Continuous positive airway pressure at 10 cm H(2)O during cardiopulmonary bypass improves postoperative gas exchange

Postbypass pulmonary dysfunction including atelectasis and increased shunting is a common problem in the intensive care unit. Negative net fluid balance and continuous positive airway pressure (CPAP) have been used to reduce the adverse effects of cardiopulmonary bypass (CPB) on the lung. To determine whether CPAP at 10 cm H(2)O during CPB results in improved postoperative gas exchange in comparison with deflated lungs during CPB, we examined 14 patients scheduled for elective cardiac surgery. Seven patients received CPAP at 10 cm H(2)O during CPB, and in the other seven patients, the lungs were open to the atmosphere (control). Measurements were taken before and after CPB, after thoracic closure, and 4 h after CPB in the intensive care unit. CPAP at 10 cm H(2)O resulted in significantly more perfusion of lung areas with a normal ventilation/perfusion distribution (V(A)/Q) and significantly less shunt and low V(A)/Q perfusion 4 h after CPB in comparison with the control group. Consequently, arterial oxygen partial pressure was significantly higher and alveolar-arterial oxygen partial pressure difference was significantly smaller. We conclude that CPAP at 10 cm H(2)O during CPB is a simple maneuver that improves postoperative gas exchange. IMPLICATIONS: Inflation of the lungs at a pressure of 10 cm H(2)O as compared with leaving the lungs deflated during cardiopulmonary bypass was examined. Lung inflation during bypass resulted in significantly improved postoperative gas exchange.     

Adult respiratory distress syndrome: improved oxygenation during high-frequency jet ventilation/continuous positive airway pressure

The role of high-frequency jet ventilation (HFJV)/continuous positive airway pressure (CPAP) and HFJV/intermittent mandatory ventilation (IMV) in the treatment of surgical patients with the adult respiratory distress syndrome were evaluated. To compare the efficacy of HFJV to IMV at a constant FiO2 and positive end-expiratory pressure, patients in surgical intensive care were randomized to receive IMV/CPAP therapy or one of three modes of HFJV: (1) HFJV/CPAP alone, (2) HFJV/CPAP + IMV (1), or (3) HFJV/CPAP + IMV (2). Each patient served as his own control. During comparison of HFJV/CPAP + IMV (1) to HFJV/CPAP + IMV (2) (n = 9) and HFJV/CPAP to HFJV/CPAP + IMV (1) (n = 7), cardiac output, PaCO2, PaO2, PvO2, and variables consisting of intrapulmonary shunt fraction (Qsp/Qt), PaO2/FiO2 ratio, and A-a gradient were calculated. The subgroup placed on HFJV/CPAP demonstrated a fall in PaO2 of 13 torr (p = NS; n = 5). HFJV/CPAP + IMV (1) compared with HFJV/CPAP significantly (p less than 0.005) increased PaO2 by 52 +/- 24 torr and decreased Qsp/Qt by 8.9 +/- 1.0 (p less than 0.025). Cardiac output remained unchanged. Comparison of HFJV/CPAP + IMV (2) to HFJV/CPAP + IMV (1) demonstrated a significant improvement in oxygenation (p less than 0.025), but of lesser magnitude (8.4 +/- 11 torr). PaO2/FiO2 ratio and A-a gradient improved in both IMV (1) and IMV (2) subgroups. Oxygenation and ventilation/perfusion (V/Q) matching significantly improved with HFJV/CPAP + IMV (1), to a greater magnitude than with HFJV/CPAP + IMV (2) or HFJV/CPAP alone, and was the preferred method of ventilatory support.     

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.     

