Continuous positive airway pressure facilitates spontaneous breathing in weaning chronic obstructive pulmonary disease patients by improving breathing pattern and gas exchange

OBJECTIVE: To elucidate the effects of continuous positive airway pressure (CPAP) on breathing pattern, gas exchange and the ability to sustain spontaneous breathing (SB) in chronic obstructive pulmonary disease (COPD) patients with dynamic hyperinflation. DESIGN: Prospective study with two randomised trials of SB without and with CPAP in each patient. SETTING: Medical intensive care units (ICUs) in two university hospitals. PATIENTS: Nine dynamically hyperinflated, intubated COPD patients recuperating from acute exacerbation. INTERVENTIONS: One SB trial with CPAP (5-7.5 cmH2O), one without (control) in each patient. MEASUREMENTS: airway opening pressure, gas flow and thus breathing pattern, oxygen uptake, carbon dioxide excretion, arterial blood gases, dyspnoea and respiratory drive (P100). RESULTS: With CPAP, intrinsic positive end-expiratory pressure (PEEPi) fell from 11.4 to 6.3 cm H2O (p < 0.05). Eight patients sustained SB with CPAP for the maximum time planned (30 min), one failed after 18 min. In contrast, only four patients successfully completed the control trial, the others failing after 5-18 min (p < 0.05). Dyspnoea-gauged on a visual analogue scale by five patients--was less severe or occurred later with CPAP. Breathing with CPAP tended to be slower (18.9 vs 22.2 min(-1), p < 0.05) and deeper (tidal volume 370 vs 323 ml). At the end of the control run, PaCO2 was higher (60 vs 55 mmHg, p < 0.05) and still rising while being stable at the end of the CPAP trial. CONCLUSION: CPAP helps severely ill COPD patients sustain SB. Apparently it does so by promoting slower, deeper breathing and thus facilitating carbon dioxide elimination.     

Extubation outcome following a spontaneous breathing trial with automatic tube compensation versus continuous positive airway pressure

OBJECTIVE: We hypothesized that the additional use of automatic tube compensation (ATC) during a spontaneous breathing trial with continuous positive airway pressure (CPAP), by minimizing respiratory work, would result in more patients undergoing successful extubation. DESIGN: Prospective, randomized, controlled study. SETTING: A ten-bed, general intensive care department at a tertiary-care hospital. PATIENTS: Adult patients (n=99) who had undergone mechanical ventilation for >24 hrs and met defined criteria for a weaning trial. INTERVENTIONS: Patients were randomized to undergo a 1-hr spontaneous breathing trial with either ATC with CPAP (ATC group, n=51) or CPAP alone (CPAP group, n=48). ATC was provided by commercially available mechanical ventilators. Patients tolerating the spontaneous breathing trial underwent immediate extubation. The primary outcome measure was successful extubation, defined as the ability to maintain spontaneous breathing for 48 hrs after discontinuation of mechanical ventilation and extubation. MEASUREMENTS AND MAIN RESULTS: There were no significant differences in demographic, respiratory, or hemodynamic characteristics between the two groups at the start of the spontaneous breathing trial. There was a trend for more patients in the ATC group to tolerate the breathing trial and undergo extubation (96% vs. 85%; p=.08). The rate of reintubation was 14% in the ATC group and 24% in the CPAP group (p=.28). Significantly more patients in the ATC group thus met the criteria for successful extubation (82% vs. 65%; p=0.04). CONCLUSION: This is the largest single-center study to date assessing the use of commercially available ATC and suggests that this might be a useful mode for performing a spontaneous breathing trial preceding extubation in a general intensive care population.     

Gastric intramucosal pH and intraluminal PCO2 during weaning from mechanical ventilation

