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Three Part Question

In [patients undergoing cardiac surgery] is [CPAP or continued ventilation] more beneficial than lung deflation in [improving post-operative lung function].

Clinical Scenario

You are about to perform four coronary arterial bypass grafts on a 78-year-old ex steel worker. He has a 60 pack per year history of smoking and his lung function tests are significantly abnormal with an FEV1 only 40% of his predicted values. His coronary arterial targets are small and you feel that an on-pump technique is the only option. You discuss the case with the anaesthetist and he asks whether he could keep ventilating while the patient is on bypass in order to improve his post-operative lung function. You have significant concerns that this may make the case even more difficult but rather than refusing this request you resolve to search the literature for evidence that this will improve post-operative lung function as your anaesthetist suggests.

Search Strategy

Medline 1966 to Jun 2005 using the OVID interface
[exp cardiopulmonary bypass/OR cardiopulmonary by-pass.mp] AND [exp ventilation/OR ventilation.mp OR exp lung/OR lung.mp] AND [exp respiratory function tests/OR respiratory function.mp OR gas exchange.mp OR oxygenation.mp OR lung function.mp].

Search Outcome

A total of 187 relevant papers were found from which 9 papers were selected as representing the best evidence on this topic

Relevant Paper(s)

Author, date and country	Patient group	Study type (level of evidence)	Outcomes	Key results	Study Weaknesses
Lamarche et al, 2004, Canada	Six groups (n=5) of 30 Landrace swine: 1. Control 2. Without CPB 3. CPB 150 min with no reperfusion 4. CPB 150 min with reperfusion 60 min 5. CPB 150 min with ventilation with reperfusion 60 min 6. CPB 150 min with NO inhalation + ventilation with 60 min of reperfusion	Experimental study (level 5)	Serum ostoecalcin	Decreased in both groups by 30%. (P<0.05).	No cross clamping of the aorta No cardioplegia used
			Endothelial function	Endothelium-dependant relaxations to acetylcholine is prevented by ventilation during CPB. Ventilation and NO inhalation during CPB has the same effect on endothelium as ventilation alone
			Pa02	Non-ventilated animals had a decrease in the PaO2 following separation from CPB, the PaO2/FiO2 being as low as 166. In the ventilation group, this ratio was never lower than 330.
			N-telopeptide cross-links	Decreased only in the calcium group by 30% (P<0.05).
			Fractional calcium absorption	Increased by 34% in the calcitriol group.
			Spinal and femoral neck bone mineral densities	No difference in both groups.
			Calcitriol increases fracture rate	32% 2 year incidence.
Magnusson et al, 1998, Sweden	In 6 pigs, CPAP with 5 cm H2O pressure was applied during CPB In another 6 pigs the lungs were open to the atmosphere during CPB	Experimental study (level 5)	Incidence of hip fracture.	47 persons (control group) and 50 (vitamin D3 group). No difference in two groups (p=0.66, log-rank test).
			V-Q distribution	Intrapulmonary shunt increased by and PaO2 decreased after CPB in both groups
			V-Q distribution	Intrapulmonary shunt increased by and PaO2 decreased after CPB in both groups
			Incidence of hip fracture.	47 persons (control group) and 50 (vitamin D3 group). No difference in two groups (p=0.66, log-rank test).
			Incidence of all non-vertebral fractures.	76 persons (control group) and 69 (vitamin D3 group). No difference in two groups (p=0.60, log-rank test).
			Post-op atelectasis by CT scan	14.5±5.5% in the CPAP group and 18.7±5.2% in the controls (P=0.20)
			Post-op atelectasis by CT scan	14.5±5.5% in the CPAP group and 18.7±5.2% in the controls (P=0.20)
			Incidence of all non-vertebral fractures.	76 persons (control group) and 69 (vitamin D3 group). No difference in two groups (p=0.60, log-rank test).
Massoudy et al, 2003, Germany	34 consecutive patients undergoing CABG Group 1 ECC: 24 pts had CBP using bilateral extra-corporeal circulation (ECC: additional cannulation of pulmonary artery left atrium and lungs perfused and ventilated during bypass) Group 2 CPB: 10 were operated using conventional CPB	Cohort study (level 3b)	Extravascular thermal volume	Increase from 4.8±0.2 ml/kg to 6.7±0.4 ml/kg, with conventional CPB but remained unchanged	Only transient and clinically small improvement in PaO2/FiO2 ratio was found
			Extravascular thermal volume	Increase from 4.8±0.2 ml/kg to 6.7±0.4 ml/kg, with conventional CPB but remained unchanged
			Haemodynamic parameters	No significant differences in Cardiac Index PA pressure or SVRI 4 h post-operatively
