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Ann Thorac Surg 2001;71:1415-1420
© 2001 The Society of Thoracic Surgeons

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Original article: cardiovascular

High lactate predicts the failure of intraaortic balloon pumping after cardiac surgery

^a Department of Intensive Care, Austin & Repatriation Medical Center, Heidelberg, Victoria, Australia
^b Department of Cardiothoracic Surgery, Austin & Repatriation Medical Center, Heidelberg, Victoria, Australia

Accepted for publication January 19, 2001.

Address reprint requests to Dr Bellomo, Intensive Care Unit (Austin Campus), Austin & Repatriation Medical Center, Heidelberg, Victoria 3084, Australia
e-mail: rb{at}austin.unimelb.edu.au

	Abstract

Top Abstract Introduction Material and methods Results Comment References

 
Background. Despite the use of intraaortic balloon pump (IABP) support in complex cardiac surgical patients, morbidity and mortality rates are high. More advanced mechanical cardiovascular support should be considered in those patients who are highly likely to die despite IABP support. We sought to identify early, readily available prognostic markers for patients receiving IABP support.

Methods. A retrospective analysis was performed on 39 patients requiring IABP support following cardiac surgery for more than 2 years. The accuracy and predictive ability of multiple potential markers of mortality were statistically assessed.

Results. Sixty-seven percent of the patients were successfully weaned from IABP support and 46% survived to hospital discharge. Serious complications occurred in 13% of patients. Serum lactate more than 10 mmol/L in the first 8 hours of IABP support predicted a 100% mortality. Base deficit more than 10 mmol/L, mean arterial pressure less than 60 mm Hg, urine output less than 30 mls/h for 2 hours, and dose of epinephrine or norepinephrine more than 10 µg/min were other highly predictive prognostic markers.

Conclusions. Morbidity and mortality rates remain high despite IABP support following cardiac surgery. Mortality can be predicted by the presence of elevated serum lactate, elevated base deficit, hypotension, oliguria and large vasopressor doses, any of which should prompt appropriate consideration as to other mechanical cardiovascular support.

	Introduction

Top Abstract Introduction Material and methods Results Comment References

 
Cardiac surgery is an important procedure for many patients with heart disease. Low cardiac output syndrome and frank cardiogenic shock, however, are serious and life-threatening complications of this surgery. The intraaortic balloon pump (IABP) is the most commonly used mechanical support device for patients who develop these complications and use of the IABP is increasing [1]. Its predominant actions are improvement of the myocardial oxygen supply-demand relationship and reduction of left ventricular afterload [2]. Suggested indications include preoperative myocardial ischemia or cardiac failure, failure to wean from cardiopulmonary bypass (CPB) and postoperative low cardiac output syndrome [3]. The IABP is likely to be superior to pharmacologic support alone for myocardial failure following cardiac surgery [4] although this has not been rigorously studied.

Despite IABP support for patients with cardiac dysfunction following cardiac surgery, mortality [5] and complication rates [6] remain high. Although myocardial ischemia is reduced by the IABP, its direct effect on cardiac output is variable [5]. Other devices, such as ventricular assist devices (VAD) or extracorporeal membrane oxygenation (ECMO), have greater effects on augmentation of cardiac output, however, these devices are highly invasive and their role has not been clearly defined. It is therefore important to be able to predict at an early stage following cardiac surgery which patients do poorly despite receiving IABP support, so that additional or alternative therapies can be considered. Furthermore, those expected to survive on IABP support might avoid the complications of more invasive mechanical support if a more favorable prognosis can be predicted.

Accordingly, we sought to identify early outcome predictors in patients receiving IABP support by concentrating upon variables which are commonly measured and are easily available soon after IABP support is commenced.

	Material and methods

Top Abstract Introduction Material and methods Results Comment References

 
A retrospective analysis was performed on a 2-year period at our tertiary level hospital (Austin and Repatriation Medical Center, Melbourne, Victoria, Australia). Using routinely collected data on 1,146 total cardiac surgical patients admitted to the Intensive Care Unit (ICU), patients receiving IABP support were identified and the medical records were inspected. Data regarding a number of variables were recorded. Clinical variables included patient demographics, type and severity of heart disease, type of surgery performed, and indication for the IABP. Postoperative parameters included duration of IABP support, use of vaso-active medications, hourly urine output and development of common complications. Hemodynamic variables, which had been measured from pulmonary arterial and systemic arterial catheters, were also noted. Biochemical variables included arterial blood gas analysis and serum lactate measurements. Survival to hospital discharge and the success of weaning the patient from the IABP were also recorded.