Prediction of lung cancer spread and functional resectability by 133Xe radiospirometry

Sixty-six patients with primary lung cancer who underwent thoracotomy were studied to determine the correlations among 133Xe radiospirometry, surgical procedures and histological extension of the lung cancer. Disturbance in the regional perfusion (Q per cent) was more prominent than disturbance of the regional ventilation (V per cent), as the pathological stage and t factor proceeded, while V per cent and Q per cent were disturbed almost equally in relation to the pathological n factor. Lobectomy was impossible in patients with a Q per cent of less than 33 per cent of the total, but low perfusion did not necessarily contraindicate surgery. The predicted postoperative FEV1.0 was calculated according to the equation of (1-b/a x (V per cent or Q per cent)) x (preoperative FEV1.0), where a and b were the number of subsegments in the lung lobes on the involved side and the resected lobe. The predicted and actually measured postoperative FEV1.0 showed significant correlations (less than 0.001) in both equations. We conclude that Q per cent reflects a complex pattern of lung cancer spread more sensitively than does V per cent, and the significance of V per cent and Q per cent in terms of prediction of postoperative EFV1.0 seems to be equivocal.     

Prone position improves gas exchange – but how?

* Mechanically ventilated patients with severe acute lung insufficiencies dramatically improve their gas exchange when treated in the prone position. * ventilation heterogeneity is greater in the supine then in the prone position during CMV. * the dominant dorsal Q while supine is not turned into a dominant ventral Q in the prone position. * in the presence of an abdominal distension, the prone position more clearly improves gas exchange than at normal abdominal pressures. * CPAP enhances the dominant dorsal lung perfusion while supine. In the prone position lung perfusion is more uniform. * V/Q matching is improved in the prone position during CMV.     

Continuous positive airway pressure for treatment of respiratory complications after abdominal surgery: a systematic review and meta-analysis

OBJECTIVE: We evaluated the potential benefit of continuous positive airway pressure (CPAP) to prevent postoperative pulmonary complications (PPCs), atelectasis, pneumonia, and intubation in patients undergoing major abdominal surgery. SUMMARY BACKGROUND DATA: PPCs are common during the postoperative period and may be associated with a high morbidity rate. Efficacy of CPAP to prevent PPCs occurrence is controversial. METHODS: Medical literature databases were searched for randomized controlled trials examining the use of CPAP versus standard therapy in patients undergoing abdominal surgery. The meta-analysis estimated the pooled risk ratio and the number needed to treat to benefit (NNTB) for PPCs, atelectasis, and pneumonia. RESULTS: The meta-analysis was carried out over 9 randomized controlled trials. Overall, CPAP significantly reduced the risk of (1) PPCs (risk ratio, 0.66; 95% confidence interval [CI], 0.52-0.85) with a corresponding NNTB of 14.2 (95% CI, 9.9-32.4); (2) atelectasis (risk ratio, 0.75; 95% CI, 0.58-0.97; NNTB, 7.3; 95% CI, 4.4-64.5); (3) pneumonia (risk ratio, 0.33; 95% CI, 0.14-0.75; NNTB, 18.3; 95% CI, 14.4-48.8). In all cases the variation in risk ratio attributable to heterogeneity was negligible, although there was some evidence of publication bias. CONCLUSIONS: This systematic review suggests that CPAP decreases the risk of PPCs, atelectasis, and pneumonia and supports its clinical use in patients undergoing abdominal surgery.     

Regional structure-function correlations in chronic obstructive lung disease measured with positron emission tomography.

BACKGROUND: Positron emission tomography, performed with isotopes of very short half life, can be used to relate local lung tissue density to local ventilation and to the ventilation:perfusion ratio. This method has been used in 10 patients with severe chronic airflow obstruction and differing values for carbon monoxide transfer factor (TLCO) and transfer coefficient (KCO). METHODS: Ventilation (VA) and the ventilation:perfusion ratio (V/Q), lung density, and blood volume were measured regionally in a single transaxial section at mid-heart level with the patients in a supine position. Alveolar volume, extravascular tissue lung density, and perfusion (Q) were derived. Twenty five regions with abnormalities in the ventilation images were analysed. RESULTS: Tissue density showed a negative correlation with the ratio V/Q (r = 0.55) and a positive correlation with Q (r = 0.59) and blood volume (r = 0.65). In four patients with a low carbon monoxide transfer factor (TLCO) and transfer coefficient (KCO) < 50% predicted many regions with low VA had low tissue density and normal or high V/Q. On the other hand, in four patients with TLCO and KCO > 50% predicted many regions with low VA had normal or high tissue density and low values of V/Q. The other two patients had patterns between these two extremes. Individual ratios between mean values of tissue density and V/Q had a positive correlation with KCO (% pred; r = 0.79). CONCLUSIONS: These findings link structural differences with distinctive functional patterns; they reinforce the view that bronchial inflammation or oedema predominate in some patients with chronic airflow obstruction, whereas alveolar destruction is the major feature in others.     

