volume control with 50% inspiratory time group. Plateau pressure did not differ significantly among the groups. Gradually during 6 h, Pao2 in the pressure regulated volume control group decreased from 688 +/- 39 to a significantly lower 304 +/- 199 mm Hg (P < 0.05) (mean +/- SD). The static compliance of the respiratory system for the pressure regulated volume control group also ended significantly lower after 6 h (P < 0.05). Wet to dry ratio for the pressure regulated volume control group was larger than for other groups (P < 0.05). Macroscopically and histologically, the lungs of the pressure regulated volume control group showed more injury than the other groups.     

Effects of Spontaneous Breathing during Airway Pressure Release Ventilation on Intestinal Blood Flow in Experimental Lung Injury

Background: In critical illness, the gut is susceptible to hypoperfusion and hypoxia. Positive-pressure ventilation can affect systemic hemodynamics and regional blood flow distribution, with potentially deleterious effects on the intestinal circulation. The authors hypothesized that spontaneous breathing (SB) with airway pressure release ventilation (APRV) provides better systemic and intestinal blood flow than APRV without SB.

Methods: Twelve pigs with oleic acid-induced lung injury received APRV with and without SB. When SB was abolished, either the tidal volume or the ventilator rate was increased to maintain pH and arterial carbon dioxide tension constant as compared to APRV with SB. Systemic hemodynamics were determined by double indicator dilution. Blood flow to the intestinal mucosa-submucosa and muscularis-serosa was measured using colored microspheres.

Results: Systemic blood flow increased during APRV with SB. During APRV with SB, mucosal-submucosal blood flow (ml [middle dot] g-1 [middle dot] min-1) was 0.39 +/- 0.21 in the stomach, 0.76 +/- 0.35 in the duodenum, 0.71 +/- 0.35 in the jejunum, 0.71 +/- 0.59 in the ileum, and 0.63 +/- 0.21 in the colon. During APRV without SB and high tidal volumes, it decreased to 0.19 +/- 0.03 in the stomach, 0.42 +/- 0.21 in the duodenum, 0.37 +/- 0.10 in the jejunum, 0.3 +/- 0.14 in the ileum, and 0.41 +/- 0.14 in the colon (P < 0.001, respectively). During APRV without SB and low tidal volumes, the respective mucosal-submucosal blood flows decreased to 0.24 +/- 0.10 (P < 0.01), 0.54 +/- 0.21 (P < 0.05), 0.48 +/- 0.17 (P < 0.01), 0.43 +/- 0.21 (P < 0.01), and 0.50 +/- 0.17 (P < 0.001) as compared to APRV with SB. Muscularis-serosal perfusion decreased during full ventilatory support with high tidal volumes in comparison with APRV with SB.     

Pulmonary gas distribution during ventilation with different inspiratory flow patterns in experimental lung injury -- a computed tomography study

BACKGROUND: There is still controversy about the optimal inspiratory flow pattern for ventilation of patients with acute lung injury. The aim of this study was to compare the effects of pressure-controlled ventilation (PCV) with a decelerating inspiratory flow with volume-controlled ventilation (VCV) with constant inspiratory flow on pulmonary gas distribution (PGD) in experimentally induced ARDS. METHODS: Sixteen adult sheep were randomized to be ventilated with PCV or VCV after surfactant depletion by repeated bronchoalveolar lavage. Positive end-expiratory pressure (PEEP) was increased in a stepwise manner from zero end-expiratory pressure (ZEEP) to 7, 14 and 21 cm H(2)O in hourly intervals. Respiratory rate, inspiration-to-expiration ratio and tidal volume were kept constant. Central hemodynamics, gas exchange and airway pressures were measured. Electron beam computed tomographic (EBCT) scans of the entire lungs were performed at baseline (preinjury) and each level of end-expiratory pressure during an inspiratory and expiratory hold maneuver. The lungs were three-dimensionally reconstructed and volumetric assessments were made separating the lungs into four subvolumes classified as overinflated, normally aerated, poorly aerated and nonaerated. RESULTS: Pressure-controlled ventilation led to a decrease in peak airway pressure and an increase in mean airway pressure. No differences between groups were found regarding plateau pressures, hemodynamics and gas exchange. Recruitment, defined as a decrease in expiratory lung volume classified as nonaerated, was similar in both groups and predominantly associated with PEEP. Overinflated lung volumes were increased with PCV. CONCLUSIONS: In this model of acute lung injury, ventilation with decelerating inspiratory flow had no beneficial effects on PGD when compared with ventilation with constant inspiratory flow, while the increase in overinflated lung volumes may raise concerns regarding potential ventilator-associated lung injury.     