			Haemodynamic parameters	No significant differences in Cardiac Index PA pressure or SVRI 4 h post-operatively
			Intra-op fluid balance	+1,955±233 ml in group 1 vs +2,654±210 ml (P<0.05) in group 2
			Intra-op fluid balance	+1,955±233 ml in group 1 vs +2,654±210 ml (P<0.05) in group 2
			Inflammatory cytokines	No difference
			Inflammatory cytokines	No difference
Gilbert et al, 1996, USA	18 patients undergoing elective CABG with CPB Group 1: CPAP applied to 9 patients during CPB Group 2: No CPAP applied to 9 patients during CPB	Cohort study (level 3b)	Lung resistance and elastance	Increased equally after CPB in both groups. The increase was relatively less with intact pleurae or net negative fluid balance
			Lung resistance and elastance	Increased equally after CPB in both groups. The increase was relatively less with intact pleurae or net negative fluid balance
Loeckinger et al, 2000, Austria	14 patients undergoing elective cardiac surgery Group 1:7 patients received CPAP at 10cm H2O during CPB Group 2: In 7 patients, the lungs were open to the atmosphere	PRCT (level 2b)	V-Q distribution	More perfusion of lung areas with a normal V/Q distribution and less shunt in CPAP group	No difference in post-operative outcome found although study was too small to find any clinical outcome differences
			V-Q distribution	More perfusion of lung areas with a normal V/Q distribution and less shunt in CPAP group
			PaO2 4 hours post-surgery	Controls: PaO2 99±9 mmHg CPAP PaO2 123±23 mmHg P<0.05
			PaO2 4 hours post-surgery	Controls: PaO2 99±9 mmHg CPAP PaO2 123±23 mmHg P<0.05
			Shunt fraction	Controls: 8.1±3.7 CPAP: 14.1±46 P<0.05
			Shunt fraction	Controls: 8.1±3.7 CPAP: 14.1±46 P<0.05
			Haemodynamic variables	CI, SVRI, CVP, MAP all similar between groups
			Haemodynamic variables	CI, SVRI, CVP, MAP all similar between groups
Cogliati et al, 1996, Italy	30 patients undergoing elective CABG Group 1 (n=101): lungs deflated Group 2 (n=10): PEEP=5 cm H2O and FiO2=1.0 Group 3 (n=10): PEEP=5 cm H2O and FiO2=0.21	PRCT (level 2b)	PaO2, P(A-a)O2, Qs/Qt and Cstat	Minor impairment of gas exchange in the third group of patients. Lung inflation with air effectively preserves respiratory system mechanics
			PaO2, P(A-a)O2, Qs/Qt and Cstat	Minor impairment of gas exchange in the third group of patients. Lung inflation with air effectively preserves respiratory system mechanics
Berry et al, 1993, Australia	61 patients undergoing elective CABG Group 1 (n=17) No CPAP during CPB Group 2 (n=22): 5cm H2O CPAP (FiO2 0.21) Group 3 (N=22) 5 cm H2O of CPAP (FiO2 1.0)	PRCT (level 2b)	P(A-a)O2 measured at 30 mins	Group 1: 43.3 kPa Group 2: 35.5 kPa Group 3: 36.8 kPa P=0.036	No sample size calculations given to support the null hypothesis that there is no difference between groups
			P(A-a)O2 measured at 30 mins	Group 1: 43.3 kPa Group 2: 35.5 kPa Group 3: 36.8 kPa P=0.036
			P(A-a)O2 measured at 4 hours	Group 1: 28.7 kPa Group 2: 35.5 kPa Group 3: 28.3 kPa P=0.32
			P(A-a)O2 measured at 4 hours	Group 1: 28.7 kPa Group 2: 35.5 kPa Group 3: 28.3 kPa P=0.32
			Time to extubation	Not affected by the use of CPAP
			Time to extubation	Not affected by the use of CPAP
Zabeeda et al, 2003, Israel	75 patients undergoing CABG (15 patients in each group) Group 1:high frequency low volume ventilation with FiO2 1.0, frequency 100 breaths per min Group 2:high frequency low volume ventilation with FiO2 0.21 Group 3:5cm H2O CPAP with FiO2 1.0 Group 4: 5 cm H2O CPAP with FiO2 0.21 Group 5: disconnected from ventilator	Single blind PRCT (level 2b)	Compliance and mean airway pressures	No difference	Number of grafts and aortic cross clamping times significantly shorter in groups 1 and 5
			PaO2 and P(A-a)O2	Group 3 had higher PaO2 and lower P(A-a)O2 5 min after weaning from CPB. but this became non-significant after chest closure
			Extubation time	Similar in all groups
Boldt et al, 1990, Germany	90 patients undergoing CABG (15 patients in each group) Group 1:lungs collapsed Group 2:CPAP with PEEP 5 cm H2O and with FiO2 1.0 Group 3: 5 cm H2O CPAP with FiO2 0.21 Group 4:CPAP with PEEP 15 cm H2O CPAP with FiO2 1.0 Group 5: CPAP with PEEP 15 cm H2O and FiO2 0.21 Group 6: mechanical ventilation PEEP 5 cm H2O and FiO2 1.0	Single blind PRCT (level 2b)	Extravascular lung water	Group 4 had an increase of 35% and Group 5 had an increase of 45% in extravascular lung water as compared to other groups. This remained higher at 5 h post surgery
			PaO2	Greatest decrease in groups 4 (–109 mmHg) and group 5 (–130 mmHg). It was least pronounced in group 3 (–33 mmHg)