True shunt in relation to venous admixture in an experimental porcine model of early ARDS

Using the multiple inert gas elimination technique, ventilation/perfusion (V/Q) relationships were studied in an experimental porcine model of the early Adult Respiratory Distress Syndrome (ARDS) to establish the nature of the increased venous admixture. Six animals served as controls and revealed no major changes apart from a 10% decrease in cardiac output during the 4-h observation period. All control animals showed a shift to a higher mean V/Q of perfusion (Qmean) and a maintained log standard deviation (QSD) throughout the experiment. The distribution was unimodal and centered around a V/Q ratio of 1.0. The share of perfusion to V/Q ratio less than 0.005 (i.e. true shunt, Qs) remained unchanged at 6-7% of cardiac output. Nine animals, given a continuous infusion of E. coli endotoxin, showed a significant decrease of 53% in cardiac output (Qt) at 4 h. Mean pulmonary artery pressure (MPAP) showed a 2-phase reaction with a peak level at 0.5 h, and a second gradual increase from 2 h onwards. Venous admixture doubled at 0.5 h, after which it declined but remained elevated throughout the observation period. All endotoxin animals showed a shift in perfusion to a higher Qmean with a significantly wider QSD at 0.5, 2 and 4 h. The distribution was unimodal and centered around a V/Q ratio of 1.0. True shunt was unchanged at 6-7% of cardiac output throughout the study. The increase in venous admixture in this experimental ARDS model is consequently explained by the widening of the V/Q scatter and is due to a perfusion shift to lower ventilation/perfusion ratios rather than to an increase in true shunt.     

Continuous positive airway pressure by mask in patients after coronary surgery

Thirty patients who underwent coronary artery bypass grafting were randomized to receive 30% oxygen by mask either with an ambient airway pressure or with 7.4 mmHg (1 kPa) continuous positive airway pressure (CPAP) for 8 h after extubation. Arterial blood oxygen tension (Pao₂) decreased remarkably in the control group after extubation (from 19.2± 5.3 kPa to 12.4 ± 2.7 kPa) but less in the CPAP group (from 16.4 ± 3.3 kPa to 14.0 ± 2.1 kPa). On the second postoperative morning Pao₂ was equally low in both groups (control: 8.4 ± 1.5 kPa, CPAP: 8.9 ± 1.9 kPa). Atelectatic areas were seen with similar frequency in both groups, 17% (whole material) on the first and 50% on the second postoperative morning. Atelectasis was more common in patients with internal thoracic artery grafting and/or pleural drainage. In conclusion, CPAP therapy was well tolerated, and minimized the decrease in Pao₂ after extubation, but could not prevent the poor oxygenation or the late development of atelectatic areas on the second postoperative day.     

Postoperative CPAP and BiPAP use can be safely omitted after laparoscopic Roux-en-Y gastric bypass

