The effect of ventilation on systemic blood gases in the presence of left ventricular ejection during cardiopulmonary bypass

J THoRAc CARDIOVASC SURG 90:287-290, 1985

The effect of ventilation on systemic blood gases in the presence of left ventricular ejection during cardiopulmonary bypass The effectof pulmonary ventilation uponsystemic arterial blood gases duringcardiopulmonary bypass in the presence of left ventricular ejection was evaluated in 20 adult male patients undergoing coronary artery bypass grafting. Following rewarming, establishment of a sinus rhythm,and production of a pulse

pressure of at least 20 mm Hg on the arterial pressure trace caused by left ventricular ejection, arterial blood gases were obtained from the arterial and venous extracorporeal circuits and the radial arterial cannula. Patients werethen randomly assigned to a nonventilation (n = 10)or a ventilation (n = 10)group. The ventilation group was given 10 breaths/min with 100% oxygen at a tidal volume of 10 ml/kg, Whereas the nonventilation group received apneic oxygenation at zero end-expiratory pressure. Mter 5 minutes the arterial blood gas data wereagain obtained. Significant fmdings (p < 0.05)included decreases in systemic carbondioxide tension and increases in systemic pH in the ventilation groupand decreases in systemic oxygen tension in the nonventilation group. Although the changes in the arterial blood gases were significant, thesechangesoccurredwell within the limitsof clinical acceptability. It is concluded that left ventricular ejection for short periods duringfuD cardiopulmonary bypass does not necessitate pulmonary ventilation.

Roger A. Moore, M.D., John D. Gallagher, M.D., Bruce P. Kingsley, M.D.,* Gerald Lemole, M.D., Deanna Kerns, B.S., and Donald L. Clark, M.D., Browns Mills. N. J.

During cardiopulmonary bypass pulmonary ventilation is commonly withheld until phasic increases in the arterial blood pressure (override) are noted, which indicate left ventricular ejection.' Presumably ventilation during left ventricular ejection allows gas exchange in blood flowing through the pulmonary circulation that has bypassed the extracorporeal circuit. However, ventilation can obstruct the surgical field, which makes surgical repair more difficult. This study was designed to evaluate the effect of ventilation on systemic blood gases in the presence of override during cardiopulmonary bypass. From the Departments of Anesthesiology and Thoracic and Cardiovascular Surgery, Deborah Heart and Lung Center, Browns Mills, N. 1., and the Department of Anesthesiology, University of Pennsylvania. Supported by a grant from the Deborah Foundation. Received for publication Sept. 6, 1984. Accepted for publication Nov. 6, 1984. Address for reprints: Roger A. Moore, M.D., Department of Anesthesia, Deborah Heart and Lung Center, Trenton Road, Browns Mills, N. J. 08015.

*Attending anesthesiologist, Center, Mesa, Ariz.

Deseret Samaritan Hospital and Health

Materials and methods After institutional review board approval and routine individual patient consent were obtained, 20 adult patients scheduled for coronary artery bypass grafting who fulfilled the study criteria were admitted into the study group. Study criteria included being male between 50 and 75 years of age, no evidence of cardiac valve dysfunction, a left ventricular ejection fraction greater than 50%, and no history of pulmonary disease. One hour prior to the scheduled operation all patients were premedicated with morphine 0.1 mg/kg (maximum 10 mg) and scopolamine 0.01 mg/kg (maximum 0.4 mg) administered intramuscularly. When patients arrived at the operating room, routine monitoring was established, which included a radial arterial catheter, a flow-directed pulmonary arterial catheter with four lumina, electrocardiographic leads II and V s, and rectal and nasopharyngeal temperature probes. After the radial arterial catheter was inserted, a nitroglycerin infusion was started at a rate of 0.5 J.Lg/kg/min. Fentanyl, diazepam, and pancuronium bromide were used as anesthetics. Extracorporeal circulation (ECC) was established with a Shiley S-100A bubble oxygenator (Shiley Inc., 287