Comparisons were made between patients who were survivors and those who died (nonsurvivors). In addition, patients who were successfully weaned from IABP support initially (regardless of survival to hospital discharge) were compared with those who failed to be weaned. Successful weaning was defined as lack of obvious hemodynamic deterioration following removal of the IABP device with survival for at least 48 hours thereafter. Finally, patients who had IABP before surgery were compared to those in whom IABP was initiated in the operating room or immediately thereafter. Parameters expected to be predictive of mortality were selected and analyzed as tests of mortality. Such parameters were required to occur within the first 8 hours of IABP support after cardiac surgery. ² testing with Yates correction factors for small numbers was performed. Continuous variables were subjected to two-sample t tests. A p value less than 0.05 was regarded as significant. Positive and negative predictive values were calculated for significant prognostic indicators. This analysis was repeated for parameters expected to be predictive of unsuccessful weaning from IABP support.

Unstable angina was defined as angina at rest. Left ventricular dysfunction was defined as a cardiac index less than 1.8 l · min^-1 m^-2 or a pulmonary capillary wedge pressure more than 22 mm Hg (as measured by a pulmonary artery catheter during the period of induction of anesthesia) or a left ventricular ejection fraction less than 30% (measured preoperatively). "Redo" surgery was defined as cardiac surgery following any previous cardiac surgical procedure. Acute renal failure (ARF) was defined as urine output less than 30 mls/h with any of a creatinine more than 300 µmol/L (if preoperative creatinine < 150 µmol/L), a creatinine rise more than 250 µmol/L (if preoperative creatinine > 150 µmol/L) or the need for continuous renal replacement therapy. Serious complications were those that contributed to death or required surgical intervention. Minor complications were all others.

The patients had standard cardiopulmonary bypass and myocardial protection techniques. Cardiopulmonary bypass was performed using a membrane oxygenator (Monolyth, Biomedica, Mirandola, Italy) with a pump rate set at 2.4 L min^-1 m^-2, and a minimum core body temperature of 30°C. The pump prime consisted of 500 mL of a urea-linked polygeline solution and 1,000 mls of Ringer’s lactate solution, 10,000 U of sodium heparin and 40 mmol of sodium bicarbonate. Cardioplegia was administered in 800 mL of blood and contained 100 mL of trometamol, 100 mg of lignocaine HCl, 20 mmol of KCl and 10 mmol of MgCl₂. Cardioplegia was delivered via the antegrade route and the coronary sinus cannula (approximately 500 mL at a temperature of 14 to 18°C. Further cardioplegia solution (which consisted of 600 mL of blood with 5 mmol of KCl and 20 mmol of MgCl₂ at a temperature of 14 to 18°C) was infused via the coronary sinus cannula at approximately 25 minute intervals. A "hot shot" (of 600 mL of blood with 5 to 10 mmol of MgCl₂ at 34 to 36°C) was given via the aortic root immediately before aortic cross clamp removal.

	Results

Top Abstract Introduction Material and methods Results Comment References

 
Over the 2-year period, 39 patients received IABP support during their intensive care unit admission for cardiac surgery. The mean age of these patients was 70.4 years (range 53 to 84). Twenty-eight (72%) were male and 11 (28%) were female. The IABP was inserted preoperatively in 10 (26%) patients (for either unstable angina, acute myocardial infarction with cardiogenic shock or a failed coronary angioplasty), 18 (46%) at the time of weaning from CPB, and 11 (28%) during the postoperative ICU course (4 for cardiogenic shock and 7 because of the need for high-dose inotropic agents to maintain adequate resuscitation).

Preoperative ischemia was common, as 20 patients (51%) had unstable angina and 11 (28%) had an acute myocardial infarction in the 28 days before surgery.