Comment(s)

Seven clinical studies were found in 202 patients. The studies investigated a wide variety of ventilatory strategies during cardiopulmonary bypass (CPB) including CPAP with positive airway pressures of 5–15 cm H2O, high frequency low volume ventilation, inspired oxygen concentrations from 21% to 100% and bilateral extra-corporeal circulation using the lungs to oxygenate the blood while on bypass. In addition, two relevant animal studies were also included. In an experimental study in pigs by Lamarche et al, ventilation during CPB prevented the occurrence of endothelial dysfunction arising after reperfusion of the pulmonary arterial tree. It also significantly improved the PaO2/FiO2 ratio. However, this model did not include aortic cross-clamping and cardioplegic arrest. Magnusson et al, assessed the utility of continuous positive airway pressure (CPAP) in 6 pigs. Unfortunately no difference in either the intrapulmonary shunt fraction or the occurrence of atelectasis post-operatively as demonstrated by CT scanning was found. In a non-randomised series of 34 low-risk patients undergoing conventional elective CABG patients, Massoudy et al demonstrated that the increase in extravascular thermal volume noted after conventional CPB was not observed in patients undergoing bilateral extra-corporeal circulation with continuous pulmonary perfusion and ventilation. This technique (the Drew Technique) uses the patients own lungs to oxygenate the lungs while on CPB. However, inflammatory markers, SVRI, CI, and PaO2/FiO2 ratio were not significantly improved post-surgery. Gilbert et al performed a study comprising of 18 patients undergoing CABG. Nine patients had CPAP during CPB and 9 patients did not. However, no difference in lung resistance was noted between patients receiving CPAP and those receiving no CPAP. The authors concluded that low levels of CPAP applied during CPB did not significantly change either mechanical properties or oxygenation. Loeckinger et al reported in an RCT of 14 patients undergoing elective cardiac surgery that CPAP at 10 cm H2O during CPB improves postoperative gas exchange. At 4 h post-surgery arterial oxygenation was improved by about 20%, the shunt fraction was halved, and blood flow to underventilated areas of lung was significantly reduced. They concluded that the simple manoeuvre of 10 cm H2O of CPAP was a simple intervention to improve post-operative respiratory function. In an RCT of 30 patients undergoing elective myocardial revascularization, Cogliati et al showed that lung inflation with air (static inflation with PEEP=5 cm H2O and FiO2=0.21) during CPB, effectively preserved respiratory system mechanics as compared to deflated lungs or lungs inflated with 100% O2. While all groups showed a deterioration in lung function post-operatively, this deterioration was least significant in the CPAP group with FiO2 of 21%. Berry et al randomised 61 patients to no CPAP, CPAP at 5 cm H2O ventilating with air or CPAP at the same level with 100% oxygen. They found that alveolar-arterial oxygen partial pressure difference (PAO2-PaO2) was improved for the CPAP groups at 30 min post-surgery but not at 4 or 8 h. The time to extubation and early extubation were also not affected by the use of CPAP. In addition, there were several cases where the surgeon demanded that the lungs be deflated due to difficult surgical access. Zabeeda et al studied 75 patients who were split into 5 groups receiving CPAP, and high frequency ventilation with either 21% or 100% inspired oxygen. The alveolar-arterial oxygen gradient was lower and the PaO2 was higher 5 min after bypass in patients receiving CPAP (100% O2), but this difference disappeared on chest closure and post-operatively. No other differences were found between groups in this study. Berry et al studied 90 patients undergoing CABG using a range of CPAP, continuous ventilation and non-ventilated techniques. There was an increase in extravascular lung water in all groups, and decrease in post-operative oxygenation, being most pronounced in patients with 15 cm H2O of CPAP as compared to those with 5 cm H2O of CPAP or controlled mechanical ventilation. No significant improvements compared with no ventilation were demonstrated.

