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.     

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.     

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.     

Improved flow and pressure capabilities of the Datex-Ohmeda SmartVent anesthesia ventilator

STUDY OBJECTIVE: To compare the flow and pressure capabilities of the Datex-Ohmeda SmartVent (Ohmeda 7900, Datex-Ohmeda, Madison, WI) to previous Ohmeda (7810 and 7000, Datex-Ohmeda, Madison, WI) anesthesia ventilators. To determine airway pressure and minute ventilation thresholds for intraoperative use of a critical care ventilator. DESIGN: Three anesthesia ventilators and one critical care ventilator (Siemens Servo 900C, Siemens, Solna, Sweden) were studied in a lung model. Retrospective medical record review. SETTING: Research Laboratory and Critical Care Unit of a Level I Trauma Center. PATIENTS: 145 mechanically ventilated patients treated for acute respiratory failure who underwent 200 surgical procedures. INTERVENTIONS: The effect of increasing pressure on mean inspiratory flow was determined by cycling each ventilator through increasing restrictors. Maximum minute ventilation was measured at low compliance (10-30 mL/cm H2O), positive end-expiratory pressure (PEEP) (0-20 cm H2O), and increased airway resistance (approximately 19 and approximately 36 cm H2O/L/sec) in a mechanical lung model. MEASUREMENTS AND MAIN RESULTS: Flow, volume, and pressure were measured with a pulmonary mechanics monitor (BICORE CP-100, Thermo Respiratory Group, Yorba Linda, CA). Preoperative peak airway pressure and minute ventilation (VE) were extracted from the medical record. Mean inspiratory flow declined with increasing pressure in all anesthesia ventilators. The SmartVent and the 7810 produced greater mean inspiratory flow than did the 7000 ventilator. As compliance progressively decreased, the Siemens, the SmartVent, and the 7810 ventilators maintained VE compared to the 7000 ventilator. The Siemens and the SmartVent maintained VE with PEEP, compared to the 7810 and 7000 ventilators. During increased airway resistance, maximal VE was lower for all ventilators. The SmartVent met the ventilation requirements in 90% of the patients compared to 67% of patients with the 7000 ventilator. CONCLUSION: The improved pressure and flow capabilities of the SmartVent increase the threshold for using a critical care ventilator intraoperatively to a peak airway pressure > 65 cm H2O and/or VE > 18 L/min.     

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 H(2)O) 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 (r(2) = 0.86 +/- 0.17), consistent with Starling resistor behavior. At end-tidal isoflurane of 1.2%, critical pressure was 1.1 +/- 3.5 cm H O; at 0.4% it decreased to -0.2 +/- 3.6 cm H(2)O ( < 0.05), indicating decreased airway collapsibility. This decrease was associated with a decrease in end-expiratory esophageal pressure of 0.6 +/- 0.9 cm H(2)O ( < 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. CONCLUSIONS: Isoflurane anesthesia is associated with decreased muscle activity and increased collapsibility of the upper airway. In this state it adopts the behavior of a Starling resistor. The decreased collapsibility observed with decreasing anesthetic depth was not a consequence of neuromuscular activity, which was unchanged. Rather, it may be related to increased lung volume and its effect on airway wall longitudinal tension. The predominant site of collapse is the soft palate.     

Influence of pneumoperitoneum and patient positioning on respiratory system compliance

