Year : 2007 | Volume
| Issue : 1 | Page : 10
The effect of positive pressure ventilatory patterns on post-bypass lung functions
Mohamed Essam Abdel-Meguid1, Emad El-Din Mansour2, Khaled M Abdallah1
1 Anaesthesia Department, College of Medicine, King Fahad Cardiac Center, King Saud University, Saudi Arabia
2 Consultant Cardiac Anaesthesia, King Khalid University Hospital, Saudi Arabia
Mohamed Essam Abdel-Meguid
Anaesthesia Department, College of Medicine, King Fahad Cardiac Center, King Saud University, Riyadh
Source of Support: None, Conflict of Interest: None
|Date of Web Publication||5-Oct-2009|
| Abstract|| |
Background: This study aimed at evaluating the effect of application of different patterns of positive ventilatory pressure either during or after cardiopulmonary bypass (CPB) on lung functions.
Methods: 30 patients undergoing coronary artery revascularisation under the management of CPB were randomly allocated into 3 groups. Group I (VCM) 10 patients were subjected to manual vital capacity manoeuvre (VCM) before weaning off the CPB. Group II (CPAP) 10 patients were subjected to continuous positive airway pressure (CPAP) of 10 cmH 2 O during CPB. Group III (PEEP) 10 patients were subjected to positive end expiratory pressure (PEEP) of 7 cmH 2 O after weaning off the CPB. Measurements included the PO 2 , PCO 2 , together with derived calculated parameters as the alveolar-arterial oxygen difference [P (A-a) DO2] and shunt fraction, as well as the dynamic lung compliance being recorded directly from the anaesthetic and ventilatory equipments. All readings were taken on closed chest and on FiO 2 of 0.5. Intraoperative anaesthetic and surgical data as well as postoperative extubation time and length of ICU stay were also evaluated.
Results: Statistical analysis of ventilatory parameters showed no significant differences for both PO 2 and PCO 2 in between the studied groups. Alveolar-Arterial oxygen difference mean values were comparable in the 3 studied groups. The mean values of intrapulmonary shunt fraction showed a significant difference in relation to the baseline values in Group I (VCM) and Group III (PEEP) at 30 minutes after ICU admission and 4 hours post CPB with estimated P value <0.01 and < 0.05 respectively, while in Group II (CPAP) mean values started to be significant after chest closure with a p value <0.05, but there was no significant intergroup differences with a P value > 0.01. Dynamic lung compliance mean values showed no intergroup statistical significance.
Conclusion: Maintenance of Ventilatory parameters was achieved in all the positive pressure ventilatory methods applied, either being applied during or after CPB.
Keywords: Cardiopulmonary bypass; Vital capacity manoeuvre; CPAP; PEEP
|How to cite this article:|
Abdel-Meguid ME, Mansour EE, Abdallah KM. The effect of positive pressure ventilatory patterns on post-bypass lung functions. Saudi J Anaesth 2007;1:10
|How to cite this URL:|
Abdel-Meguid ME, Mansour EE, Abdallah KM. The effect of positive pressure ventilatory patterns on post-bypass lung functions. Saudi J Anaesth [serial online] 2007 [cited 2020 May 31];1:10. Available from: http://www.saudija.org/text.asp?2007/1/1/10/56268
| Introduction|| |
Pulmonary dysfunction after cardiopulmonary bypass (CPB) is considered one of the major consequences in cardiac surgery that results either from increased lung water content or development of postoperative atelectasis, both of which will result in decreased lung compliance and in return result in deficient gas exchange as a result of increased shunt fraction  . The main clinical manifestation of post-pump lung syndrome is defective oxygenation that may result in a spectrum of consequences resulting in prolonged postoperative mechanical ventilation and failure of fast tracking concept in cardiac surgery. Atelectasis and decreased lung compliance is the main feature of post-pump lung syndrome  , thus with prevention of those consequences proper oxygenation could be achieved and hence shortening of intubation time postoperatively as well as shortening of length of ICU stay. Application of positive ventilatory pressure using continuous positive airway pressure (CPAP) during CPB was proved to have beneficial effects on maintenance of oxygenation parameters (2,3). Manual application of Positive ventilatory pressure on the conclusion of CPB using the vital capacity manoeuvre (VCM) showed significant improvement in ventilatory parameters when compared to deflated lungs throughout the CPB interval  . In the present study comparison was made between different patterns positive ventilatory pressure being applied either during or after CPB including the application of positive end expiratory pressure (PEEP) after weaning off CPB which was not studied before on humans.
