Postoperative hypothermia and patient outcomes after elective cardiac surgery

Transcription

1 doi: /j x ORIGINAL ARTICLE Postoperative hypothermia and patient outcomes after elective cardiac surgery D. Karalapillai, 1 D. Story, 2 G. K. Hart, 3,4 M. Bailey, 5 D. Pilcher, 6,7 D. J. Cooper 8 and R. Bellomo 8 1 Consultant Anaesthetist, 2 Associate Professor and Head of Research, Department of Anaesthesia and Pain Management, 3 Associate Professor and Deputy Director, Department of Intensive Care, Austin Health, Heidelberg, Australia 4 Chair, 5 Associate Professor and Senior Statistical Consultant, 6 Director Adult Patient Database, Australian and Intensive Care Society Centre for Outcome and Resources Evaluation, 8 Professor and Head of Intensive Care research, Australian and Intensive Care Research Centre, Department of Epidemiology and Preventive Medicine, School of Public Health and Preventive Medicine, Monash University Alfred Hospital, Melbourne, Australia 7 Staff Specialist, Alfred Hospital, Melbourne, Australia Summary Hypothermia after elective cardiac surgery is an important physiological abnormality and is associated with increased morbidity and mortality. The Australian and New Zealand intensive care adult patient database was studied to obtain the lowest and highest temperature in the first 24 h after surgery. Hypothermia was defined as core temperature < 36 C; transient hypothermia as temperature < 36 C that was corrected within 24 h; and persistent hypothermia as hypothermia that was not corrected within 24 h. Hypothermia occurred in out of a total of consecutive patients (66%) and was persistent in 111 (0.3%). Transient hypothermia was not independently associated with increased hospital mortality (OR = 0.9, 95% CI ), whereas persistent hypothermia was associated with markedly increased risk of death (OR = 6.3, 95% CI = ). Hypothermia is common in postoperative cardiac surgery patients during the first 24 h after ICU admission but, if transient, is not independently associated with an increased risk of death.... Correspondence to: Dr D. Karalapillai dharshi.karalapillai@austin.org.au Accepted: 21 April 2011 Hypothermia is thought to be a common and important physiological abnormality in surgical patients admitted to intensive care units (ICUs). Previous studies of postoperative hypothermia in ICU patients have used a single temperature observation (often on admission) and examined small patient groups, usually from one centre [1 6]. Despite these limitations, in studies of heterogeneous cohorts of postoperative patients, hypothermia was independently associated with complications and increased mortality [2, 6]. This association appears biologically plausible, and is supported by evidence from numerous studies [5 15]. Although this may reflect the fact that hypothermia is a marker of illness severity, a randomised controlled trial has suggested that the morbidity associated with hypothermia may be reduced by active warming [12]. Patients undergoing cardiac surgery may be at particular risk of hypothermia [6]. In a large singlecentre study, patients after coronary artery surgery using cardiopulmonary bypass were at greater risk of complications and mortality if their ICU admission temperature was < 36 C [6]. A more recent multicentre study of more than 2000 patients having off-pump coronary artery surgery [15] found that half were hypothermic on arrival in the ICU and that both hypothermia and hyperthermia were associated with increased morbidity and mortality. These studies, however, used a single measurement on arrival in ICU to define hypothermia and did not include patients having more complex 780 Anaesthesia Ó 2011 The Association of Anaesthetists of Great Britain and Ireland

2 D. Karalapillai et al. Æ Hypothermia after cardiac surgery cardiac surgery (valve replacement or repair, combined valve and coronary artery surgery or thoracic aortic surgery). They also failed to assess whether the association with mortality continued when postoperative hypothermia was prolonged. We tested the hypothesis that, in patients admitted to the ICU after all types of elective cardiac surgery, hypothermia might be both common and independently associated with increased mortality, and that this association would apply to both transient and persistent hypothermia. We used data from a large Australian and New Zealand multicentre ICU database to test this hypothesis. Methods We performed a retrospective observational study of prospectively collected postoperative ICU data. The study data were collected and submitted to the Australian and New Zealand Intensive Care Society (ANZICS) database by participating units after collection by trained data collectors. The ANIZCS Centre for Outcomes and Resource Evaluation (CORE) Management Committee granted access to the data in accordance with standing protocols. Data are collected under the Quality