Coronary artery bypass grafting (CABG) was performed in 31 patients (79%), valvular surgery in 3 (8%), and a combination procedure in 5 patients (13%). Surgical, anesthetic, perfusion, and IABP techniques were standard throughout the study period. The mean CPB time was 142.1 minutes (range 63 to 332) and the mean aortic cross-clamp time was 79.9 minutes (range 35 to 204). During the first 8 hours in the ICU, the mean cardiac index was 2.37 ± 0.9 L/m²/min. Furthermore, 36.8% of patients had a cardiac index below 2 L/m²/min on IABP and 47.6% before IABP. The mean pulmonary artery occlusion pressure (PAOP) was 18.8 ± 5 mm Hg and 42.1% of patients had a PAOP above 20 mm Hg. The average mean arterial pressure (MAP) was 68.3 ± 12.7 mm Hg (23.6% of patients with MAP < 60 mm Hg) and the mean lactate was 9.6 ± 4.1 mmol/L with 56.4% of patients on more than 10 µg/min of combined epinephrine and norepinephrine infusion to sustain their circulation.

Of the 39 patients, 26 (67%) were successfully weaned from the IABP. Subsequently, 8 died within the same hospital admission, leaving 18 survivors to hospital discharge (46%). No patient required recommencement of IABP support following successful weaning. Table 1 shows a comparison of survivors and nonsurvivors. The groups were statistically similar, except for the CPB time which was longer in nonsurvivors (p = 0.02). Table 2 shows a comparison of patients successfully weaned from IABP support and those who failed to be weaned from IABP support. Again, the patient groups were statistically similar, however, patients in the successfully weaned group were more likely to have had CABG surgery alone (p = 0.03), preoperative unstable angina (p = 0.03) or a shorter CPB time (p = 0.01). Patients in whom IABP was initiated before surgery had a lower mortality than those in whom IABP was initiated in the operating room or immediately thereafter (22.2% versus 63.3%; p = 0.054), they also had a shorter CPB time (142 ± 70 versus 157 ± 72 minutes; p = 0.0054), a better lowest MAP in the ICU (77 ± 17 versus 65 ± 9 mm Hg; p = 0.05) and a lower base deficit (5.2 ± 2.8 versus 10.3 ± 6; p = 0.017).

Complications of IABP support are listed in Table 5. Serious complications occurred in 13% of patients while minor complications occurred in 31%. Electrical supply failure in one patient led to cessation of IABP function and cardiac arrest (which possibly contributed to the patient’s death the following day), while 4 patients required surgery or died as a result of significant bleeding from the insertion site. Difficult insertion was included as a complication, however, all patients were eventually able to have the IABP inserted once vascular access was obtained. Balloon rupture occurred in 4 cases, however, all were late in the course of IABP support and after detection and removal of the device, successful weaning occurred in all patients. Lower limb ischemia, which occurred in 3 patients, was reversible in all patients after device removal.

	Comment

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With results such as these, it is extremely important to identify any factors which predict whether survival is more or less likely to occur. Other investigators have identified various markers which predict mortality. Preoperative clinical factors previously described include age [8], female sex [8], renal dysfunction [8, 9], prior acute myocardial infarction [10], presence of left ventricular aneurysm [11], use of nitroglycerin [12], and left ventricular failure as assessed by functional class [10, 12] or by ventriculography [9, 10]. Peri-operative surgical factors previously described are combined CABG and valvular surgery [13], prolonged CPB time [10], and prolonged aortic cross-clamp time [10]. Factors related to the IABP device which predict poor survival are small balloon size [13], transthoracic insertion [12], and prolonged duration of support [10, 13]. Postoperative factors previously described include requirement for pacing to assist weaning from CPB [8], use of isoproterenol or digoxin [8], need for dialysis [9], lack of recovery of cardiac function within 24 hours [9], and documentation of a perioperative acute myocardial infarction [11]. The need for operative or postoperative insertion of the IABP device (as opposed to preoperative insertion) is also a poor prognostic indicator [12]. One group of investigators has developed a scoring system utilizing early postoperative cardiac output and systemic vascular resistance to assist in prognostication [14].