BACKGROUND: Obstructive sleep apnea (OSA) is prevalent in the morbidly obese population. The need for routine preoperative testing for OSA has been debated in bariatric surgery publications. Most investigators have advocated the use of continuous positive airway pressure (CPAP) or bi-level positive airway pressure (BiPAP) in the postoperative setting; however, others have reported pouch perforations or other gastrointestinal complications as a result of their use. From a review of our experience, we present an algorithm for the safe postoperative treatment of patients with OSA without the use of CPAP or BiPAP. METHODS: From January 2003 to December 2007, 1095 laparoscopic Roux-en-Y gastric bypasses were performed at our institution. Preoperative testing for OSA was not routinely performed. A prospective database was maintained. The data included patient demographics, co-morbidities (including OSA and CPAP/BiPAP use), perioperative events, complications, and follow-up information. Patients with known OSA were not given CPAP/BiPAP after surgery. They were observed in a monitored setting during their inpatient stay, ensuring continuous oxygen saturation of >92%. All patients used patient-controlled analgesia, were trained in the use of incentive spirometry, and ambulated within a few hours of surgery. The outcomes were compared between the OSA patients using preoperative CPAP/BiPAP versus those with OSA without preoperative CPAP/BiPAP versus patients with no history of OSA. RESULTS: A total of 811 patients were included in the study group with no known history of OSA. Of the 284 patients with a confirmed diagnosis of OSA, 144 were CPAP/BiPAP dependent. Statistically significant differences were present in age distribution and gender, with men having greater CPAP/BiPAP dependency. No significant differences were found in body mass index, length of stay, pulmonary complications, or deaths. One pulmonary complication occurred in the OSA, CPAP/BiPAP-dependent group, three in the OSA, non-CPAP group, and six in the no-known OSA group. No anastomotic leaks or deaths occurred in the series. CONCLUSION: Postoperative CPAP/BiPAP can be safely omitted in laparoscopic Roux-en-Y gastric bypass patients with known OSA, provided they are observed in a monitored setting and their pulmonary status is optimized by aggressive incentive spirometry and early ambulation.     

End-expiratory lung volume and ventilation distribution with different continuous positive airway pressure systems in volunteers

Background: Continuous positive airway pressure (CPAP) has been shown to improve oxygenation and a number of different CPAP systems are available. The aim of this study was to assess lung volume and ventilation distribution using three different CPAP techniques. Methods: A high‐flow CPAP system (HF‐CPAP), an ejector‐driven system (E‐CPAP) and CPAP using a Servo 300 ventilator (V‐CPAP) were randomly applied at 0, 5 and 10 cmH₂O in 14 volunteers. End‐expiratory lung volume (EELV) was measured by N₂ dilution at baseline; changes in EELV and tidal volume distribution were assessed by electric impedance tomography. Results: Higher end‐expiratory and mean airway pressures were found using the E‐CPAP vs. the HF‐CPAP and the V‐CPAP system (P<0.01). EELV increased markedly from baseline, 0 cmH₂O, with increased CPAP levels: 1110±380, 1620±520 and 1130±350 ml for HF‐, E‐ and V‐CPAP, respectively, at 10 cmH₂O. A larger fraction of the increase in EELV occurred for all systems in ventral compared with dorsal regions (P<0.01). In contrast, tidal ventilation was increasingly directed toward dorsal regions with increasing CPAP levels (P<0.01). The increase in EELV as well as the tidal volume redistribution were more pronounced with the E‐CPAP system as compared with both the HF‐CPAP and the V‐CPAP systems (P<0.05) at 10 cmH₂O. Conclusion: EELV increased more in ventral regions with increasing CPAP levels, independent of systems, leading to a redistribution of tidal ventilation toward dorsal regions. Different CPAP systems resulted in different airway pressure profiles, which may result in different lung volume expansion and tidal volume distribution.     

The effect of intraoperative PEEP ventilation and postoperative CPAP breathing on postoperative lung function following upper abdominal surgery

The problem of how to improve postoperative pulmonary function after upper abdominal surgery was investigated in a randomized study involving 64 patients who were subjected to various treatment regimens designed to increase airway pressure. Intraoperative ventilation was carried out with either zero or positive end exspiratory pressure, and postoperatively either CPAP or a nasal oxygen catheter were applied. The following groups were formed: 1. ZEEP /O2-catheter; 2. PEEP/O2-catheter; 3. ZEEP /CPAP; 4. PEEP/CPAP. The typical reduction in vital capacity occurred postoperatively, the lowest value being recorded on the 2nd postoperative day, an alteration in respiratory pattern with reduced tidal volume and increased respiratory rate together with an initially low, later normal alveolar ventilation, and an initial hypoxaemia which was at first associated with a moderate hypercapnia, on the second postoperative day with a normocapnia, were observed. At no point in time could a difference be found between the 4 groups, no measurable improvement in respiratory function being found as a result of the treatment given. Intra- and post-operative increase in airway pressure was however found to be associated with a reduction in the incidence of post-operative pulmonary complications.     