OBJECTIVE: To study the value of gastric intramucosal pH and gastric intraluminal PCO2 measurements to predict weaning outcome from mechanical ventilation. DESIGN: Prospective clinical study. SETTING: Intensive care medicine department of a university hospital. PATIENTS: Nineteen adult critically ill patients who were mechanically ventilated because of acute respiratory failure and were considered ready to be weaned. INTERVENTIONS: The patients were weaned with: synchronized intermittent mandatory ventilation plus positive end-expiratory pressure (SIMV+PEEP) or continuous positive airway pressure with pressure support ventilation (CPAP+PSV). A gastric tonometer was placed in all the patients. Tonometric, respiratory, and hemodynamic variables were measured during the weaning process. MEASUREMENTS: Hemodynamic variables, respiratory mechanics, pulmonary gas exchange, respiratory muscle force, spontaneous pattern of breathing, and the central control of breathing were recorded. Simultaneously, the intramucosal pH and gastric intraluminal PCO2 were measured. MAIN RESULTS: Eleven patients were successfully extubated and eight failed. The patients who failed showed higher values of mouth occlusion pressure, respiratory rate, and effective inspiratory impedance (mouth occlusion pressure/mean inspiratory flow). The intramucosal pH was initially 7.19 +/- 0.22 and decreased to 7.10 +/- 0.16 during the weaning process in patients who failed (p < .05). At the same time, the intramucosal pH showed a nonsignificant change from 7.36 +/- 0.07 to 7.32 +/- 0.07 in the patients who were successfully extubated. The intramucosal pH was statistically different when both groups were compared during the initial and the final evaluations (p < .05). For the initial evaluation, the sensitivity and specificity to predict weaning failure when the intramucosal pH was < or =7.30 were 0.88 (95% confidence interval [CI], 0.66-1) and 0.82 (95% CI, 0.59-1), respectively. The gastric intraluminal PCO2 was higher in patients who failed (p < .05). When gastric intraluminal PCO2 was . or =40 torr during the initial evaluation, weaning failure occurred with a sensitivity of 1 (95% CI, 0.31-1) and a specificity of 0.55 (95% CI, 0.26-0.84). CONCLUSIONS: Weaning failure was associated with gastric intramucosal acidosis. The intramucosal pH and gastric intraluminal PCO2 may be helpful to predict weaning outcome. Further controlled clinical trials in a larger group of patients are needed.     

Differences in respiratory parameters during continuous positive airway pressure and pressure support ventilation in infants and children

The extubation criteria of pressure support ventilation (PSV) in infants and children were not yet established. We studied the differences in respiratory parameters during continuous positive airway pressure (CPAP) using a constant flow type ventilator and PSV using a demand valve type ventilator. Nineteen children (1.9+/-2.9 years old) who were ready to extubate were studied. All patients had recovered from their respiratory failure and had finished the weaning process of the ventilatory support. They were scheduled for extubation on the next day when their ventilatory mode had attained to a PSV of 3 cmH2O with a positive end-expiratory pressure (PEEP) of 3 cmH2O. On the extubation day, tidal volume (TV) and respiratory frequency (RR) were measured with a respiratory monitor at two modes (CPAP of 3 cmH2O and PSV), and the duty ratio (DR) and mean inspiratory flow (MF) were calculated. The sequence of the ventilatory mode was random. No case required reintubation. TV was 61.6+/-54.9 during CPAP and 67.7+/-61.4 ml during PSV, and RR was 38.5+/-10.6 and 37.1+/-8.8 beats/min., respectively. DR was 0.382+/-0.067 and 0.359+/-0.085, and MF was 96.6+/-78.3 and 101.0+/-69.0 ml/sec., respectively. The measured parameters and calculated values showed no significant difference between CPAP and PSV. It was found that the respiratory parameters were almost the same with CPAP and PSV immediately before the extubation, and the previous extubation criteria of CPAP can be used.     

Flow-triggering reduces inspiratory effort during weaning from mechanical ventilation

Objective To investigate whether a new flow-triggered (FT) system can reduce the patient's inspiratory effort compared to a traditional pressure-triggered (PT) system during weaning from mechanical ventilation.Design Prospective study.Setting Intensive care unit of a General Hospital.Patients and participants 10 mechanically ventilated patients, without chronic airway disease, ready to wean.Measurements Minute ventilation, breathing pattern, lung mechanics, inspiratory work of breathing (WI) and pressure time product (PTP) of Ppl were obtained in two conditions: 1) unsupported spontaneous breathing through the ventilator circuit (SB); 2) spontaneous breathing with continuous positive airway pressure set at 5 cmH₂O (CPAP). Two triggering systems, namely PT and FT, were used in each condition.Results Though there was no change in breathing pattern, minute ventilation, and lung mechanics, the magnitude of the inspiratory effort decreased significantly with FT compared to PT in both instances. The added resistance (total flow resistance minus pulmonary resistance) decreased by 37% on average when FT replaced PT. PTP decreased, on average, 27% and 15% during SB and CPAP, respectively, with FT compared to PT (p<0.05). A similar significant decrease was observed in WI.Conclusion The new FT system, i.e. flow-by system, reduces the unintentional ventilatory workload upon the patients' inspiratory muscles compared to traditional PT system during weaning from mechanical ventilation.     