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and venous circuits of the oxygenator and from. the radial artery catheter were obtained (presampling point). Blood samples were placed on ice immediately and analyzed within one-half hour for pH, Pco., and POz values with the Instrumentation Laboratory system 1303 blood gas pH analyzer (Instrumentation Laboratory Inc., Lexington, Mass.) and oxygen saturation and hemoglobin with the Instrumentation Laboratory 282IL Co-Oximeter. Patients were then randomly assigned to either a ventilation (VE) or nonventilation (NVE) group. Patients in the VE group received two large breaths of 100% oxygen to expand the lungs and were ventilated at a rate of 10 breaths/min and a tidal volume of 10 ml/kg measured by Wright's spirometry. Patients in the NVE group continued to receive apneic oxygenation at zero airway pressure. After 5 minutes, hemodynamic, temperature, and blood gas data were obtained for both groups (postsampling point). Hemodynamic data in the VE group were obtained at end-expiration. Statistical comparisons of demographic information between the VE and NVE groups were performed with the unpaired t test. Comparisons of hemodynamic, temperature, and blood gas data within each group (i.e., presampling versus postsampling point) and between groups were performed by two-factor mixed-design analysis of repeated measures; significant F ratios were evaluated with a Bonferroni test. Significance was considered to exist with a p value less than 0.05.

Irvine, Calif.); blood flows were maintained over 2.0 L/rnin/m. 2 Gas flow to the oxygenator was kept within 1 L of the total blood flow with 100%oxygen (no carbon dioxide was added). During ECC pulmonary management consisted of apneic oxygenation with oxygen flowed into the anesthetic circle system at 0.2 L/min and a zero circuit pressure. At the end of ECC and after rewarming to a rectal temperature greater than 34 0 C, removal of the aortic cross-clamp, and establishment of a regular cardiac rhythm, the extracorporeal venous return was partially obstructed until left ventricular ejection was evident on the radial arterial pressure trace. Venous obstruction was adjusted to provide at least a 20 mm Hg pulse pressure difference between systolic and diastolic pressure (override) superimposed on the pressure produced by the continuing ECC. Extracorporeal blood flow was maintained over 2 L/min/m z and Ringer's lactate solution was added to the extracorporeal circuit if necessary. Throughout the remainder of the study the heart was not manipulated, no drugs were given, and extra corporeal blood and gas flows were held constant. After 5 minutes of at least 20 mm Hg override, systemic arterial blood pressures (systolic, mean, and diastolic), pulmonary arterial blood pressures (systolic and diastolic), central venous pressure (mean), heart rate, nasal and rectal temperatures, and blood pH, carbon dioxide tension (Pco.), oxygen tension (Po.), oxygen saturation, and hemoglobin from the arterial

Table I. Demographic data for ventilation (VE) and nonventilation (NVE) groups

VE NVE

Age (yr)

Weight (kg)

59.g ± 1.94 62.10 ± 2.59

gO.25 ± 3.76 80.47 ± 1.96

Bypass grajts

Perfusion blood flow

(no.)

iLfmin)

Perfusion oxygen flow (L/minl

Override (mm Hg)

± 0.2 3.5 ± 0.48

4.17 ± 0.17 4.45 ± 0.19

4.11 ± O.3g 4.3 ± 0.26

36.4 ± 3.52 35.0 ± 2.72

n

Legend: 1\11 values arc expressed as the mean of the sample ± the standard error of the mean.

Table II. Hemodynamic and temperature data for ventilation (VE) and nonventilation (NVE) groups at the presampling and postsampling points Systemic arterial pressure (mm Hg) Systolic

VE Presampling point Postsampling point

I

Mean

I

Diastolic

Pulmonary artery pressure tmm Hg) Systolic

I

Diastolic

Central venous pressure (mm Hg)

99 ± 4.37 97.6 ± 4.2\

73.7 ± 2.12 70.1 * ± 2.19

62.9 ± 2.13 60.4 ± 2.00

15.7 ± 1.58 14.9 ± 1.53

11.2± 1.21 11.8 ± 1.32

7.60 ± 1.7\ 7.7 ± 1.65

95.4 ± 4.97 85.8* ± 4.34

69.1 ± 3.67 64.1* ± 3.48

60.4 ± 4.21 54.7* ± 3.01

13.0 ± 2.29 13.9 ± 2.29

8.60 ± 1.77 9.0 ± 1.76

6.40 ± 1.69 6.40 ± 1.97

NVE Presampling point Postsampling point For legend see Table I. *Postsampling value significantly less than presampling. value (p

< 0.05).

