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PCO2
From the Department of Anesthesiology, Research Center, Montreal Heart Institute, 5000 Belanger Street East, Montreal, Quebec, H1T 1C8, Canada.
Address correspondence to: Dr. A. Denault, Phone: 514-376-3330; Fax: 514-376-1355; E-mail: denault{at}videotron.ca
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Abstract |
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PCO2), may be used as index to evaluate the adequacy of the cardiac output to the oxygen consumption. To determine the incidence of elevated PCO2 and its relationship with difficult separation from bypass (DSB) in patients undergoing cardiac surgery, we conducted a prospective observational cohort study. Methods: Data were collected from 58 consecutive patients undergoing various cardiac operations requiring cardiopulmonary bypass (CPB). During the procedure, arterial and venous blood gases and lactate were sampled. Blood was drawn after induction of anesthesia, during bypass and at the closure of the chest wall. Difficult separation from bypass was defined as a systolic arterial pressure < 80 mmHg, and diastolic pulmonary artery pressure > 15 mmHg during progressive separation from CPB with inotropic or mechanical support of cardiac function, or hemodynamic instability resulting in reintroduction of extra-corporeal circulation or insertion of an intra-aortic balloon pump.
Results: In our study, 65% of the samples were associated with elevated PCO2 (>6mmHg). Variables associated with difficult weaning were LVEF, duration of bypass and aortic cross-clamping, pre-bypass PCO2 and in-bypass lactate values (P < 0.05). Multivariable analysis identified the pre-bypass PCO2 and the duration of bypass as predictors of DSB.
Conclusion: Elevated PCO2 is frequently observed during cardiac surgery and values obtained before bypass were associated with DSB. The PCO2 gradients could be used as marker of the adequacy of tissue perfusion during cardiac surgery.
VENO-ARTERIAL PCO2 difference (PCO2) is an index that may be used to evaluate the adequacy of the cardiac output (CO)1 and to identify the critical oxygen delivery point below which the oxygen consumption-delivery (VO2/DO2) relationship becomes dependant.2,3
The role and importance of PCO2 in cardiac surgery has been studied only in the postoperative period.1,4–7 We report our pre-bypass observations on the relationship between PCO2 and the risk of difficult separation from bypass (DSB).
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Methods |
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Patient's age, sex, height, weight and pre-operative left ventricular ejection fraction (LVEF) were recorded. Standard monitoring for cardiac surgery was used. The CPB circuit was primed with Ringer's lactate. Arterial and venous blood gases were drawn after induction of anesthesia and every 20 min during cardiopulmonary bypass (CPB). During CPB, the CO was maintained at 2.4 L•m-2 or according to the mean arterial pressure. A bubble oxygenator, or a membrane in high-risk patients, was used according to the surgeon's choice. Vesical temperature was maintained between 32-34°C during CPB. Hypothermia (21°C) was required in two aortic procedures. After collecting arterial and mixed venous blood gases from the distal port of the pulmonary artery catheter, samples were analyzed at 37°C using 
-stat management. (STAT-9, Nova biomedical, Waltham, MA). The manufacturer's precision for PCO2 is 2.4% (0.6 to 1.6 mmHg for values between 23.5 to 42.6 mmHg). In addition, lactate was obtained from the same sample. During the pre- and in-bypass periods, the maximal values of PCO2 and lactate were recorded. During CPB, the CO was noted at the time of the blood gas withdrawal. CO determination before and after CPB was obtained by the thermodilution method. The durations of CPB, clamping time, duration of stay in the ICU and in the hospital were recorded and the patients were followed-up until discharge from hospital.
The normal value in humans for PCO2 is 4 to 6 mmHg.8 Clinicians were not blinded to PCO2 and lactate measurements, but the values were not used to guide the selection of vasoactive medication during separation from bypass. In addition, the anesthesiologists were unaware of the potential relationship between DSB and those indices at the time of the study. Delayed separation from bypass was defined as a systolic arterial pressure < 80 mmHg, and diastolic pulmonary artery pressure > 15 mmHg during separation from CPB without inotropic or mechanical support of cardiac function9 or hemodynamic instability resulting in the reintroduction of CPB or the insertion of an intraaortic balloon pump (IABP). In patients with DSB, a low peripheral arterial blood pressure was confirmed by central aortic pressure measurement. Patients requiring large amounts of vasopressors (norepinephrine or epinephrine >4 µg•min–1), inotropes (dobutamine >10 µg•kg–1•min–1 or the use of amrinone and milrinone) and/or IABP after CPB were included in the group DSB.