Clinical Bottom Line

A wide range of ventilatory strategies while on cardiopulmonary bypass have been attempted including CPAP with positive airway pressures of 5–15 cm H2O, high frequency low volume ventilation (100 breaths per min), inspired oxygen concentrations from 21% to 100% and bilateral extra-corporeal circulation using the lungs to oxygenate the blood while on bypass. While some small and transient benefits for CPAP with 10 cm H2O have been demonstrated no convincing clinical benefits for any of these strategies have been shown and thus ventilation while on Cardiopulmonary bypass cannot be supported as a strategy to improve post-operative lung function.

References

1. Lamarche Y, Gagnon J, Malo O, Blaise G, Carrier M, Perrault LP. Ventilation prevents pulmonary endothelial dysfunction and improves oxygenation after cardiopulmonary bypass without aortic cross-clamping. Eur J Cardiothorac Surg 2004;26:554–563.
2. Magnusson L, Zemgulis V, Wicky S, Tyden H, Hedenstierna G. Effect of CPAP during cardiopulmonary bypass on postoperative lung function. An experimental study. Acta Anaesthesiol Scand 1998;42:1133–1138.
3. Massoudy P, Piotrowski JA, van de Wal HCJM, Giebler R, Marggraf G, Peters J, Jakob HG. Perfusing and Ventilating the patient's lungs during bypass ameliorates the increase in extravascular thermal volume after coronary bypass grafting. Ann Thorac Surg 2003;76:516–522.
4. Gilbert TB, Barnas GM, Sequeira AJ. Impact of pleurotomy, continuous positive airway pressure, and fluid balance during cardiopulmonary bypass on lung mechanics and oxygenation. J Cardiothorac Vasc Anesth 1996;10:844–849.
5. Loeckinger A, Kleinsasser A, Lindner KH, Margreiter J, Keller C, Hoermann C. Continuous positive airway pressure at 10 cm H(2)O during cardiopulmonary bypass improves postoperative gas exchange. Anesth Analg 2000;91:522–527.
6. Cogliati AA, Menichetti A, Tritapepe L, Conti G. Effects of three techniques of lung management on pulmonary function during cardiopulmonary bypass. Acta Anaesthesiol Belg 1996;47:73–80.
7. Berry CB, Butler PJ, Myles PS. Lung management during cardiopulmonary bypass: is continuous positive airways pressure beneficial. Br J Anaesth 1993;71:864–868.
8. Zabeeda D, Gefen R, Medalion B, Khazin V, Shachner A, Ezri T. The effect of high-frequency ventilation of the lungs on postbypass oxygenation: A comparison with other ventilation methods applied during cardiopulmonary bypass. Cardiothorac Vasc Anesth 2003;17:40–44
9. Boldt J, King D, Scheld HH, Hempelmann G. Lung management during cardiopulmonary bypass: influence on extravascular lung water. J Cardiothorac Anest 1990;4:73–79.