| Patients and methods|| |
After obtaining our hospital Research Review Board approval , the study was instituted by November 2003 and ends by May 2004. An informed patient consent was taken from eligible patients before being enrolled to the study.
In a randomised prospective observational study, 30 patients scheduled for elective cardiac revascularisation procedure that was previously determined to be under the management of cardiopulmonary bypass (CPB) whether by the tepid CPB method or by the on pump beating heart technique were enrolled to the study. Randomisation of patients was taken through sealed envelops. Exclusion criteria included pre-existing pulmonary disease, poor LV function with EF < 30%, Morbid obese patients with a BMI > 35, lengthy CPB time >120 min, severe haemodynamic instability necessitating either a high inotropic support or application of a ventricular assist device as intra-aortic balloon pump (IABP), or prolonged ventilatory support due to any other reason rather than ventilatory derangements.
Patients were randomly allocated into 3 groups, group I (VCM) 10 patients were subjected to manual ventilation using the vital capacity manoeuvre (VCM) just before weaning off the CPB and before resuming mechanical ventilation while lungs being deflated all through the CPB interval. VCM was achieved by manually ventilating lungs to an airway pressure of 40 cmH 2 O for 15 seconds. Patients in group II (CPAP) 10 patients were subjected to a CPAP of 10 cmH 2 O all through CPB interval and then to resume mechanical ventilation directly without applying VCM. Patients in group III (PEEP) 10 patients were subjected to a PEEP of 7 cmH 2 O on resuming mechanical ventilation provided that haemodynamic parameters permit the application of such pressure. PEEP was continued throughout the postoperative period till weaning from mechanical ventilation to be weaned down to 5 cmH 2 O before extubation.
Data collection of ventilatory parameters involved PO 2 and PCO 2, alveolar - arterial oxygen difference [P (A-a) DO2], shunt fraction and dynamic lung compliance (DLC). Shunt fraction was calculated based on the three-compartment model proposed by Riley and colleagues (5,6). Dynamic lung compliance (DLC) in ml/cmH 2 O was directly recorded from the anaesthetic and ventilatory equipments. Data were collected at the following specified time points:
Times from ICU admission to extubation as well as the length of ICU stay were also recorded. All patients received premedication in the form of lorazepam 2 mg orally at night of the operation in addition to intramuscular morphine sulphate 0.1 mg/kg 1 h prior to transfer to the operating room (OR). On receiving patient in OR, standard monitoring connected and a large bore peripheral venous as well as 20-gauge radial arterial cannulae were inserted. Induction follows with Sufentanil 1-1.5 ug/Kg, Midazolam 0.05-0.1 mg/Kg and Rocuronium 0.9 mg/Kg. Patients were anaesthetically maintained on total intravenous infusion of same inducing agents supplemented with Sevoflurane guided by the Bispectral index (Aspect industries, USA) monitoring in a range of reading 40-60. The lungs were mechanically ventilated with controlled mode delivering a tidal volume of 8ml/Kg while the respiratory rate was adjusted to keep end tidal CO 2 of 32-36 mmHg. Ventilatory parameters were recorded through the anaesthesia machine ventilatory monitoring system (Datex-Ohmeda, Type 5). All ventilatory and oxygenation data were recorded on FiO 2 of 0.5 and in closed chest to provide proper standardization.