Effects of patient-triggered automatic switching between mandatory and supported ventilation in the postoperative weaning period

OBJECTIVE: To compare two ventilator settings in the postoperative weaning period. Patient-triggered automatic switching between controlled ventilation and supported spontaneous breathing (Automode, AM) was compared to synchronised intermittent mandatory ventilation (SIMV) with stepwise manual adjustment of mandatory frequency according to the breathing activity. DESIGN: Prospective clinical investigation. SETTING: Eighteen-bed intensive care unit in a university hospital. PATIENTS: Forty postoperative patients with healthy lungs who had undergone brain tumour surgery. INTERVENTIONS: Randomisation either to the AM or SIMV weaning procedure after entering the ICU. MEASUREMENTS AND RESULTS: Total weaning time and number of manipulations on the ventilator were observed. Cardiocirculatory and respiratory parameters were measured consecutively at five points during the weaning period. No significant differences were seen for cardiocirculatory parameters, airway pressures and oxygenation between the two groups. There was a trend to shorter weaning times with AM (136 +/- 46 min vs 169 +/- 68 min, n.s.), the average number of manipulations on the ventilator was lower (0.55 +/- 0.69 vs 5.05 +/- 1.19,p < 0.001) and arterial partial pressure of carbon dioxide (PaCO2) levels showed fewer variations in the late phase of the weaning period (39.5 +/- 3.1 vs 38.3 +/- 7.2, p < 0.001 for differences in variance). CONCLUSIONS: Automatic, patient-triggered switching between controlled and supported mode of ventilation can be used for postoperative weaning of neurosurgical patients with healthy lungs. Compared to a SIMV weaning procedure, fewer manipulations on the ventilator are necessary and individual adaptation of ventilation seems to be more accurate.     

Hypercapnia test as a predictor of success in spontaneous breathing trials and extubation

BACKGROUND: The ventilatory capacity of the respiratory neuromuscular system can be studied with the hypercapnia test. OBJECTIVE: To determine whether decreased response to the hypercapnia test is associated with failure to pass a spontaneous breathing trial (SBT) or extubation failure. METHODS: We studied 103 intubated patients ready for SBT. We used a hypercapnia test in which we approximately doubled the dead space and thus caused re-inhalation of expired air. We calculated 3 ratios: the ratio of P(0.1) (airway occlusion pressure 0.1 s after the onset of inspiratory effort) during hypercapnia test to baseline P(0.1); the ratio of the change in minute volume [DeltaV(E)] to the change in P(aCO(2)) (we call this ratio the hypercapnic ventilatory response); and the ratio of the change in P(0.1) [DeltaP(0.1)] to the change P(aCO(2)) (we call this ratio the hypercapnic-respiratory-drive response). RESULTS: Thirty-six patients failed the SBT, and 11 patients failed extubation. The mean values for the SBT/extubation-success group, the extubation-failure group, and the SBT-failure group, respectively, were: ratio of hypercapnia-test P(0.1) to baseline P(0.1): 4.3 +/- 2.7, 3.7 +/- 1.3, and 3.0 +/- 1.8 (P = .03); hypercapnic ventilatory response: 0.60 +/- 0.35 L/min/mm Hg, 0.50 +/- 0.26 L/min/mm Hg, and 0.31 +/- 0.21 L/min/mm Hg (P < .001); hypercapnic respiratory-drive response: 0.48 +/- 0.24 cm H(2)O/mm Hg, 0.42 +/- 0.19 cm H(2)O/mm Hg, and 0.27 +/- 0.15 cm H(2)O/mm Hg (P < .001). For predicting SBT/extubation success, the sensitivities and specificities, respectively, were: ratio of hypercapnia-test P(0.1) to baseline P(0.1) 0.80 and 0.47; hypercapnic ventilatory response 0.86 and 0.53; hypercapnic respiratory-drive response 0.82 and 0.55. CONCLUSIONS: The SBT/extubation-failure patients had less response to the hypercapnia test than did the SBT/extubation-success patients, and the hypercapnia test was not useful in predicting SBT or extubation success.     