STUDY OBJECTIVE: To investigate the influence of pneumoperitoneum (PP) and posture on respiratory compliance and ventilation pressures. DESIGN: Prospective, single blind trial. PATIENTS: 10 female ASA physical status I and II patients scheduled for elective gynecologic laparoscopy. SETTING: University medical center. INTERVENTIONS: Anesthesia was performed as total IV anesthesia (TIVA) with propofol, alfentanil, and atracurium. After induction of anesthesia and orotracheal intubation, the lungs were ventilated to maintain partial pressure of CO(2) (P(ET)CO(2)) of 30 +/- 3 mmHg. Ventilation was kept constant. As gas mixture oxygen and air 1:1 was used without positive end-expiratory pressure (PEEP). MEASUREMENTS: Measurements were taken before and after creation of pneumoperitoneum with an intraabdominal pressure (IAP) of 10 mmHg, of 15 mmHg in 20 degrees head-down tilt, then in 20 degrees head-up tilt, and after deflation of PP. We determined peak inspiratory pressure (PIP), mean airway pressure (mPaw), P(ET)CO(2), expiratory minute volume (V(E)), heart rate (HR), and systolic (SBP), diastolic (DBP), and mean arterial pressure (MAP). Respiratory system compliance (C(eff rs)) was calculated as quotient of tidal volume (V(T)) and PIP. MAIN RESULTS: After creation of PP (IAP 10 mmHg), there was a significant increase of median PIP (3 cmH(2)O), mPaw (1 cm H(2)O) and arterial pressure (BP), (MAP by 7 mmHg), C(eff rs) decreased by 6 mL. cm H(2)O(-1). Increase of IAP to 15 mmHg led to a further increase of PIP (2 cm H(2)O) and mPaw (1 cm H(2)O), and a further decrease of C(eff rs) by 5 mL cm H(2)O(-1); BP decreased (MAP by 5.5 mmHg). Head-up or head down positions showed no significant hemodynamic or pulmonary changes. P(ET)CO(2)increased from 29.5 to 36 mmHg at an IAP of 15 mmHg, but then no further changes were noticed. Five minutes after deflation of pneumoperitoneum all values returned to baseline levels. CONCLUSIONS: Creation of PP at an IAP of 15 mmHg reduced respiratory system compliance, and increased peak inspiratory and mean airway pressures, which quickly returned to normal values after deflation. Head-down or head-up position did not further alter those parameters.     

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.     

Pressure and flow limitations of anesthesia ventilators

The effect of increasing airway pressure on the mean inspiratory flow and maximum minute ventilation (VE) capabilities of five anesthesia ventilators (Ohio Anesthesia, Airshields Ventimeter, Ohmeda 7000, Draeger AV-E and Siemens 900D) was compared to identify mechanical factor(s) limiting intraoperative ventilation of the lungs of patients with acute respiratory failure. The effect of increasing airway pressure on mean inspiratory flow was determined by cycling each ventilator through increasing restrictors. Maximum VE was measured under three study conditions using a test lung: 1) low compliance (10-30 ml/cmH2O) and minimal airflow resistance; 2) positive end-expiratory pressure (PEEP) of 0, 10, and 20 cmH2O at a compliance of 20 ml/cmH2O with minimal airflow resistance; and 3) increased resistance (19 +/- 11 cmH2O.1(-1).s-1) and compliance of 30 ml/cmH2O. As airway pressure increased from 0 to 80 cmH2O, mean inspiratory flow decreased markedly for all ventilators except the Siemens. The Siemens ventilator delivered the greatest VE under all three conditions and maintained VE when airway pressure increased due to decreased compliance or the application of PEEP; all other ventilators markedly decreased VE under these conditions. The addition of airway resistance reduced maximal VE for all ventilators by limiting the maximal inspiratory duty cycle (T1/TTOT). Thus, mean inspiratory flow of conventional anesthesia ventilators decreases with increasing airway pressure. The decreased inspiratory flow limits maximum VE when airway pressure is elevated because of decreased lung-thorax compliance and/or increased airway resistance, such as that characterizing patients with acute respiratory failure. Significant airway resistance further limits maximum VE by limiting the maximal T1/TTOT that can be used without increasing end-expiratory lung pressure.(ABSTRACT TRUNCATED AT 250 WORDS)     

Respiratory mechanics during xenon anesthesia in pigs: comparison with nitrous oxide.