Effects of spontaneous breathing during airway pressure release ventilation on intestinal blood flow in experimental lung injury

BACKGROUND: In critical illness, the gut is susceptible to hypoperfusion and hypoxia. Positive-pressure ventilation can affect systemic hemodynamics and regional blood flow distribution, with potentially deleterious effects on the intestinal circulation. The authors hypothesized that spontaneous breathing (SB) with airway pressure release ventilation (APRV) provides better systemic and intestinal blood flow than APRV without SB. METHODS: Twelve pigs with oleic acid-induced lung injury received APRV with and without SB. When SB was abolished, either the tidal volume or the ventilator rate was increased to maintain pH and arterial carbon dioxide tension constant as compared to APRV with SB. Systemic hemodynamics were determined by double indicator dilution. Blood flow to the intestinal mucosa-submucosa and muscularis-serosa was measured using colored microspheres. RESULTS: Systemic blood flow increased during APRV with SB. During APRV with SB, mucosal-submucosal blood flow (ml. g-1. min-1) was 0.39 +/- 0.21 in the stomach, 0.76 +/- 0.35 in the duodenum, 0.71 +/- 0.35 in the jejunum, 0.71 +/- 0.59 in the ileum, and 0.63 +/- 0.21 in the colon. During APRV without SB and high tidal volumes, it decreased to 0.19 +/- 0.03 in the stomach, 0.42 +/- 0.21 in the duodenum, 0.37 +/- 0.10 in the jejunum, 0.3 +/- 0.14 in the ileum, and 0.41 +/- 0.14 in the colon (P < 0.001, respectively). During APRV without SB and low tidal volumes, the respective mucosal-submucosal blood flows decreased to 0.24 +/- 0.10 (P < 0.01), 0.54 +/- 0.21 (P < 0.05), 0.48 +/- 0.17 (P < 0.01), 0.43 +/- 0.21 (P < 0.01), and 0.50 +/- 0.17 (P < 0.001) as compared to APRV with SB. Muscularis-serosal perfusion decreased during full ventilatory support with high tidal volumes in comparison with APRV with SB. CONCLUSION: Maintaining SB during APRV was associated with better systemic and intestinal blood flows. Improvements were more pronounced in the mucosal-submucosal layer.     

Effects of the Beach Chair Position, Positive End-expiratory Pressure, and Pneumoperitoneum on Respiratory Function in Morbidly Obese Patients during Anesthesia and Paralysis

Background: The authors studied the effects of the beach chair (BC) position, 10 cm H2O positive end-expiratory pressure (PEEP), and pneumoperitoneum on respiratory function in morbidly obese patients undergoing laparoscopic gastric banding.

Methods: The authors studied 20 patients (body mass index 42 +/- 5 kg/m2) during the supine and BC positions, before and after pneumoperitoneum was instituted (13.6 +/- 1.2 mmHg). PEEP was applied during each combination of position and pneumoperitoneum. The authors measured elastance (E,rs) of the respiratory system, end-expiratory lung volume (helium technique), and arterial oxygen tension. Pressure-volume curves were also taken (occlusion technique). Patients were paralyzed during total intravenous anesthesia. Tidal volume (10.5 +/- 1 ml/kg ideal body weight) and respiratory rate (11 +/- 1 breaths/min) were kept constant throughout.