Weaning from mechanical ventilation

Practice guidelines on weaning should be based on the results of several well-designed randomized studies performed over the last decade. One of those studies demonstrated that immediate extubation after successful trials of spontaneous breathing expedites weaning and reduces the duration of mechanical ventilation as compared with a more gradual discontinuation of ventilatory support. Two other studies showed that the ability to breathe spontaneously can be adequately tested by performing a trial with either T-tube or pressure support of 7 cmH₂O lasting either 30 or 120 min. In patients with unsuccessful weaning trials, a gradual withdrawal for mechanical ventilation can be attempted while factors responsible for the ventilatory dependence are corrected. Two randomized studies found that, in difficult-to-wean patients, synchronized intermittent mandatory ventilation (SIMV) is the most effective method of weaning.     

Comparison of the oxygen cost of breathing exercises and spontaneous breathing in patients with stable chronic obstructive pulmonary disease

BACKGROUND AND PURPOSE: The oxygen demand of breathing exercises and the clinical implications have not been studied in detail. In this study, the oxygen cost of 3 common breathing exercises believed to reduce oxygen cost (ie, work of breathing) was compared with that of spontaneous breathing in patients with chronic obstructive pulmonary disease (COPD). SUBJECTS: Thirty subjects with stable, moderately severe COPD participated. METHODS: Oxygen consumption (VO2) and respiratory rate (RR) during spontaneous breathing at rest (SB) were recorded for 10 minutes. Subjects then performed 3 breathing exercises in random order, with a rest between exercises: diaphragmatic breathing (DB), pursed-lip breathing (PLB), and a combination of DB and PLB (CB). Oxygen consumption and RR were measured. RESULTS: Mean VO2 ( SD) was lower during the breathing exercises (165.8 +/- 22.3 mL O2/min for DB, 164.8 +/- 20.9 mL O2/min for PLB, and 167.7 +/- 20.7 mL O2/min for CB) compared with SB (174.5 +/- 25.2 mL O2/min). Correspondingly, mean RR (+/- SD) was higher during SB (17.3 +/- 4.23 breaths/min), followed by DB (15.0 +/- 4.32 breaths/min), PLB (12.8 +/- 3.53 breaths/min), and CB (11.2 +/- 2.7 breaths/min). DISCUSSION AND CONCLUSION: Given that patients do not spontaneously adopt the breathing pattern with the least O2 and the lowest RR, the results suggest that determinants of the breathing pattern other than metabolic demand warrant being a primary focus in patients with COPD.     

Pulse and Systolic Pressure Variation Assessment in Partially Assisted Ventilatory Support

OBJECTIVE: The use of pulse pressure variation (PPV) and systolic pressure variation (SPV) is possible during controlled ventilation (MV). Even in acute respiratory failure, controlled MV tends to be replaced by assisted ventilatory support. We tested if PPV and SPV during flow triggered synchronized intermittent mechanical ventilation (SIMV) could be as accurate as in controlled MV. METHODS: Prospective case-controlled study. Thirty patients who met criteria of weaning from controlled MV. Twenty minutes pressure support ventilation with 3 min(-1) flow triggered SIMV breathes (10 ml kg(-1)) T1, then three consecutive breaths in controlled MV (respiratory rate 12 min(-1),10 ml kg(-1)) T2. PPV and SPV were measured in T1 and T2. Correlation and Bland-Altman analysis were used to compare respective values of PPV and SPV in the two modes of ventilation. RESULTS: Significant correlations were found between dynamic indices in SIMV during pressure support ventilation and those in controlled MV mode. The mean differences between two measurements were: PPV 0.6+/-2.8% (limit of agreement: -5.0 and 6.2), SPV 0.5+/-2.3 mmHg (limit of agreement: -4.0 and 5.1). CONCLUSIONS: PPV and SPV measured during SIMV fitted with the findings in controlled MV. Dynamic indexes could be accurately monitored in patients breathing with assisted respiratory assistance adding an imposed large enough SIMV breath.     