BACKGROUND: Because of its high density and viscosity, xenon (Xe) may influence respiratory mechanics when used as an inhaled anesthetic. Therefore the authors studied respiratory mechanics during xenon and nitrous oxide (N2O) anesthesia before and during methacholine-induced bronchoconstriction. METHODS: Sixteen pentobarbital-anesthetized pigs initially were ventilated with 70% nitrogen-oxygen. Then they were randomly assigned to a test period of ventilation with either 70% xenon-oxygen or 70% N2O-oxygen (n = 8 for each group). Nitrogen-oxygen ventilation was then resumed. Tidal volume and inspiratory flow rate were set equally throughout the study. During each condition the authors measured peak and mean airway pressure (Pmax and Pmean) and airway resistance (R(aw)) by the end-inspiratory occlusion technique. This sequence was then repeated during a methacholine infusion. RESULTS: Both before and during methacholine airway resistance was significantly higher with xenon-oxygen (4.0 +/- 1.7 and 10.9 +/- 3.8 cm H2O x s(-1) x 1(-1), mean +/- SD) when compared to nitrogen-oxygen (2.6 +/- 1.1 and 5.8 +/- 1.4 cm H2O x s(-1) x l(-1), P < 0.01) and N2O-oxygen (2.9 +/- 0.8 and 7.0 +/- 1.9, P < 0.01). Pmax and Pmean did not differ before bronchoconstriction, regardless of the inspired gas mixture. During bronchoconstriction Pmax and Pmean both were significantly higher with xenon-oxygen (Pmax, 33.1 +/- 5.5 and Pmean, 11.9 +/- 1.6 cm H2O) when compared to N2O-oxygen (28.4 +/- 5.7 and 9.5 +/- 1.6 cm H2O, P < 0.01) and nitrogen-oxygen (28.0 +/- 4.4 and 10.6 +/- 1.3 cm H2O, P < 0.01). CONCLUSIONS: Airway pressure and resistance are increased during xenon anesthesia. This response is moderate and not likely to assume major importance for the general use of xenon in anesthesia.     

Isoflurane inhalation enhances increased physiologic deadspace volume associated with positive pressure ventilation and compromises arterial oxygenation

Abnormally increased physiologic deadspace volume (Vd(phys)), consisting of alveolar deadspace volume and airway deadspace volume, is one of several causative factors predisposing to compromised arterial blood gas exchange. We compared the effects of two methods of general anesthesia on Vd(phys) when combined with positive pressure ventilation (PPV): total IV anesthesia (TIVA) and inhaled anesthesia with isoflurane. Forty patients with no history of pulmonary pathology undergoing elective surgery in the supine position were studied. A crossover design was used, and all patients received both anesthetic methods sequentially in randomized order. PPV and TIVA significantly increased Vd(phys) compared with baseline (preoperative and breathing spontaneously) from 164 +/- 60 mL to 264 +/- 79 mL (P < 0.05). Isoflurane inhalation combined with PPV significantly enhanced this increase, resulting in a twofold increase in Vd(phys) to 315 +/- 80 mL (P < 0.05). Also, alveolar deadspace volume increased by more than 200% with isoflurane. Furthermore, isoflurane inhalation (1.15% end-tidal concentration) resulted in impaired arterial oxygenation, as evidenced by a significant decrease in the Pao(2)/fractional inspired oxygen concentration ratio compared with baseline values from 387 +/- 35 to 310 +/- 70 (P < 0.05). Although significant increases in Vd(phys) resulted with PPV combined with TIVA, these adverse changes were much less compared with isoflurane inhalation and PPV. These findings may apply to subjects with compromised pulmonary function (i.e., acute respiratory distress syndrome or severe inhalational burn injury).     

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 at different concentrations of propofol anesthesia

BACKGROUND: This study investigated the effect of varying concentrations of propofol on upper airway collapsibility and the mechanisms responsible for it. METHODS: Upper airway collapsibility was determined from pressure-flow relations at three concentrations of propofol anesthesia (effect site concentration = 2.5, 4.0, and 6.0 mug/ml) in 12 subjects spontaneously breathing on continuous positive airway pressure. At each level of anesthesia, mask pressure was transiently reduced from a pressure sufficient to abolish inspiratory flow limitation (maintenance pressure = 12 +/- 1 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. Electromyographic activity of the genioglossus muscle (EMGgg) was obtained via intramuscular electrodes in 8 subjects. RESULTS: With increasing depth of anesthesia, (1) critical closing pressure progressively increased (-0.3 +/- 3.5, 0.5 +/- 3.7, and 1.4 +/- 3.5 cm H2O at propofol concentrations of 2.5, 4.0, and 6.0 microg/ml respectively; P < 0.05 between each level), indicating a more collapsible upper airway; (2) inspiratory flow at the maintenance pressure significantly decreased; and (3) respiration-related phasic changes in EMGgg at the maintenance pressure decreased from 7.3 +/- 9.9% of maximum at 2.5 microg/ml to 0.8 +/- 0.5% of maximum at 6.0 microg/ml, whereas tonic EMGgg was unchanged. Relative to the levels of phasic and tonic EMGgg at the maintenance pressure immediately before a decrease in mask pressure, tonic activity tended to increase over the course of five flow-limited breaths at a propofol concentration of 2.5 microg/ml but not at propofol concentrations of 4.0 and 6.0 microg/ml, whereas phasic EMGgg was unchanged. CONCLUSIONS: Increasing depth of propofol anesthesia is associated with increased collapsibility of the upper airway. This was associated with profound inhibition of genioglossus muscle activity. This dose-related inhibition seems to be the combined result of depression of central respiratory output to upper airway dilator muscles and of upper airway reflexes.     