Results: In the supine position, respiratory function was abnormal: E,rs was 21.71 +/- 5.26 cm H2O/l, and end-expiratory lung volume was 0.46 +/- 0.1 l. Both the BC position and PEEP improved E,rs (P < 0.01). End-expiratory lung volume almost doubled (0.83 +/- 0.3 and 0.85 +/- 0.3 l, BC and PEEP, respectively; P < 0.01 vs. supine zero end-expiratory pressure), with no evidence of lung recruitment (0.04 +/- 0.1 l in the supine and 0.07 +/- 0.2 in the BC position). PEEP was associated with higher airway pressures than the BC position (22.1 +/- 2.01 vs. 13.8 +/- 1.8 cm H2O; P < 0.01). Pneumoperitoneum further worsened E,rs (31.59 +/- 6.73; P < 0.01) and end-expiratory lung volume (0.35 +/- 0.1 l; P < 0.01). Changes of lung volume correlated with changes of oxygenation (linear regression, R2 = 0.524, P < 0.001) so that during pneumoperitoneum, only the combination of the BC position and PEEP improved oxygenation.     

Effects of the beach chair position, positive end-expiratory pressure, and pneumoperitoneum on respiratory function in morbidly obese patients during anesthesia and paralysis

BACKGROUND: The authors studied the effects of the beach chair (BC) position, 10 cm H2O positive end-expiratory pressure (PEEP), and pneumoperitoneum on respiratory function in morbidly obese patients undergoing laparoscopic gastric banding. METHODS: The authors studied 20 patients (body mass index 42 +/- 5 kg/m2) during the supine and BC positions, before and after pneumoperitoneum was instituted (13.6 +/- 1.2 mmHg). PEEP was applied during each combination of position and pneumoperitoneum. The authors measured elastance (E,rs) of the respiratory system, end-expiratory lung volume (helium technique), and arterial oxygen tension. Pressure-volume curves were also taken (occlusion technique). Patients were paralyzed during total intravenous anesthesia. Tidal volume (10.5 +/- 1 ml/kg ideal body weight) and respiratory rate (11 +/- 1 breaths/min) were kept constant throughout. RESULTS: In the supine position, respiratory function was abnormal: E,rs was 21.71 +/- 5.26 cm H2O/l, and end-expiratory lung volume was 0.46 +/- 0.1 l. Both the BC position and PEEP improved E,rs (P < 0.01). End-expiratory lung volume almost doubled (0.83 +/- 0.3 and 0.85 +/- 0.3 l, BC and PEEP, respectively; P < 0.01 vs. supine zero end-expiratory pressure), with no evidence of lung recruitment (0.04 +/- 0.1 l in the supine and 0.07 +/- 0.2 in the BC position). PEEP was associated with higher airway pressures than the BC position (22.1 +/- 2.01 vs. 13.8 +/- 1.8 cm H2O; P < 0.01). Pneumoperitoneum further worsened E,rs (31.59 +/- 6.73; P < 0.01) and end-expiratory lung volume (0.35 +/- 0.1 l; P < 0.01). Changes of lung volume correlated with changes of oxygenation (linear regression, R2 = 0.524, P < 0.001) so that during pneumoperitoneum, only the combination of the BC position and PEEP improved oxygenation. CONCLUSIONS: The BC position and PEEP counteracted the major derangements of respiratory function produced by anesthesia and paralysis. During pneumoperitoneum, only the combination of the two maneuvers improved oxygenation.     

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.     

Effects of end-inspiratory and end-expiratory pressures on alveolar recruitment and derecruitment in saline-washout-induced lung injury -- a computed tomography study