- Baseline reading after induction of anaesthesia and before sternotomy
- After chest closure
- 30 min after admission to ICU
- 4 hours post CPB
- 1 hour post-extubation
Anticoagulation was induced with 300 units/kg of unfractionated heparin IV push before cannulation of the aorta where a celite-activated coagulation time of > 400 sec must be achieved. The CPB circuit consisted of a membrane oxygenator (Medtronic cardiovascular, Brooklyn Park, MN), non-occlusive roller pump and arterial filter. The oxygenator was primed with 2000 ml of crystalloid solution, 100 ml mannitol 20%, NaHCO 3 50 mEq, unfractionated heparin and solu-medrol 500 mg. Tepid technique was used while maintaining temperature at 32-33 C o with a pump flow rates of 2.4-2.8 L/min/m 2 to maintain a mean arterial pressure of 60-80 mmHg. Myocardial preservation achieved through warm blood cardioplegia I the tepid CPB technique, while in other cases normothermic on pump beating heart technique was used for revascularisation.
During CPB, the lungs were deflated in group I (VCM) and group III (PEEP), while in group II (CPAP) a CPAP was adjusted to 10 cmH 2 O using the pop-off valve. On going weaning off the CPB, patients in group I resumed mechanical ventilation after application of VCM , patients in group II resumed mechanical ventilation directly without application of VCM, while patients in group III resumed mechanical ventilation with application of PEEP of 7 cmH 2 O. Ventilation adjusted as pre-bypass parameters to be continued in the ICU and till fulfilling the criteria for weaning from mechanical ventilation which include appropriate sensorium, haemodynamic stability with CI of > 2.1 L/min/m 2 , minimal chest tube output, urine output > 0.5 ml/kg/hr, temperature > 35.5 o C and stable ventilatory parameters with PO 2 > 60mmHg, PCO 2 < 40 mmHg, pH 7.36-7.4, SpO 2 > 95% all this while maintained on pressure support ventilation for at least of 15 min and maintaining stability.
Data were analysed using a statistical software package (GraphPad InStat; version 3.00 for Windows, GraphPad Software Inc., San Diego, California, USA). Data expressed as mean (SD) unless otherwise indicated. One way analysis of variance (ANOVA) was used to compare the mean values between the studied groups. For significant findings a post-ANOVA pair wise comparisons of means was conducted. Chi-square test and student's t-test were applied when appropriate. P values < 0.05 were considered significant.
| Results|| |
Results showed that the three groups were comparable with regard to patient demography [Table 1], anaesthetic requirements, and surgical management (Number of grafts and Total CPB time) [Table 2].
Considering the mean values of PO 2 and PCO 2 , results showed no statistical significance to the baseline values or intergroup differences at any time of recording [Table 3].
Regarding the mean alveolar-arterial oxygen difference [P (A-a) DO 2 ], results revealed insignificant intergroup differences at any time point of assessment or to the baseline values [Table 3].
For the calculated intrapulmonary shunt fraction, it was significantly high in group I (VCM) and group III (PEEP) 30 minutes after ICU admission and at 4 hours post-bypass when compared to the baseline values (P < 0.01 , < 0.05 respectively), while in group II (CPAP) the mean shunt fraction increased significantly after chest closure compared with the baseline value (P < 0.05). While, there was no significant intergroup difference at any time point of recording (P > 0.05) [Table 3].
Regarding the DLC, the 3 studied groups showed insignificant differences whether to baseline values or with intergroup comparison at any definite point of recording [Table 3].
The postoperative parameters were comparable in the three studied groups with no significant statistical differences [Figure 1].
| Discussion|| |
There is good evidence that early extubation is safe and well tolerated after cardiac surgery, while it requires identifying eligible patients and adapting both surgical and anaesthetic management to serve this process  . Ventilatory management providing optimum gas exchange parameters is crucial to achieve early extubation, specially when it was proved that impaired pulmonary gas exchange was found to be a major consequence after cardiac surgical procedures performed under the management of CPB  .