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.     

Pressure-controlled inverse ratio ventilation after cardiac surgery.

BACKGROUND AND OBJECTIVE: Pressure-controlled inverse ratio ventilation was compared with controlled mechanical ventilation in patients after cardiac surgery. METHODS: Ten patients were ventilated after sternal closure using a Siemens Servo 900C ventilator to a target end-tidal PCO2 of 4.0 kPa. They were randomized to receive controlled mechanical ventilation or pressure-controlled inverse ratio ventilation. CO2-based data were recorded on a laptop personal computer, which together with arterial PCO2 permitted measurement of the respiratory dead space. Once measurements were complete the ventilator was switched to the other mode and new measurements taken. RESULTS: PaCO2 and VCO2 were virtually the same in both modes. Peak airway pressure (17.2 +/- 2.7 vs. 20.8 +/- 2.5 cmH2O, P < 0.01) and minute ventilation (4.9 +/- 1.1 vs. 5.3 +/- 1.1 cmH2O, P < 0.01) were less during pressure-controlled inverse ratio ventilation. Physiological dead space fraction (0.39 +/- 0.06 vs. 0.51 +/- 0.05, P < 0.001), airway dead space (56 +/- 15 vs. 81 +/- 15 mL, P < 0.001) and alveolar dead space fraction (0.25 +/- 0.07 vs. 0.31 +/- 0.09, P < 0.01) were all less during pressure-controlled inverse ratio ventilation. There were no differences in heart rate or mean arterial pressure. CONCLUSIONS: The prolonged inspiratory period and pressure-controlled flow pattern of pressure-controlled inverse ratio ventilation reduce the alveolar and airway dead spaces, and give lower peak airway pressures, compared with conventional ventilation, in cardiac surgical patients.     

Use of the cuffed oropharyngeal airway as an alternative to the laryngeal mask airway with positive-pressure ventilation.

BACKGROUND: The cuffed oropharyngeal airway is a modified Guedel-type oral airway with a cuff at its distal end. The objectives of this study were to compare the ability of the cuffed oropharyngeal airway and the laryngeal mask airway to provide positive-pressure ventilation during general anesthesia, and to assess their relative ease of use and ability to reduce total fresh gas flow rates. METHODS: In this prospective, randomized study, a cuffed oropharyngeal airway (n = 25) or a laryngeal mask airway (n = 25) device was inserted after induction of anesthesia intravenously using 2 mg/kg propofol. While anesthesia was maintained with sevoflurane and nitrous oxide, the leak pressure, leak fraction (the fractional difference between the inspired and expired tidal volume), minimum fresh gas flow rate, and need for airway manipulations were determined. The anesthesia provider who inserted the device completed an evaluation form at the end of the 15-min study period. RESULTS: Positive-pressure ventilation was established successfully on the first attempt in 92% of the patients when the cuffed oropharyngeal airway was used and in 88% of the patients when the laryngeal mask airway device was used. However, manipulations of the airway device were necessary more frequently (8 vs. 1 patient; P < 0.05) and the leak pressure was less (22 +/- 6 cm water vs. 26 +/- 5 cm water; P < 0.05) with the cuffed oropharyngeal airway than with the laryngeal mask airway. In addition, the leak fraction (0.19 +/- 0.18 vs. 0.31 +/- 0.22; P < 0.05) and the minimum fresh gas flow rate (1.3 +/- 1.5 vs. 2.4 +/- 2.5; P = 0.12) were less in the laryngeal mask airway group. CONCLUSIONS: Positive-pressure ventilation is possible with the laryngeal mask airway and cuffed oropharyngeal airway devices. Although the cuffed oropharyngeal airway can be inserted easily by inexperienced users with a high first-attempt success rate (> 90%), manipulations of the device may be required to maintain a patent airway. The laryngeal mask airway device allows positive-pressure ventilation at slightly greater peak inspiratory pressures.     