Assurance Legislation of the Commonwealth of Australia, with government support and funding. The ANZICS CORE database is a high quality national database of patients admitted to ICU from > 100 ICUs in Australia and New Zealand [16]. Data were retrieved from patients admitted to the ICU over an 8-year period between 2000 and We included all consecutive patients whose admission source was the operating room after elective cardiac surgery. Postoperative temperatures were measured using a pulmonary artery catheter thermistor, which is standard practice in this group of patients in Australia and New Zealand. This method of measuring temperature is considered the gold standard of core temperature estimation in postoperative patients [17, 18]. Normothermia was defined as a lowest temperature 36 C and hypothermia as < 36 C, as is consistent with previous studies [1, 2]. Measurements were recorded at intervals of 1 4 h. We excluded patients with a temperature > 38 C and readmissions, because of the known association between hyperthermia and increased mortality [15]. Patients whose surgical management included intra-operative deep hypothermic circulatory arrest were not studied. We used the lowest and highest temperature within the first 24 h of intensive care admission as the primary study variables. In-hospital mortality was the primary outcome measure. Secondary outcome measures were ICU mortality and ICU and hospital length of stay. We compared the characteristics and outcomes of three groups: patients without hypothermia; patients who had their lowest temperature in the hypothermic range but whose highest temperature was normal (transient hypothermia); and patients who had both lowest and highest temperatures in the hypothermic range (persistent hypothermia). Univariate comparisons for overall group differences were performed using chi-squared tests for equal proportion, ANOVA for continuously normally distributed variables and Kruskal Wallis tests otherwise. We determined the relationship between temperature and mortality using logistic regression analysis adjusting for age, sex, illness severity, type of surgery, ICU dependency and requirement for mechanical ventilation of the lungs. As the most accurate measure of illness severity, the acute physiology and chronic health evaluation (APACHE III score), contains a temperature component, an adjusted (corrected) APACHE III score was derived whereby the temperature component was removed [19]. A two-sided p value of 0.05 was considered to be statistically significant for main effects, but to account for multiple comparisons, a Bonferroni correction was applied, in which a reduced p value of p = (0.05 3) was used to indicate statistical significance for post-hoc comparisons. All analysis was performed using SAS version 9.1 (SAS Institute Inc., Cary, NC, USA). Results We studied patients admitted to 60 ICUs in Australia and New Zealand after elective cardiac surgery (Table 1). The mean (SD) lowest temperature was 35.6 C (0.8) and the mean highest temperature was 37.4 C (0.4). In total, (66%) patients experienced hypothermia in the 24 h after ICU admission. However, only 111 (0.3%) had persistent hypothermia. Persistently hypothermic patients were older, more likely to be female and had higher corrected APACHE III scores than normothermic patients (p < 0.001). Overall, 661 out of patients died in hospital (1.5%). Mortality was significantly higher in the persistent hypothermia group (Table 2). Non-survivors were older, more likely to be female, had higher APACHE scores, lower temperatures and were more likely to be persistently hypothermic. Anaesthesia Ó 2011 The Association of Anaesthetists of Great Britain and Ireland 781

3 D. Karalapillai et al. Æ Hypothermia after cardiac surgery Anaesthesia, 2011, 66, pages Table 1 Baseline data in patients admitted to the ICU after elective cardiac surgery between 2000 and 2008 in 60 centres in Australia and New Zealand. Values are mean (SD) or number (proportion). Hypothermia defined as any temperature < 36 C in the first 24 h after surgery. Total (n = ) Normothermia (n = ) Transient hypothermia (n = ) Persistent hypothermia (n = 111) Age; years 66 (13) 65 (13) 67 (12) 69 (13) Male (71%) (73%) (70%) 72 (65%) Surgery type Coronary artery surgery (62%) 8676 (60%) (63%) 47 (42%) Isolated valve surgery (25%) (25%) 7205 (25%) 18 (16%) Combined coronary and valve surgery 2603 (6%) 769 (5%) 1827 (6%) 7 (6%) Other 3069 (7%) 1497 (10%) 1533 (5%) 39 (35%) APACHE III score 46 (15) 43 (15) 48 (15) 61 (27) Corrected APACHE III score 42 (15) 40 (15) 42 (15) 52 (27) Highest temperature; C 37.4 (0.4) 37.4 (0.4) 37.3 (0.4) 35.5 (0.5) Lowest temperature; C 35.6 (0.8) 36.0 (0.3) 35.2 (0.6) 34.4 (1.3) Lungs ventilated (99%) (99%) (96%) 110 (99%) Table 2 Characteristics of survivors and non-survivors in patients who underwent cardiac surgery between 2000 and 2008 in 60 ICU s