(S-133) LACTIC ACIDOSIS AFTER CARDIOPULMONARY BYPASS - PATTERNS, OUTCOME AND CORRELATING FACTORS

(S-133) Naughton, P.J., Sunday 9:15

TITLE: LACTIC ACIDOSIS AFTER CARDIOPULMONARY BYPASS - PATTERNS, OUTCOME AND CORRELATING FACTORS

AUTHORS: Paul J. Naughton, MB Bch BAO, Michael S. O'Connor, DO, John P. Yared, MD
AFFILIATION: Cleveland Clinic Foundation, Cleveland, OH.

INTRODUCTION: Lactic acidemia is a marker for tissue ischemia and anaerobic metabolism. The peak serum lactate concentration has been shown to have prognostic implications regarding survival in a number of clinical states associated with low tissue perfusion including septic, cardiogenic and hemorrhagic shock, and acute pulmonary embolus¹. Unexplained lactic acidosis reportedly occurs in 15% of patients following cardiac surgery². We aimed to analyze the pattern and duration of lactic acidosis, the implications of, and assess for any influencing factors for its development in patients after cardiopulmonary bypass (CPB).
METHODS: In our intensive care unit (ICU), serum lactate is routinely measured in patients with metabolic acidemia after CPB, and followed every 6 hours until serum concentrations fall below 4mmol/L. We collected data from patients with lactic acidemia (serum lactate>4mmol/ L) over a six month period from May through October 1998. In addition to this data, preoperative and intraoperative factors were analyzed for each patient from our cardiothoracic database that is maintained on a daily basis by trained data assistants.
RESULTS: Data from a total of 60 patients was analyzed. The majority of patients, 40/60 (66%) had coronary artery bypass grafting (CABG). Combined CABG and valve surgery was performed on 23 patients. Table 1 indicates serum lactate levels at admission and at 6, 12, and 18 hours.

Table 1:

Time	Admission	6 Hours	12 Hours	18 Hours
Serum Lactate mmol/L
X±SD	9.7±3.6	7.8±4.4	4.6±2.6	3.2±3.1
(Range)	(4.4-20.6)	(1.7-24.2)	(1.1-12.2)	(0.8-22.6)

Mortality occurred in 3/60 (5%) patients. All 3 patients had a serum lactate >10mmol/L on admission to ICU, yielding a positive predictive value (ppv) for mortality of 12%, and a negative predictive value (npv) of 100%. At 12 hours, the ppv for mortality increased (ppv=66%) if the serum lactate concentration remained >10mmol/L. The results of linear regression analysis for a number of variables on lactic acidemia are shown in table II. Other variables that did not show any relationship to the serum concentration of lactate included the type of procedure performed, hematocrit, and preoperative serum bilirubin concentrations. On arrival in ICU, 49/60 (81%) patients were receiving inotropic support. This positively correlated (r=0.86, P=0.021) with the presence of lactic acidemia.

Table II: Results of separately regressing Lactic Acidosis on five explanatory variables.

Explanatory Variable	Regression Coefficient	Standard error	t	P
Cardiac Index	0.19	0.49	.24	>0.2
CPB time	0.005	0.007	3.4	>0.2
Cross-clamp time	-0.005	0.01	0.5	.>0.2
Core temperature	-0.489	0.6	0.8	>0.2
Pre-operative creatinine	-0.42	0.41	1.04	>0.2
Glucose	0.02	0.004	5.68	<0.001

CONCLUSION: In this group of patients, an elevated lactate at admission to ICU after cardiopulmonary bypass was not a prognostic indicator of mortality. A serum lactate concentration greater than 10mmol/L at 12 hours, however, may be predictive. Univariable linear regression analysis using a range of predictor variables failed to describe a relationship to an elevated lactate in these patients.
REFERENCES:

1.) Crit Care Med 1992; 20: 80.

2.) Anesth Analg 1997; 84: SCA1-SCA127.