The effects of thoracic epidural analgesia with bupivacaine 0.25% on ventilatory mechanics in patients with severe chronic obstructive pulmonary disease

Optimal analgesia is important after thoracotomy in pulmonary-limited patients to avoid pain-related pulmonary complications. Thoracic epidural anesthesia (TEA) can provide excellent pain relief. However, potential paralysis of respiratory muscles and changes in bronchial tone might be unfavorable in patients with end-stage chronic obstructive pulmonary disease (COPD). Therefore, we evaluated the effect of TEA on maximal inspiratory pressure, pattern of breathing, ventilatory mechanics, and gas exchange in 12 end-stage COPD patients. Pulmonary resistance, work of breathing, dynamic intrinsic positive end-expiratory pressure, and peak inspiratory and expiratory flow rates were evaluated by assessing esophageal pressure and airflow. An increase in minute ventilation (7.50 +/- 2.60 vs 8.70 +/- 2.10 L/min; P = 0.04) by means of increased tidal volume (0.46 +/- 0.16 vs 0.53 +/- 0.14 L/breath; P = 0.003) was detected after TEA. These changes were accompanied by an increase in peak inspiratory flow rate (0.48 +/- 0.17 vs 0.55 +/- 0.14 L/s; P = 0.02) and a decrease in pulmonary resistance (20.7 +/- 9.9 vs 16.6 +/- 8.1 cm H(2)O. L(-1). s(-1); P = 0.02). Peak expiratory flow rate, dynamic intrinsic positive end-expiratory pressure, work of breathing, PaO(2), and maximal inspiratory pressure were unchanged (all P > 0.50). We conclude that TEA with bupivacaine 0.25% can be used safely in end-stage COPD patients. IMPLICATIONS: Thoracic epidural anesthesia with bupivacaine 0.25% does not impair ventilatory mechanics and inspiratory respiratory muscle strength in severely limited chronic obstructive pulmonary disease patients. Thus, thoracic epidural anesthesia can be used safely in patients with end-stage chronic obstructive pulmonary disease.     

Respiratory Mechanics during Xenon Anesthesia in Pigs: Comparison with Nitrous Oxide

Background: Because of its high density and viscosity, xenon (Xe) may influence respiratory mechanics when used as an inhaled anesthetic. Therefore the authors studied respiratory mechanics during xenon and nitrous oxide (N2O) anesthesia before and during methacholine-induced bronchoconstriction.

Methods: Sixteen pentobarbital-anesthetized pigs initially were ventilated with 70% nitrogen-oxygen. Then they were randomly assigned to a test period of ventilation with either 70% xenon-oxygen or 70% N2O-oxygen (n = 8 for each group). Nitrogen-oxygen ventilation was then resumed. Tidal volume and inspiratory flow rate were set equally throughout the study. During each condition the authors measured peak and mean airway pressure (Pmax and Pmean) and airway resistance (Raw) by the end-inspiratory occlusion technique. This sequence was then repeated during a methacholine infusion.

Results: Both before and during methacholine airway resistance was significantly higher with xenon-oxygen (4.0 +/- 1.7 and 10.9 +/- 3.8 cm H2O [middle dot] s-1 [middle dot] l-1, mean +/- SD) when compared to nitrogen-oxygen (2.6 +/- 1.1 and 5.8 +/- 1.4 cm H2O [middle dot] s-1 [middle dot] l-1, P < 0.01) and N2O-oxygen (2.9 +/- 0.8 and 7.0 +/- 1.9, P < 0.01). Pmax and Pmean did not differ before bronchoconstriction, regardless of the inspired gas mixture. During bronchoconstriction Pmax and Pmean both were significantly higher with xenon-oxygen (Pmax, 33.1 +/- 5.5 and Pmean, 11.9 +/- 1.6 cm H2O) when compared to N2O-oxygen (28.4 +/- 5.7 and 9.5 +/- 1.6 cm H2O, P < 0.01) and nitrogen-oxygen (28.0 +/- 4.4 and 10.6 +/- 1.3 cm H2O, P < 0.01).     