Effects of spontaneous breathing during airway pressure release ventilation on renal perfusion and function in patients with acute lung injury

OBJECTIVE: Controlled mechanical ventilation can impair systemic and renal blood flow and function, which may be aggravated by respiratory acidosis. We hypothesized that partial ventilatory support using airway pressure release ventilation (APRV) with spontaneous breathing provides better cardiopulmonary and renal function than full ventilatory support using APRV without spontaneous breathing. DESIGN: Prospective randomized study. SETTING: Intensive care unit of a university hospital. PATIENTS: Twelve patients with acute lung injury (ALI). INTERVENTIONS: Airway pressure release ventilation with and without spontaneous breathing, maintaining either the same minute ventilation (V(E)) or the same airway pressure (Paw) limits. MEASUREMENTS: Systemic hemodynamics were estimated by double-indicator dilution, effective renal blood flow (ERBF) by para-aminohippurate, and glomerular filtration rate (GFR) by inulin clearance. RESULTS: Compared to APRV with spontaneous breathing, cardiac index (CI) was decreased when the upper Paw limit was increased to provide the same V(E) (4.26+/-1.21 l min(-1) m(-2)vs 3.72+/-0.99 l min(-1) m(-2); p<0.05) while CI was increased when Paw limits were held constant (4.91+/-1.41 l min(-1) m(-2); p<0.05). Effective renal blood flow and GFR were higher during APRV with spontaneous breathing (858+/-388 ml min(-1) m(-2) and 94+/-47 ml min(-1) m(-2)) than during APRV without spontaneous breathing and the same V(E) (714+/-236 ml min(-1) m(-2)and 82+/-35 ml min(-1) m(-2)) or the same Paw (675+/-287 ml min(-1) m(-2) and 80+/-41 ml min(-1) m(-2); p<0.05). Urine volume did not change. CONCLUSIONS: Spontaneous breathing during APRV was associated with better renal perfusion and function than APRV without spontaneous breathing applying either the same V(E) or the same Paw limits. Maintaining spontaneous breathing during ventilatory support may, therefore, be advantageous in preventing deterioration of renal function in patients with ALI.     

Bedside assessment of the work of breathing

The oxygen consumption (VO2) of healthy volunteers and patients recovering from respiratory failure in the ICU was measured by indirect calorimetry during complete mechanical (VO2vent) and spontaneous (VO2wean) ventilation. The work of breathing was calculated as the difference in VO2 between spontaneous and mechanical ventilation and expressed as a percentage of VO2vent (% delta VO2). The average % delta VO2 for eight normal healthy volunteers was 3.7 +/- 1.8%, while it was 7.7 +/- 8.8% with the Bechman and Utah metabolic carts for patients recovering from ventilatory failure who were weaned successfully from respiratory support based on clinical criteria within 24 h of their metabolic study. In patients who were not weaned successfully, the average % delta VO2 measured was 24.7 +/- 12.3%. Indirect calorimetry is a relatively simple, reliable bedside technique for determining the oxygen cost of breathing. In our sample, the oxygen cost of breathing was a reliable predictor of weaning and extubation in patients recovering from respiratory failure. This measurement may be clinically useful in identifying the patient who cannot sustain spontaneous ventilation because of excessive respiratory work.     

Effects of noninvasive positive pressure ventilatory support in non-COPD patients with acute respiratory insufficiency after early extubation

Objective: To investigate the effects of noninvasive positive pressure ventilation (NPPV) on pulmonary gas exchange, breathing pattern, intrapulmonary shunt fraction, oxygen consumption, and resting energy expenditure in patients with persistent acute respiratory failure but without chronic obstructive pulmonary disease (COPD) after early extubation. Design: Prospective study. Setting: Multidisciplinary intensive care unit of a university hospital. Patients: 15 patients after prolonged mechanical ventilation (> 72 h) with acute respiratory insufficiency after early extubation. Interventions: Criteria for early extubation were arterial oxygen tension (PaO₂) L 40 mm Hg (fractional inspired oxygen 0.21), arterial carbon dioxide tension (PaCO₂) K 55 mm Hg, pH > 7.32, respiratory rate K 40 breaths per min, tidal volume (V_T) L 3 ml/kg, rapid shallow breathing index K 190 and negative inspiratory force L 20 cmH₂O. After extubation, two modes of NPPV were applied [continuous positive airway pressure (CPAP) of 5 cmH₂O and pressure support ventilation (PSV) with 15 cmH₂O pressure support]. Measurements and main results: Oxygenation and ventilatory parameters improved during both modes of NPPV (p < 0.05): increase in PaO₂ of 11 mm Hg during CPAP and 21 mm Hg during PSV; decrease in intrapulmonary shunt fraction of 7 % during CPAP and 12 % during PSV; increase in tidal volume of 1 ml/kg during CPAP and 4 ml/kg during PSV; decrease in respiratory rate 6 breaths/min during CPAP and 9 breaths/min during PSV. Oxygen consumption (15 % during CPAP, 22 % during PSV) and resting energy expenditure (12 % during CPAP, 20 % during PSV) were reduced (p < 0.05). PaCO₂ decreased, whereas minute ventilation and pH increased during PSV (p < 0.05). The median duration of NPPV was 2 days. Two patients had to be reintubated. Conclusions: In non-COPD patients with persistent acute respiratory failure after early extubation, NPPV improved pulmonary gas exchange and breathing pattern, decreased intrapulmonary shunt fraction, and reduced the work of breathing. Received: 14 May 1999 Final revision received: 25 June 1999 Accepted: 6 July 1999     