Positive ventilatory pressure was proposed to have a role in improvement of gas exchange explained by the reduction in lung water which in return provides a more compliant interstitial space allowing better gas exchange  . This was supported by many clinical trials one of which studied 14 patients, 7 being subjected to CPAP during CPB, while the other control 7 patients, the lungs were kept deflated all through the CPB time, and they demonstrated that application of CPAP 10 cmH 2 O during CPB provides better oxygenation and less shunt fraction than their control group  . On the other hand, other trials failed to prove such effect of CPAP application during CPB. That in one of the clinical studies they failed to demonstrate positive results with application of CPAP of 5 cmH 2 O during CPB  , also others found that low levels of CPAP being applied during CPB did not improve whether the oxygenation or the mechanical ventilatory parameters  .
While considering the VCM which was proved to have a beneficial role in gas exchange and improvement in lung compliance in an experimental study done on pigs models demonstrated that VCM effectively prevents post-CPB atelectasis (10,11), also in another clinical trail on humans undergoing cardiac surgical procedure comparing the VCM to a control group where no re-expansion manoeuvre was used , and their results showed a significant difference in ventilatory parameters specially in shunt fraction  . In the present study the comparison was made between the application of both VCM and CPAP, and our results showed no statistical significant differences in the ventilatory parameters between the two positive pressure ventilatory methods applied , apart from the increase in the shunt fraction which was significantly higher than the baseline in both groups, yet it was within the normal physiological range (2-5 %)  thus with no clinical significance.
The influence of the application of PEEP after CPB has not been previously studied in humans undergoing cardiac surgery, which may be due to the fear of its deleterious haemodynamic effects in such a critical period, and that was the idea in the present study of applying a minimal pressure of 7 cmH 2 O in a trial to fasten the slow re-expansion of alveoli in the post-bypass period and to compare its ventilatory effects with other ventilatory patterns being applied. PEEP when applied was readily tolerated by the patients and showed a comparable effects on the ventilatory parameters compared to the other two studied groups.
Considering the postoperative parameters recorded, the studied groups were comparable regarding the extubation time and length of ICU, that from the ventilatory point of view, their parameters were maintained all through the postoperative period, thus once fulfilling the criteria for extubation, weaning from mechanical ventilation was conducted.
The present study showed that different ventilatory patterns could be applied to maintain lung functions all through the perioperative period and the comparison was done to the VCM as a ventilatory pattern being now the standard technique applied in patients ongoing weaning off CPB and before resuming mechanical ventilation.
Based on clinical evidence a CPAP when applied all through CPB time will eventually prohibit the conventional endotracheal suctioning before ongoing weaning off the CPB which may have a prophylactic role in preventing mechanical lobular and even lobar lung collapse. This suctioning manoeuvre when applied will alleviate all the beneficial effects of CPAP during CPB. On the other hand, application of PEEP in the post-bypass period may have undesirable haemodynamic effects that may result from application of such positive pressure in the immediate post-bypass period with its well known criticality from the haemodynamic point. Here, the VCM could be considered a safe and practical positive pressure ventilatory method if being applied cautiously so that over inflation of the lungs do not interrupt the course of the left internal mammary artery (LIMA) after being harvested and anastomosed to the native coronary vessel.
In conclusion, this study has shown that application of VCM at the end of CPB and before resuming mechanical ventilation gives comparable ventilatory parameters as for the application of positive ventilatory pressure whether being applied during (CPAP) or after (PEEP) the CPB. Thus, the application of such manoeuvre will provide a safe and reliable maintenance in gas exchange parameters of patients being subjected to CPB. The application of minimal PEEP in the post-bypass period in an attempt to help in alveolar recruitment and prevention of atelectasis still needs further clinical investigations.
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[Table 1], [Table 2], [Table 3]