Statistical analysis
The number of patients to be included was based on a difference in 2 mmHg using an of 0.05 and a ß of 0.20. Based on these assumptions, 15 patients per group were necessary. Pre- and in-bypass variables were related with the presence or absence of DSB by using chi-square and Fisher's exact test. Continuous variables were entered in a logistic regression analysis to determine independent predictors of DSB. Univariate analysis were used for preliminary selection (alpha level of 0.25) of factors with P < 0.1 to be included in a stepwise hierarchical regression. The final model included all factors significant at the 0.05 alpha level. Age, gender, weight, and height were included in the model to attenuate potential sampling biases. Results are expressed as mean ± standard deviation.
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Results |
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: DSB was observed in 15 patients (26%): four died and five required installation of an IABP . Sixty-five percent of the samples were associated with elevated PCO2.
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PCO2 and duration of bypass as predictors of a DSB (Table II).
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Discussion |
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PCO2 in 37 patients, which was more than half of our cohort. In patients with DSB, we observed similar risk factor as previously described such as LVEF, duration of bypass and clamping. However, in those patients with DSB, the PCO2 before and during bypass was also higher. This is the first study to report such an association. These observations support a relationship with DSB and an unmatched VO2/DO2. This mismatch will be associated with an increase in PCO2.
Ariza studied 10 patients after CPB and found that the PCO2 was not associated with inadequate tissue perfusion: only two patients had elevated PCO2 but all their patients were low risk with good LVEF and none required inotropes post operatively.4 Lebuffe et al. and Ruokonen et al.1,6 observed an association between low systemic and regional blood flow and increased PCO2 after cardiac surgery and Cavaliere5 observed an association between post-op high PCO2 and complications. In a randomized study involving 393 patients using goal-oriented hemodynamic therapy, Pölönen et al.7 observed that PCO2 was higher in the control group and this group had higher morbidity and longer ICU stay.
There are several limitations to our study. This was an observational study. The PaCO2 is influenced also by CO2 production and alveolar ventilation.10 The adequate interpretation of PCO2 requires a steady state of these two factors. Finally, DSB was the only outcome measured.
In summary, elevated PCO2 is frequently observed during cardiac surgery. High pre-CPB PCO2 was an independent predictor for DSB. Further studies are required to confirm this observation, to determine the mechanism of its increase and the benefit of its correction during cardiac surgery.
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Acknowledgments |
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Accepted for publication November 1, 2000.
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References |
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2 Zhang H, Vincent J-L. Arteriovenous differences in PCO2 and pH are good indicators of critical hypoperfusion. Am Rev Respir Dis 1993; 148: 867–71.[Medline]
3 Van Der Linden P, Rausin I, Deltell A, et al. Detection of tissue hypoxia by arteriovenous gradient for PCO2 and pH in anesthetized dogs during progressive hemorrhage. Anesth Analg 1995; 80: 269–75.[Abstract]
4 Ariza M, Gothard JWW, MacNaughton P, Hooper J, Morgan CJ, Evans TW. Blood lactate and mixed venous-arterial PCO2 gradient as indices of poor peripheral perfusion following cardiopulmonary bypass surgery. Intensive Care Med 1991; 17: 320–4.[Medline]
5 Cavaliere F, Martinelli L, Guarneri S, Varano C, Rossi M, Schiavello R. Arterial-venous PCO2 gradient in early postoperative hours following myocardial revascularization. J Cardiovasc Surg 1996; 37: 499–503.[Medline]
6 Ruokonen E, Soini HO, Parviainen I, Kosonen P, Takala J. Venoarterial CO2 gradient after cardiac surgery: relation to systemic and regional perfusion and oxygen transport. Shock 1997; 8: 335–40.[Medline]
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Pölönen P, Ruokonen E, Hippeläinen M, Pöyhönen M, Takala J. A prospective, randomized study of goal-oriented hemodynamic therapy in cardiac surgical patients. Anesth Analg 2000; 90: 1052–9.
8 Adrogué HJ, Rashad MN, Gorin AB, Yacoub J, Madias NE. Assessing acid-base status in circulatory failure. Differences between arterial and central venous blood. N Engl J Med 1989; 320: 1312–6.[Abstract]
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Hardy J-F, Searle N, Roy M, Perrault J. Amrinone, in combination with norepinephrine, is an effective first-line drug for difficult separation from cardiopulmonary bypass. Can J Anaesth 1993; 40: 495–501.
10 Idris AH, Staples ED, O'Brien DJ, et al. Effect of ventilation on acid-base balance and oxygenation in low blood-flow states. Crit Care Med 1994; 22: 1827–34.[Medline]
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