Effects of nasal continuous positive airway pressure on awake ventilatory responses to hypoxia and hypercapnia in patients with obstructive sleep apnea

This study was aimed to examine the short- and long-term effects of nasal continuous positive airway pressure (CPAP) on the chemosensitivity to hypoxia and hypercapnia in the patients with obstructive sleep apnea (OSA). Awake ventilatory responses to hypoxia and hypercapnia were examined in 28 patients (3 female) with moderate to severe OSA. All these tests were examined before and after 2 weeks of nasal CPAP. In 10 patients these tests were repeated after 3-6 months of nasal CPAP. All were also tested for spirometry and arterial blood gas analysis. Patients were middle-aged (48.9 +/- 9.9 years) and their mean apnea-hypopnea index was 58.3 +/- 20.4/hour. After 2 week of nasal CPAP, PaO2 significantly increased (77.7 +/- 11.8 vs. 84.6 +/- 9.8 mmHg) and PaCO2 significantly decreased (44.9 +/- 3.8 vs. 42.3 +/- 3.7 mmHg). The ventilatory response to hypoxia significantly decreased (0.80 +/- 0.51 vs. 0.61 +/- 0.51 liter/min/%) whereas the ventilatory response to hypercapnia significantly increased after 2 weeks (1.47 +/- 0.73 vs. 1.80 +/- 0.76 liter/min/mmHg). Similar findings were also observed after 3-6 months of nasal CPAP in 10 OSA patients. Nasal CPAP treatment can alter the ventilatory responses in patients with OSA.     

Restoration of volume by crystalloid versus colloid after coronary artery bypass: hemodynamics, lung water, oxygenation, and outcome

We compared Ringer's acetate-gluconate solution with 6% dextran-70 infused during rewarming after coronary bypass surgery. In a randomized study, 18 patients received 56 +/- 15 ml/kg of crystalloid (group 1), and 14 patients received 16 +/- 6 ml/kg of dextran (group 2). Data were taken at the following intervals: 4 to 5 h after terminating the cardiopulmonary bypass, after rewarming, the next morning on controlled ventilation and continuous positive airway pressure (CPAP) breathing, and after extubation. The patients were followed for 14 days. Prophylactic nitroglycerin infusion may have increased the need for plasma expansion. After volume loading, the stroke index increased in both groups, but the left ventricular stroke work index increased in group 2 only. After transition to the CPAP mode, hydrostatic pressures increased, more in group 2, doubling the pulmonary shunt flow. Pulmonary extravascular thermal volume did not change in either group. We conclude that hemodynamic stability occurred faster with dextran, and ventilatory weaning was somewhat easier with crystalloid.     

Physiologic effects of transtracheal open ventilation in postextubation patients with high upper airway resistance