Activity, air boots, and aspirin as thromboembolism prophylaxis in knee arthroplasty. A multiple regimen approach

In this prospective study 90 patients underwent 100 total knee replacements. Each patient walked and practiced rehabilitation exercises one day preoperatively and the morning of surgery. The contralateral calf was intermittently pumped during surgery followed by bilateral pulsatile calf compression postoperatively. Aspirin was administered in the recovery room and continued 600 mg twice a day until discharge. Early activity and ambulation postoperatively were expected. Routine deep venous thrombosis screening tests were not performed. No physiologically significant postoperative venous thrombosis on any of these patients were observed. A Doppler test was performed on eight patients, venogram on two patients, and V/Q lung scan on two patients in whom a thromboembolic disease problem was clinically suspected. All of these tests were negative except for one V/Q scan.     

Safety and efficacy of postoperative continuous positive airway pressure to prevent pulmonary complications after Roux-en-Y gastric bypass

Continuous positive airway pressure (CPAP) is used to prevent apneic arrest and/or hypoxia in patients suffering from obstructive sleep apnea. This modality has not been universally accepted for patients following upper gastrointestinal surgery because of concerns that pressurized air will inflate the stomach and proximal intestine, resulting in anastomotic disruption. This study was performed to assess the safety and efficacy of postoperative CPAP for patients undergoing a gastrojejunostomy as part of a Roux-en-Y gastric bypass (RYGB) procedure. A total of 1067 patients (837 women [78%] and 230 men [22%]) were prospectively evaluated for the risk of developing anastomotic leaks and pulmonary complications after the RYGB procedure. Of the 1067 patients undergoing gastric bypass, 420 had obstructive sleep apnea and 159 were dependent on CPAP. There were 15 major anastomotic leaks, two of which occurred in CPAP-treated patients. Contingency table analysis demonstrated that there was no correlation between CPAP utilization and the incidence of major anastomotic leakage (P = 0.6). Notably, no episodes of pneumonia were diagnosed in either group. Despite the theoretical risk of anastomotic injury from pressurized air delivered by CPAP, no anastomotic leaks occurred that were attributable to CPAP. There were no pulmonary complications in a patient population that is at risk for developing them postoperatively. CPAP is a useful modality for treating hypoventilation after RYGB without increasing the risk of developing postoperative anastomotic leaks. Presented at the Forty-Second Annual Meeting of The Society for Surgery of the Alimentary Tract, Atlanta, Georgia, May 20–23, 2001 (poster presentation).     

Comparison of conventional mechanical ventilation and synchronous independent lung ventilation (SILV) in the treatment of unilateral lung injury

Eight patients presenting with severe unilateral pulmonary injury responded poorly to conventional mechanical ventilation. Synchronous independent lung ventilation (SILV) was employed to provide support of ventilation and oxygenation without creating the ventilation/perfusion (V/Q) mismatch observed during conventional ventilation. All patients demonstrated improved oxygenation (mean increase, 80 torr) during SILV with the FIO2 unchanged from previous therapy. Invasive hemodynamic monitoring in five of eight patients showed no difference in the commonly measured cardiopulmonary parameters with the two forms of mechanical ventilation. Peak inspiratory pressure (PIP), continuous positive airway pressure (CPAP), and pressure change secondary to tidal volume delivery to the uninvolved lung were significantly less during SILV. SILV is an effective method of improving oxygenation in patients with severe unilateral pulmonary injury.     