BACKGROUND: Lung protective ventilation using low end-inspiratory pressures and tidal volumes (VT) has been shown to impair alveolar recruitment and to promote derecruitment in acute lung injury. The aim of the present study was to compare the effects of two different end-inspiratory pressure levels on alveolar recruitment, alveolar derecruitment and potential overdistention at incremental levels of positive end-expiratory pressure. METHODS: Sixteen adult sheep were randomized to be ventilated with a peak inspiratory pressure of either 35 cm H2O (P35, low VT) or 45 cm H2O (P45, high VT) after saline washout-induced lung injury. Positive end-expiratory pressure (PEEP) was increased in a stepwise manner from zero (ZEEP) to 7, 14 and 21 cm of H2O in hourly intervals. Tidal volume, initially set to 12 ml kg(-1), was reduced according to the pressure limits. Computed tomographic scans during end-expiratory and end-inspiratory hold were performed along with hemodynamic and respiratory measurements at each level of PEEP. RESULTS: Tidal volumes for the two groups (P35/P45) were: 7.7 +/- 0.9/11.2 +/- 1.3 ml kg(-1) (ZEEP), 7.9 +/- 2.1/11.3 +/- 1.3 ml kg(-1) (PEEP 7 cm H2O), 8.3 +/- 2.5/11.6 +/- 1.4 ml kg(-1) (PEEP 14 cm H2O) and 6.5 +/- 1.7/11.0 +/- 1.6 ml kg(-1) (PEEP 21 cm H2O); P < 0.001 for differences between the two groups. Absolute nonaerated lung volumes during end-expiration and end-inspiration showed no difference between the two groups for given levels of PEEP, while tidal-induced changes in nonaerated lung volume (termed cyclic alveolar instability, CAI) were larger in the P45 group at low levels of PEEP. The decrease in nonaerated lung volume was significant for PEEP 14 and 21 cm H2O in both groups compared with ZEEP (P < 0.005). Over-inflated lung volumes, although small, were significantly higher in the P45 group. Significant respiratory acidosis was noted in the P35 group despite increases in the respiratory rate. CONCLUSION: Limiting peak inspiratory pressure and VT does not impair alveolar recruitment or promote derecruitment when using sufficient levels of PEEP.     

Developing a strategy to improve ventilation in an unprotected airway with a modified mouth-to-bag resuscitator in apneic patients

The strategies to ensure safety during ventilation of an unprotected airway are limiting airway pressure and/or inspiratory flow. In this prospective, randomized study we assessed the effect of face mask ventilation with small tidal volumes in the modified mouth-to-bag resuscitator (maximal volume, 500 mL) versus a pediatric self-inflatable bag versus automatic pressure-controlled ventilation in 40 adult apneic patients during induction of anesthesia. The mouth-to-bag resuscitator requires the rescuer to blow up a balloon inside the self-inflating bag that subsequently displaces air which then flows into the patient's airway. Respiratory variables were measured with a pulmonary monitor (CP-100). Mouth-to-bag resuscitator and pressure-controlled ventilation resulted in significantly lower (mean +/- sd) peak airway pressure (8 +/- 2 and 8 +/- 1 cm H(2)O), peak inspiratory flow rate (0.7 +/- 0.1 and 0.7 +/- 0.1 L/s), and larger inspiratory time fraction (33% +/- 5% and 47% +/- 2%) in comparison to pediatric self-inflating bag ventilation (12 +/- 3 cm H(2)O; 1 +/- 0.2 L/s; 27% +/- 4%; all P < 0.001). The tidal volumes were similar between groups. No stomach inflation occurred in either group. We conclude that using a modified mouth-to-bag resuscitator or automatic pressure-controlled ventilation with similar small tidal volumes during face mask ventilation resulted in an approximately 25% reduction in peak airway pressure when compared with a standard pediatric self-inflating bag.     

Mechanical versus manual ventilation via a face mask during the induction of anesthesia: a prospective, randomized, crossover study

One approach to make ventilation safer in an unprotected airway has been to limit tidal volumes; another one might be to limit peak airway pressure, although it is unknown whether adequate tidal volumes can be delivered. Accordingly, the purpose of this study was to evaluate the quality of automatic pressure-controlled ventilation versus manual circle system face-mask ventilation regarding ventilatory variables in an unprotected airway. We studied 41 adults (ASA status I-II) in a prospective, randomized, crossover design with both devices during the induction of anesthesia. Respiratory variables were measured with a pulmonary monitor (CP-100). Pressure-controlled mask ventilation versus circle system ventilation resulted in lower (mean +/- SD) peak airway pressures (10.6 +/- 1.5 cm H(2)O versus 14.4 +/- 2.4 cm H(2)O; P < 0.001), delta airway pressures (8.5 +/- 1.5 cm H(2)O versus 11.9 +/- 2.3 cm H(2)O; P < 0.001), expiratory tidal volume (650 +/- 100 mL versus 680 +/- 100 mL; P = 0.001), minute ventilation (10.4 +/- 1.8 L/min versus 11.6 +/- 1.8 L/min; P < 0.001), and peak inspiratory flow rates (0.81 +/- 0.06 L/s versus 1.06 +/- 0.26 L/s; P < 0.001) but higher inspiratory time fraction (48% +/- 0.8% versus 33% +/- 7.7%; P < 0.001) and end-tidal carbon dioxide (34 +/- 3 mm Hg versus 33 +/- 4 mm Hg; not significant). We conclude that in this model of apneic patients with an unprotected airway, pressure-controlled ventilation resulted in reduced inspiratory peak flow rates and peak airway pressures when compared with circle system ventilation, thus providing an additional patient safety effect during mask ventilation. IMPLICATIONS: In this model of apneic patients with an unprotected airway, pressure-controlled ventilation resulted in reduced inspiratory peak flow rates and lower peak airway pressures when compared with circle system ventilation, thus providing an additional patient safety effect during face-mask ventilation.     