OBJECTIVE: To investigate whether transtracheal open ventilation (TOV), pressure control ventilation (PCV) through a minitracheotomy tube (internal diameter 4 mm), is an effective method of inspiratory assistance under high upper airway resistance in postextubation patients; to compare, in a lung model study, TOV with other methods. DESIGN: Clinical study: A prospective, controlled, crossover study. Lung model study: A prospective laboratory trial. SETTING: Clinical study: A six-bed general intensive care unit in a teaching hospital. Lung model study: Animal research laboratory. PATIENTS: Clinical study: Eleven postextubation patients, who had undergone minitracheotomy for sputum retention between January 1997 and December 1997. SUBJECT: Lung model study: Two-bellows-in-a-box lung model, which included ordinary and high levels of upper airway resistance. INTERVENTIONS: Clinical study: Ventilatory settings were: assist/control (A/C) mode, 2 breaths/min of A/C back-up rate, 35-40 cm H2O of PCV, 0.6-0.8 secs of inspiratory time, and 0 cm H2O of positive end-expiratory pressure. The ventilatory parameters of TOV were compared with those of spontaneous breathing (SB). Lung model study: Effect of TOV on inspiratory assistance was compared with those of SB, open minitracheotomy, 5 L/min of transtracheal gas insufflation, and 5 and 10 cm H2O of pressure support ventilation (PSV), which simulated noninvasive positive ventilation. TOV ventilatory settings were: A/C mode; 30, 40, and 50 cm H2O of PCV, 0.9 secs of inspiratory time, and 0 cm H2O of positive end-expiratory pressure. At each ventilatory setting, we adjusted the inspiratory effort of the model to give a tidal volume of 0.5 L. MEASUREMENTS AND MAIN RESULTS: Clinical study: TOV was performed for 76.6 +/- 38.6 hrs (mean +/- sd) over 5.6 +/- 2.6 days without major complication. Peak tracheal pressure, which was measured distal to the minitracheotomy tube in six patients by a catheter pressure transducer, was 4.33 +/- 0.59 cm H2O. Inspiratory tidal volume delivered by the ventilator was 0.51 +/- 0.06 L. Respiratory rate during TOV was lower than during SB. According to esophageal pressure and respiratory inductive plethysmography, TOV reduced the patient's inspiratory work and improved the breathing pattern. Lung model study: Mean tracheal pressure during TOV and 10 cm H2O of PSV were positive values and they had larger inspiratory assistance according to the pressure-time product of pleural pressure. Although high upper airway resistance reduced the inspiratory assistance of PSV, it did not change the effects of TOV. CONCLUSIONS: TOV effectively reduced patient's inspiratory work and was more useful than open minitracheotomy and transtracheal gas insufflation. TOV also improved the breathing pattern. TOV may be useful for resolving some postextubation respiratory problems and avoiding the need for reintubation.     

BIPAP用于全身麻醉气管拔管后短期呼吸支持可行性分析

目的：比较和分析BIPAP用于全身麻醉气管拔管后短期呼吸支持的可行性。方法：60例成年择期手术病人随机分为3组（n=20），全麻气管导管拔除后分别予以BIPAP（B组）、CPAP＋PSV（C组）、SIMV＋CPAP（S组）面罩无创机械通气，记录并比较3组予以面罩无创通气后不同时刻血流动力学、血气参数、呼吸参数、并发症发生率及耐受性。结果：（1）3组无创通气后15min血压、心率较基础值明显增加（P＜0．01），S组比B组、C组增加更为明显（P＜0．01），B组、C组在30min以后血流动力学逐渐恢复至基础值，而S组在60分钟时才恢复至无创通气前水平；（2）分钟通气量相同时，B组、C组气道最高压力及平均压低于S组（P＜0．05）、且随无创通气逐渐降低，而S组变化不明显，B组自主呼吸恢复比S组明显（P＜0．05）；（3）3组无创通气后PaO2均显著增加（P＜0．01），PaCO2均在正常生理范围，但S组有升高（P＜0．05）；（4）三组并发症发生率及胃肠减压气体引流量无明显差异（P＞0．05）：（5）B组、C组比S组耐受性更好（P＜0．01）。结论：BIPAP对血流动力学、呼吸功能影响小、允许任何时相的自主呼吸、耐受性良好、并发症少、适用范围广，应用于全身麻醉后短期呼吸支持具有可行性。     

Extubation failure: diagnostic value of occlusion pressure (P0.1) and P0.1-derived parameters

OBJECTIVE: To evaluate the ability of the new, built-in occlusion pressure (P0.1) measurement to predict extubation failure. DESIGN AND SETTING: Prospective observational multicentre study in the ICU of five general hospitals. PATIENTS: Hundred thirty patients on mechanical ventilation longer than 48 h when considered ready for weaning. MEASUREMENTS AND RESULTS: Patients underwent a 30-min spontaneous breathing trial with simultaneous monitoring of occlusion pressure (P0.1) and breathing pattern (f/Vt). Sixteen patients (12%) failed the weaning trial and full ventilatory support was resumed, while 114 tolerated the trial and were extubated. Twenty-one (18%) required reintubation within 48 h. The area under the ROC curve for diagnosing extubation failure was 0.53 for f/Vt, 0.59 for P0.1 and 0.61 for P0.1*f/Vt (p=NS). Accordingly, P0.1*f/Vt more than 100 detected extubation failure with a sensitivity of 0.89, specificity of 0.35, positive predictive value of 0.21 and negative predictive value of 0.94. CONCLUSION: During a first trial of spontaneous breathing on pressure support ventilation (PSV), bedside P0.1 and P0.1*f/Vt are of little help, if any, for predicting extubation failure.     