Comparative analysis of the hemodynamic and respiratory parameters during laparoscopic versus open living donor nephrectomy: an experimental model

Increased intrabdominal pressure induced by pneumoperitoneum induces modifications in cardiovascular and respiratory systems. The aim of the study was to analyze the hemodynamic and respiratory modifications produced by pneumoperitoneum during living donor nephrectomy in a porcine experimental model. Twenty pigs underwent left nephrectomy, 10 by laparoscopy and 10 by an open approach. The following parameters were measured: mean arterial pressure (MAP), central venous pressure, cardiac output (CO), systemic vascular resistance (SVR), end tidal CO2 (ETCO2), minute volume (MV), respiratory airway pressure (RAP), and "compliance." Both groups were monitored for cardiac and respiratory systems at basal, 5, 30, and 60 minutes as well as postsurgery. The comparative analysis demonstrated increased CO with a higher difference at 30 minutes (4.33 +/- 0.73 vs 8.54 +/- 1.26 L/min, P < .001); decreased SVR (1118.81 +/- 302.52 vs 663.37 +/- 81.45 dinas x s x cm(-5), P < .001), and elevated MAP among the laparoscopic group (66.5 +/- 11.52 vs 80.25 +/- 2.49 mm Hg, P = .004). Analysis of respiratory modifications showed an initial increase in ETCO2 (44.3 +/- 2.6 vs 54.1 +/- 12.56 mm Hg, P < .035) and a higher MV administered (5.6 +/- 0.1 vs 7.01 +/- 0.96 L/min, P = .03) to the laparoscopy group. An increased RAP was observed at 5 minutes (22.11 +/- 2.76 vs 28.8 +/- 3.68 mm Hg, P < .001), in the laparoscopic group and lower levels of "compliance" at the same moment in that group (16 +/- 1.66 vs 14.9 +/- 4.07 cm H2O). Laparoscopic nephrectomy caused an increase in CO and MAP and decreased SVR. Likewise there were elevations of RAP, ETCO2, and MV and a slight decrease in the "compliance."     

Gastric distension and ventilation during laparoscopic cholecystectomy: LMA-classic vs. tracheal intubation

PURPOSE: The standard laryngeal mask airway LMA-Classic was designed as an alternative to the endotracheal tube (ETT) or the face mask for use with either spontaneous or positive pressure ventilation. Positive pressure ventilation may exploit leaks around the LMA cuff, leading to gastric distension and/or inadequate ventilation. We compared gastric distension and ventilation parameters with LMA vs ETT during laparoscopic cholecystectomy. METHODS: One hundred and one, ASA I-II adults scheduled for elective laparoscopic cholecystectomy were randomly assigned to LMA-Classic or ETT. Patients with BMI >30 kg x m(-2), hiatus hernia or gastroesophageal reflux were excluded. Following induction of anesthesia, an in-and-out orogastric tube was passed to decompress the stomach before insertion of the LMA (women size #4, men size #5) or ETT (women 7 mm, men 8 mm). Anesthesia was maintained with isoflurane in nitrous oxide and oxygen (FIO2 0.3-0.5), rocuronium and fentanyl. The surgeon, blinded to the type of airway, scored gastric distention 0-10 at insertion of the laparoscope and immediately before removal at the end of the surgical procedure. RESULTS: Incidence and degree of change in gastric distension were similar in both groups. Ventilation parameters during insufflation (mean +/- SD) for LMA and ETT were: S(P)O2 98 +/- I vs 98 +/- I, P(ET)CO2 38 +/- 4 vs 36 +/- 4 mm Hg and airway pressure 21 +/- 4 vs 23 +/- 3 cm water. CONCLUSION: Positive pressure ventilation with a correctly placed LMA-Classic of appropriate size permits adequate pulmonary ventilation. Gastric distension occurs with equal frequency with either airway device.     

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.