However, many of these parameters are not useful prognostic indicators because they are neither sufficiently discriminatory in a clinical situation nor available early in the course of IABP support after cardiac surgery. When a patient is deteriorating despite IABP support, prognostic indicators need to be readily available and highly predictive, so that other support devices can be instituted in a timely manner. If it is known, for instance, that abnormal values for a certain variable can accurately predict a mortality rate of 100%, then there can be no harm in the institution of an alternative support device, aside from financial or technical limitations. Furthermore, this information must be available early, because there may be no benefit obtained from such devices if their application is delayed until death is virtually inevitable. It may be that the institution of another support device, such as a ventricular assist device, should be implemented before the patient leaves the operating room, as even an 8-hour period of shock can lead to multiple organ failure with a low survival rate.

Our retrospective study has identified elevated serum lactate and severe metabolic acidosis as the most important prognostic markers for a high mortality rate when patients are receiving IABP support following cardiac surgery. Importantly, these paraneters are highly predictive and available both readily and early in the course of support. Serum lactate more than 10 mmol/L is able to predict a 100% mortality rate in this group of patients, which is extremely important information. Hypotension, oliguria (and development of ARF), need for large doses of vasopressor agents and prolonged CPB or aortic cross clamp time are also statistically significant but less predictive prognostic markers. With the exception of ARF, these markers are all available early following cardiac surgery, and therefore when present, can alert clinicians to the high likelihood of death.

We did not find that previously identified prognostic markers such as age, sex, prior acute myocardial infarction, left ventricular functional class, nonpreoperative insertion of the IABP, prolonged duration of IABP support or combined CABG and valvular surgery were predictive of mortality.

Negative predictive markers were not found to be as highly predictive as most positive predictors. Furthermore, outcome prediction based on the lack of a particular variable is less valuable at the bedside in the determination of need for more aggressive therapy. The presence of unstable angina however, was statistically more common in patients successfully weaned from the IABP, suggesting that this clinical historical feature can alert clinicians to a better prognosis.

Our patients had a serious complication rate of 13% and a total complication rate of 44%. Others have reported rates between 12% and 42% [15], although rarely are serious complication rates more than 12% to 15% [6, 15]. Our serious complication rate is therefore similar, but our total complication rate is higher, largely because the rate of difficult insertion was included. It is of note that clinical data significantly underestimate the number of complications found at necropsy [16].

Our study is limited by both its size and its retrospective nature. Nevertheless, it represents 2 years experience in a tertiary level cardiac surgical center and reflects the difficulty in obtaining large patient numbers for trials in this area. Due to the small patient numbers, it was impractical to perform multivariate analysis. Several of the variables found to be significant predictors of mortality were often present in an individual patient, thereby confounding the true significance of these variables. For example, when a patient had a serum lactate more than 10 mmol/L, the base deficit was always more than 10 mmol/L (although the reverse was not always true), and oliguria, hypotension, and large vasopressor dose were often present simultaneously, further reflecting the lack of independence of all the prognostic markers.

From these data, it is possible to reason that a patient requiring IABP support following cardiac surgery has a mortality rate of approximately 50%, but that if an elevated lactate or a severe metabolic acidosis develops early in the course of IABP support, the mortality rate increases to approach 100%. Therefore, patients receiving IABP support who develop these metabolic derangements can only be expected to die, and consequently should be seriously considered for application of any available device that is technically possible, even if it offers only a moderate survival advantage. For this reason, survival rates in the 30% range, which have been reported in patients receiving VAD support when IABP support has failed, become acceptable. Although only small studies have been performed, hospital survival rates have been 33% for a left VAD [17], 29% for a biventricular VAD [18], and 25% for the smaller, transvalvular left VAD, named the "Hemopump" [19]. Treatment with these or other devices would appear to offer a survival advantage to those patients who have been identified to have the poorest prognosis despite IABP support. A randomized, prospective trial comparing IABP support and any of these devices in patients who are estimated to have a high mortality rate (eg, > 60% to 70%) in the early postoperative phase may now be appropriate.

Our study results suggest that elevated serum lactate is a very powerful predictor for mortality in those patients receiving IABP support for cardiac surgery. When it is present, more aggressive mechanical cardiovascular support should be strongly considered.

	References

Top Abstract Introduction Material and methods Results Comment References

 

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Author home page(s): Jai S. Raman Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Davies, A. R. Articles by Buxton, B. F. Search for Related Content PubMed PubMed Citation Articles by Davies, A. R. Articles by Buxton, B. F. Related Collections Cardiac - physiology Mechanical Circulatory Assistance