Prediction of post-extubation work of breathing

OBJECTIVE: To evaluate which mode of preextubation ventilatory support most closely approximates the work of breathing performed by spontaneously breathing patients after extubation. DESIGN: Prospective observational design. SETTING: Medical, surgical, and coronary intensive care units in a university hospital. PATIENTS: A total of 22 intubated subjects were recruited when weaned and ready for extubation. INTERVENTIONS: Subjects were ventilated with continuous positive airway pressure at 5 cm H2O, spontaneous ventilation through an endotracheal tube (T piece), and pressure support ventilation at 5 cm H2O in randomized order for 15 mins each. At the end of each interval, we measured pulmonary mechanics including work of breathing reported as work per liter of ventilation, respiratory rate, tidal volume, negative change in esophageal pressure, pressure time product, and the airway occlusion pressure 100 msec after the onset of inspiratory flow, by using a microprocessor-based monitor. Subsequently, subjects were extubated, and measurements of pulmonary mechanics were repeated 15 and 60 mins after extubation. MEASUREMENTS AND MAIN RESULTS: There were no statistical differences between work per liter of ventilation measured during continuous positive airway pressure, T piece, or pressure support ventilation (1.17+/-0.67 joule/L, 1.11+/-0.57 joule/L, and 0.97+/-0.57 joule/L, respectively). However, work per liter of ventilation during all three preextubation modes was significantly lower than work measured 15 and 60 mins after extubation (p < .05). Tidal volume during pressure support ventilation and continuous positive airway pressure (0.46+/-0.11 L and 0.44+/-0.11 L, respectively) were significantly greater than tidal volume during both T-piece breathing and spontaneous breathing 15 mins after extubation (p < .05). Negative change in esophageal pressure, the airway occlusion pressure 100 msec after the onset of inspiratory flow, and pressure time product were significantly higher after extubation than during any of the three preextubation modes (p < .05). CONCLUSIONS: Work per liter of ventilation, negative change in esophageal pressure, the airway occlusion pressure 100 msec after the onset of inspiratory flow, and pressure time product all significantly increase postextubation. Tidal volume during continuous positive airway pressure or pressure support ventilation overestimates postextubation tidal volume.     

The spontaneous breathing pattern and work of breathing of patients with acute respiratory distress syndrome and acute lung injury

BACKGROUND: The spontaneous breathing pattern and its relationship to compliance, resistance, and work of breathing (WOB) has not been examined in patients with acute respiratory distress syndrome (ARDS) or acute lung injury (ALI). Clinically, the ratio of respiratory frequency to tidal volume (f/VT) during spontaneous breathing may reflect adaptation to altered compliance, resistance, and increased WOB. We examined the relationship between f/VT, WOB, and respiratory system mechanics in patients with ARDS/ALI. METHODS: Data from spontaneous breathing trials were collected from 33 patients (20 with ARDS, 13 with ALI) at various points in their disease course. WOB and respiratory system mechanics were measured with a pulmonary mechanics monitor that incorporates Campbell diagram software. Differences between the patients with ARDS and ALI were assessed with 2-sided unpaired t tests. Multivariate linear regression models were constructed to assess the relationship between f/VT and other pulmonary-related variables. RESULTS: Patients with ARDS had significantly lower compliance than those with ALI (24 +/- 6 mL/cm H2O vs 40 +/- 13 mL/cm H2O, respectively, p < 0.001), but this did not translate into significant differences in either WOB (1.70 +/- 0.59 J/L vs 1.43 +/- 0.90 J/L, respectively, p = 0.30) or f/VT (137 +/- 82 vs 107 +/- 49, respectively, p = 0.23). Multivariate linear regression modeling revealed that peak negative esophageal pressure, central respiratory drive, duration of ARDS/ALI, minute ventilation deficit between mechanical ventilation and spontaneous breathing, and female gender were the strongest predictors of f/VT. CONCLUSION: The characteristic rapid shallow breathing pattern in patients with ARDS/ALI occurs in the context of markedly diminished compliance, elevated respiratory drive, and increased WOB. That f/VT had a strong, inverse relationship to peak negative esophageal pressure also may reflect the influence of muscle weakness.