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.     

Collapsibility of the Upper Airway during Anesthesia with Isoflurane

Background: The unprotected upper airway tends to obstruct during general anesthesia, yet its mechanical properties have not been studied in detail during this condition.

Methods: To study its collapsibility, pressure-flow relationships of the upper airway were obtained at three levels of anesthesia (end-tidal isoflurane = 1.2%, 0.8%, and 0.4%) in 16 subjects while supine and spontaneously breathing on nasal continuous positive airway pressure. At each level of anesthesia, mask pressure was transiently reduced from a pressure sufficient to abolish inspiratory flow limitation (11.8 +/- 2.7 cm H2O) to pressures resulting in variable degrees of flow limitation. The relation between mask pressure and maximal inspiratory flow was determined, and the critical pressure at which the airway occluded was recorded. The site of collapse was determined from simultaneous measurements of nasopharyngeal, oropharyngeal, and hypopharyngeal and esophageal pressures.

Results: The airway remained hypotonic (minimal or absent intramuscular genioglossus electromyogram activity) throughout each study. During flow-limited breaths, inspiratory flow decreased linearly with decreasing mask pressure (r2 = 0.86 +/- 0.17), consistent with Starling resistor behavior. At end-tidal isoflurane of 1.2%, critical pressure was 1.1 +/- 3.5 cm H2O; at 0.4% it decreased to -0.2 +/- 3.6 cm H2O (P < 0.05), indicating decreased airway collapsibility. This decrease was associated with a decrease in end-expiratory esophageal pressure of 0.6 +/- 0.9 cm H2O (P < 0.05), suggesting an increased lung volume. Collapse occurred in the retropalatal region in 14 subjects and in the retrolingual region in 2 subjects, and did not change with anesthetic depth.     

Optimal Mean Airway Pressure during High-frequency Oscillation: Predicted by the Pressure-Volume Curve

Background: A number of groups have recommended setting positive end-expiratory pressure during conventional mechanical ventilation in adults at 2 cm H2O above the lower corner pressure (PCL) of the inspiratory pressure-volume (P-V) curve of the respiratory system. No equivalent recommendations for the setting of the mean airway pressure (Paw) during high-frequency oscillation (HFO) exist. The authors questioned if the Paw resulting in the best oxygenation without hemodynamic compromise during HFO is related to the static P-V curve in a large animal model of acute respiratory distress syndrome.

Methods: Saline lung lavage was performed in seven sheep (28 +/- 5 kg, mean +/- SD) until the arterial oxygen partial pressure/fraction of inspired oxygen ratio decreased to 85 +/- 27 mmHg at a positive end-expiratory pressure of 5 cm H2O (initial injury). The PCL (20 +/- 1 cm H2O) on the inflation limb and the point of maximum curvature change (PMC; 26 +/- 1 cm H2O) on the deflation limb of the static P-V curve were determined. The sheep were subjected to four 1-h cycles of HFO at different levels of Paw (PCL + 2, + 6, + 10, + 14 cm H2O), applied in random order. Each cycle was preceded by a recruitment maneuver at a sustained Paw of 50 cm H2O for 60 s.

Results: High-frequency oscillation with a Paw of 6 cm H2O above PCL (PCL + 6) resulted in a significant improvement in oxygenation (P < 0.01 vs. initial injury). No further improvement in oxygenation was observed with higher Paw, but cardiac output decreased, pulmonary vascular resistance increased, and oxygen delivery decreased at Paw greater than PCL + 6. The PMC on the deflation limb of the P-V curve was equal to the PCL + 6 (r = 0.77, P < 0.05).