in Australia and New Zealand. Values are mean (SD) or number (proportion). Survivors (n = ) Non-survivors (n = 661) p value Age; years 66 (12) 74 (11) < Male (71%) 397 (60%) < APACHE III 46 (15) 72 (27) < Corrected APACHE III score 41 (14) 67 (26) < Lungs ventilated (99%) 659 (99%) 0.69 Lowest temperature; C 35.6 (0.4) 35.5 (0.4) < Highest temperature; C 37.2 (0.6) 37.0 (0.8) < Normothermia (34%) 216 (33%) 0.55 Transient hypothermia (66%) 427 (65%) 0.45 Persistent hypothermia 93 (0.2%) 18 (3%) < Multivariate analysis showed that patients with transient hypothermia had similar odds ratios for hospital mortality compared with normothermic patients (Table 3). However, patients with persistent hypothermia were significantly more likely to die in hospital than normothermic patients. Other independent predictors of mortality were age, female sex and corrected APACHE III score. Discussion This study has shown that transient postoperative hypothermia is very common, but not independently associated with hospital mortality, contrary to part of our hypothesis. However, persistent hypothermia, although rare, was significantly and independently associated with increased mortality. This is the largest study (to date) of hypothermia after all types of cardiac surgery. The incidence of hypothermia of 66% is much greater than that shown by Insler et al., who found that 28% of patients had an arrival temperature of less than 36 C after isolated primary coronary bypass graft surgery [6]. Our study, however, included more complex combined and re-do surgery. Because we used the lowest and highest temperatures within the first 24 h rather than on arrival in the ICU, some patients may have become hypothermic after arrival. Similarly, Hannan et al. found that nearly 50% of patients experienced hypothermia after off-pump coronary artery surgery; this difference can also be 782 Anaesthesia Ó 2011 The Association of Anaesthetists of Great Britain and Ireland

4 D. Karalapillai et al. Æ Hypothermia after cardiac surgery Table 3 Results of multivariate analysis for hospital mortality in patients who underwent cardiac surgery between 2000 and 2008 in 60 ICU s in Australia and New Zealand. Variable Odds ratio (95% CI) p value Age; years 1.03 ( ) < Corrected APACHE III score 1.06 ( ) < Transient hypothermia vs 0.89 ( ) 0.30 normothermia Persistent hypothermia vs 6.3 ( ) < normothermia Male 0.62 ( ) < explained by the difference in surgical complexity, the use of cardiopulmonary bypass in the vast majority of our patients and the longer period of observation. The finding that transient hypothermia was not associated with increased mortality may appear to be in contrast with previous studies [6, 15]. However, these studies did not correct for illness severity scores, which is important because hypothermia may simply be a marker of illness severity. However, this study did not assess differences in morbidity between the three temperature groups. Reduced morbid cardiac events, blood loss, wound infection and longer hospital stay have all been previously described with peri-operative warming in specific surgical populations [20]. The fact that persistent hypothermia is so strongly associated with increased mortality is very relevant. Whether this association represents the fact that persistent hypothermia is a marker of illness severity, or that failing to correct hypothermia over the first 24 h in ICU has detrimental clinical effects, or both, cannot be determined. However, the seemingly favourable outcome associated with transient hypothermia suggests that, if hypothermia is short lived and can be corrected, it has no effect on mortality. However, while these observations suggest that active correction of hypothermia in ICU may be desirable, they are also consistent with the proposition that the transient hypothermia so often seen in cardiac surgery patients may have limited clinical consequences. The major strength of this study is its size and the fact that the data come from so many centres. The values were not collected to demonstrate any specific association related to hypothermia, reducing the risk of bias. Furthermore, given that over patients were included in our study, it is significantly powered to detect even a relatively weak association. A major limitation of our study, however, is that it was retrospective. Temperature measurements were not protocol based and only peak and lowest temperatures were recorded in the database, irrespective of the number of measurements taken. However, such data are more complete than in previous studies where only the ICU admission temperature was recorded. The precise timing within the first 24 h in which the lowest and highest temperatures were recorded was also not specified in our database. Therefore, the exact duration of hypothermia is not known. Other limitations relate to deficiencies in our collected data, which did