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Fig. 1. The mean with standard error of the mean pH, Pco., P0 2 and oxygen saturation obtained from the extracorporeal arterial and venous cannulas as well as the radial arterial cannula for patients in the ventilation and nonventilation groups. Significant changes (p < 0.05) between presampling and postsampling point values are indicated (*).

Results Temperature

r C)

Heart rate (beats/min)

Rectal

Nasopharyngeal

79.9 ± 4.43 83.4 ± 4.68

35.5 ± 0.22 35.9 ± 0.18

37.6 ± 0.31 37.7 ± 0.30

70.9 ± 3.54 73.7 ± 3.64

35.8 ± 0.29 35.8 ± 0.29

38.0 ± 0.26 37.7 ± 0.21

Demographic comparisons of the VE and NVE groups failed to show statistically significant differences between the two groups (Table I). Comparison of the hemodynamic and temperature data between the VE and the NVE group at both the presampling and postsampling data points failed to show significant differences between the two groups (Table II). Withingroup comparisons of the hemodynamic and temperature data between the presampling and postsampling data points revealed a significant fall (p < 0.05) in the

The Journal of Thoracic and Cardiovascular Surgery

290 Moore et ai.

systolic, mean, and diastolic systemic pressures for the NVE group and a fall (p < 0.05) of the mean systemic pressure in the VE group (Table II). Between-group comparisons (VE and NVE) of blood gas data at the presampling period showed no significant differences (p > 0.05). Between-group comparisons of blood gas data at the postsampling period showed significant differences only in the radial arterial blood gas data in which the VE group was found to have a higher pH (p < 0.01 and a lower Pco, (p < 0.01) than the NVE group. Within-group comparisons between presampling and postsampling period blood gas data for the VE group showed significant decreases in Pco, (p < 0.01) and increases in pH (p < 0.01) for arterial, radial arterial, and venous blood gases (Fig. 1). Within-group comparisons between presampling and postsampling period blood gas data for the NVE group showed a significant decrease in the P0 2 (p < 0.01) for radial arterial blood gases (Fig. 1). Comparisons of hemoglobin concentrations within and between groups failed to show significant differences. Mean hemoglobin concentrations at all study points were between 7.5 and 8.0 gmjlOO ml.

override is blood returning to the heart from the venae cavae. In our study vena caval return to the heart was artificially increased with partial obstruction of the extracorporeal venous return. The end point we chose was at least 20 mm Hg of override. Presumably, if the ejected blood was not ventilated, systemic deoxygenation and carbon dioxide retention would result. In our study the NVE group did have a significant fall in systemic oxygenation, but the resultant arterial P02 was greater than 300 mm Hg. Therefore, ventilation in the presence of override had little if any clinical effect on systemic oxygenation. In addition, ventilated patients in our study showed a significant increase in pH and fall in Pco, which indicated that carbon dioxide removal was enhanced by ventilation of the shunted blood. Since the radial arterial Pco, prior to ventilation was between 30 and 40 rnm Hg, the clinical significance of10wering the Pco, further in these patients was minimally beneficial and may have led to increased peripheral vascular resistance." Therefore, ventilation of the lungs during short periods of left ventricular ejection during ECC had little clinical significance.

Discussion

We give special thanks to the staff of the Surgery, Pulmonary, and Perfusion Departments.

A variety of techniques have been studied for the management of the lungs during hypothermic cardiopulmonary bypass to minimize postoperative pulmonary dysfunction."? Use of positive-pressure ventilation throughout ECC has been related to decreased pulmonary compliance':" 8 and increased pulmonary shunt. 6-8 Because of this, either static inflation or passive deflation of the lungs is usually used; neither method has proved superior.r" Ventilation is resumed either just prior to weaning from cardiopulmonary bypass or when the left ventricle receives sufficient volume to provide consistent ejection.' In the latter situation ventilation allows gas exchange of blood that bypasses the extracorporeal oxygenator. Blood filling the left ventricle during ECC in the presence of a competent aortic valve is made up of blood from the thebesian vessels, bronchial collateral branches, coronary sinus, and venae cavae. The combined blood flow from the thebesian vessels, bronchial collateral branches, and coronary sinus is significantly desaturated and hypercarbic.' If sufficient amounts of this blood were ejected from the left ventricle, elevations in systemic carbon dioxide and decreases in systemic oxygenation would be expected. However, the total amount of blood from these sources is relatively small and the quantity is insufficient to allow consistent left ventricular ejection.' The primary source of ejected blood during

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