Randomized clinical study of the prevention of pulmonary complications after thoracoabdominal resection by two different breathing techniques

BACKGROUND: Pulmonary complications are frequently seen after thoracoabdominal resection of the oesophagus. The aim of this study was to compare the effects of two different breathing exercise regimens applied in the immediate postoperative period on the risk of pulmonary insufficiency after thoracoabdominal resection. METHODS: Seventy patients undergoing thoracoabdominal resection for cancer of the oesophagus and cardia were randomized after operation to breathing exercises by inspiratory resistance-positive expiratory pressure (IR-PEP) (n = 36) or continuous positive airway pressure (CPAP) (n = 34). The study groups were well matched for all relevant clinical and demographic data. RESULTS: Respiratory function deteriorated significantly immediately after operation; the lowest values of forced vital capacity and peak expiratory flow were measured during the first postoperative day and oxygen saturation was lowest on days 4-6. Significantly fewer patients in the CPAP group required reintubation and prolonged artificial ventilation (P < 0.05). There were minor non-significant differences between the study groups with respect to respiratory and other postoperative variables, usually in favour of CPAP. CONCLUSION: Provision of CPAP in the immediate postoperative period decreased the risk of respiratory distress requiring reintubation and the need for artificial ventilation compared with breathing exercises by IR-PEP.     

胸部肿瘤病人围手术期血气监测及临床意义

报道１００例胸部肿瘤病人围手术期血气监测结果：术后１～４天动脉血氧分压较术前明显降低（Ｐ＜０．００１）。食管和肺手术后平均动脉血氧分压无明显差异（Ｐ＞０．０５）。术后低氧血症与年龄、术前心肺并发症及麻醉、手术引起的ＶＡ／Ｑ比例失常有关。它是引起术后心肺并发症的重要因素，所以为了减少术后并发症，术后３天内常规吸氧并保持呼吸道通畅是非常重要的。     

Effects of conventional physiotherapy, continuous positive airway pressure and non-invasive ventilatory support with bilevel positive airway pressure after coronary artery bypass grafting

BACKGROUND: Coronary artery bypass graft (CABG) surgery with the use of mammary arteries is associated with severe alteration of lung function parameters. The purpose of the present study was to compare the effect on lung function tests of conventional physiotherapy using incentive spirometry (IS) with non-invasive ventilation on continuous positive airway pressure (CPAP) and with non-invasive ventilation on bilevel positive airway pressure (BiPAP or NIV-2P), METHODS: Ninety-six patients were randomly assigned to 1 of 3 groups: NIV-2P (1 h/3 h), CPAP (1 h/3 h) and IS (20/2 h). Pulmonary function tests and arterial blood gases analyses were obtained before surgery. On the 1st and 2nd postoperative days, these parameters were collected together with cardiac output and calculation of venous admixture. RESULTS: For the 3 groups a severe restrictive pulmonary defect was observed during the 1st postoperative day. On the 2nd postoperative day, in opposition to IS, intensive use of CPAP and NIV-2P reduced significantly the venous admixture (P<0.001) and improved VC, FEV1 and PaO2 (P<0.01). CONCLUSION: We conclude that preventive use of NIV can be considered as an effective means to decrease the negative effect of coronary surgery on pulmonary function.     

Continuous Positive Airway Pressure Increases Vagal and Phrenic Nerve Activity in Cats

To investigate how continuous positive airway pressure (CPAP) changes the vagal nerve activity and whether CPAP alters the efferent phrenic nerve activity or the breathing pattern similarly before and after vagotomy, a study was made of vagal and phrenic nerve activity in chloralose-anaesthetized cats. In the vagal nerve, CPAP increased the mean impulse frequency during expiratory rest. The breath-related impulse frequency also increased with CPAP. With higher CPAP (greater than or equal to 0.5 kPa), the peak of breath-induced activity in the vagal nerve lasted longer than inspiration. In the phrenic nerve, the impulse frequency in the bursts increased almost linearly with CPAP irrespective of whether the vagal nerves were intact or not. The duration of the phrenic nerve bursts decreased with increasing CPAP when the vagal nerves were intact. When the vagal nerves were cut, the burst duration did not change. The rate of breathing was almost unchanged by CPAP regardless of whether the vagal nerves were cut or not. The inspiration/expiration ratio decreased with increasing CPAP when the vagal nerves were intact, but not when they were cut.