not include factors such as the duration of surgery, the use of intra-operative temperature monitoring or details of cardiopulmonary bypass temperature management, post-bypass temperature management and postoperative rewarming in the ICU (passive or active). A survey of 40 Australian cardiac surgery units that was conducted concurrently reported routine rewarming to a peak temperature of C, which was therefore likely to have been the most common practice in the centres that collected the data we have presented [21]. Transient hypothermia in postoperative cardiac surgery patients is common in the early postoperative period but is not associated with increased mortality. On the other hand, persistent hypothermia has a strong independent association with increased hospital mortality. Although this association may reflect the fact that persistent hypothermia is a marker of illness severity, it would seem prudent, until further information becomes available, to prevent (or minimise the duration of) hypothermia, in the first 24 h after cardiac surgery. Competing interests No external funding or competing interests declared. References 1 Karalapillai D, Story D. Hypothermia on arrival in the intensive care unit after surgery. Critical Care and Resuscitation 2008; 10: Karalapillai D, Story D, Calzavacca P, et al. Inadvertent hypothermia and mortality in postoperative ICU patients. Anaesthesia 2009; 64: Abelha FJ, Castro MA, Neves AM, et al. Hypothermia in a surgical intensive care unit. BMC Anesthesiology 2005; 5: Kongsayreepong S, Chaibundit C, Chadpaibool J, et al. Predictor of core hypothermia and the surgical intensive care unit. Anesthesia and Analgesia 2003; 96: Anaesthesia Ó 2011 The Association of Anaesthetists of Great Britain and Ireland 783

5 D. Karalapillai et al. Æ Hypothermia after cardiac surgery Anaesthesia, 2011, 66, pages Slotman GJ, Jed EH, Burchard KW. Adverse effects of hypothermia in postoperative patients. American Journal of Surgery 1985; 149: Insler SR, OConnor MS, Leventhal MJ, et al. Association between postoperative hypothermia and adverse outcome after coronary artery bypass surgery. Annals of Thoracic Surgery 2000; 70: Bush HL, Hydo LJ, Fischer E, et al. Hypothermia during elective abdominal aortic aneurysm repair: the high price of avoidable morbidity. Journal of Vascular Surgery 1995; 21: Frank SM, Higgins MS, Breslow MJ, et al. The catecholamine, cortisol, and hemodynamic response to mild hypothermia: a randomized controlled trial. Anesthesiology 1995; 82: Frank SM, Fleisher LA, Breslow MJ, et al. Perioperative maintenance of normothermia reduces the incidence of morbid cardiac events. A randomized clinical trial. Journal of the American Medical Association 1997; 277: Schmied H, Kurz A, Sessler DI, et al. Mild hypothermia increases blood loss and transfusion requirements during total hip arthroplasty. Lancet 1996; 347: Lenhardt R, Marker E, Goll V, et al. Mild intraoperative hypothermia prolongs postanesthetic recovery. Anesthesiology 1997; 87: Kurz A, Sessler DI, Lenhardt RA. Study of wound infections and temperature group: perioperative normothermia to reduce incidence of surgical wound infection and shorten hospitalization. New England Journal of Medicine 1996; 347: Nesher N, Zisman E, Wolf T, et al. Strict thermoregulation attenuates myocardial injury during coronary artery bypass graft surgery as reflected by reduced levels of cardiac specific troponin I. Anesthesia and Analgesia 2003; 96: Hohn L, Schweizer A, Kalangos A, et al. Benefits of intraoperative skin surface warming in cardiac surgical patients. British Journal of Anaesthesia 1998; 80: Hannan EL, Samdashvili Z, Wechsler A, et al. The relationship between perioperative temperature and adverse outcomes after off-pump coronary artery bypass graft surgery. Journal of Thoracic and Cardiovascular Surgery 2010; 139: Stow PJ, Hart GK, Higlett T, et al. Development and implementation of a high quality clinical database: the Australian and New Zealand Intensive Care Society Adult Patient database. Journal of Critical Care 2006; 21: Nierman DM. Tools that we use: If you can t measure it, you can t manage it. Critical Care Medicine 2007; 35: Moran JL, Peter JV, Solomon PJ, et al. Tympanic temperature measurements: are they reliable in critically ill? A clinical study of measures of agreement. Critical Care Medicine 2007; 35: Knaus WA, Wagner DP, Zimmerman JE. The APACHE III prognostic system. Risk prediction of hospital mortality for critically ill hospitalized adults. Chest 1992; 100: Leslie K, Sessler DI. Perioperative hypothermia in the high-risk surgical patient. Best Practice and Research in Clinical Anaesthesiology 2003; 17: Tuble SC, Wilcox TW, Baker RA. Australian and New Zealand perfusion survey: Management and procedure. Journal of Extra Corporeal Technology 2009; 41: Anaesthesia Ó 2011 The Association of Anaesthetists of Great Britain and Ireland