Anaesthesia & Arrhythmias

Journals Reviewed: J. Cardiothoracic and Vascular Anaesthesia Abstracted by: Dr C French; MB. BCh. (Registrar, University of the Witwatersrand) Summary of abstract A couplet of articles on rhythm - postoperative atrial arrhythmias, and intra-operative QT prolongation. (You may wish to briefly browse our editorial comment ). 1. Prophylaxis of Atrial Tachyarrhythmias after CABG An important study, considering the ever-increasing number of bypass procedures being performed, and the associated post-operative morbidity. This study shows (again) the benefits of beta-blockade in the peri-operative period. Beta blockade, but not digoxin, magnesium, or magnesium + digoxin, substantially reduced the incidence of post-operative atrial tachyarrhythmia (POAT). This translates into shortened hospital stay (7.6 versus 9.2 days, on average). This randomised, prospective trial, conducted over three years, had sufficient numbers (n=400) to yield meaningful results. The study appears well-designed, with appropriate statistical analysis. The authors have clearly devoted substantial effort to creating a well thought-out and relevant trial. Mortality is low (1/400) and the incidence of post-operative myocardial infarction acceptable. A single surgeon performed all the surgery, a further point in the study's favour. Patients were randomised to one of six regimens, two of these including post-operative beta blockade in the form of propranolol 1mg IV six hourly. Of note is the common use of beta-blockade pre-operatively, in 73 percent of all patients. This raises the possibility of some of the arrhythmias or adverse outcomes being related to withdrawal of beta blockade. One could question the wisdom of such withdrawal. The authors do not shy away from discussion of this possibly somewhat controversial move. Results are unequivocal - beta blockade (whether combined with magnesium or not) prevents nearly 50% of 'POAT', reducing the incidence from 38% in controls, to 18.6% in those receiving beta blockade. The POAT was predominantly atrial fibrillation, and most arrhythmias occurred in the first four days, peaking on day two. Older age was a strong predictor of POAT. An interesting sub-analysis is that of the patients who had withdrawal of beta-blockade. This showed a significant withdrawal effect, something which perhaps clouds an otherwise excellent study. Can one, being aware of the risk, justify withdrawing peri-operative beta blockade? Importantly, beta blocker administration was beneficial in preventing POAT even in patients not subject to a withdrawal phenomenon. It might be argued that the magnesium dose (and consequently, serum magnesium levels) in this study was not sufficient to adequately suppress arrhythmias, with mean recorded serum levels of ~ 2 mEq/litre, in response to infusion of 12g over 96 hours. Whether substantially higher doses of magnesium will have a beneficial effect is an open question, but what the study really tells us is that, in the absence of substantial contra-indications, low-dose beta blockade should be a standard of care in the peri-operative management of patients undergoing coronary artery bypass grafting. Article 1: A ß-blocker, not Magnesium, is effective prophylaxis for atrial tachyarrhythmias after coronary artery bypass surgery. J Cardiothor Vasc Anaesth 2001 April 15(2) 204-9 Article type: Clinical Study Authors: Bert AA, Reinert SE, Singh AK. 2. Volatiles and the QT interval In this study, sixty-five ASA I or II patients undergoing non-cardiac surgery had monitoring of QTc, while undergoing inhalational induction of anaesthesia. Agents used were halothane, isoflurane or sevoflurane, with random allocation (n= 22, 19 and 24, respectively). The reviewer is mildly surprised at the use of isoflurane as an inhalational induction agent! Would not intravenous induction, followed by a volatile have been more appropriate, (although the authors appear to have chosen their approach to minimise interfering variables)? The study also brings up the contrast between statistical and clinical relevance. Surprisingly (in view of its reputation as a proarrhythmic agent), halothane decreased mean QTc from 445 to 426 milliseconds, while isoflurane increased QTc by a similar amount (41 to 465 ms). Sevoflurane appeared fairly neutral. The authors discuss the relevant literature, providing few surprises - several previous studies more-or-less agree with their findings, with some discrepancies. As the authors point out, the clinical relevance of their study seems limited, especially with the multiplicity of factors contributing to the occurrence of torsades de pointes. They speculate that "Sevoflurane or halothane may be a better choice than isoflurane in patients with conditions known to prolong Q-T interval..".   Article 2: The effects of volatile anesthetics on the Q-Tc interval. J. Cardiothoracic Vasc Anaesth 2001 15(2) April 188-91 Article type: Clinical Study Authors: Güler N, et al. {Dr French also reviewed several articles on gastric tonometry, but: (a) even the editor's devious mind could not find a powerful association between this topic and arrhythmias, and (b) the articles were sufficiently poor not to merit discussion}! Editorial Comment Magnesium is a controversy-provoking metal. Although some claim that it has a well-validated role in the management of acute myocardial infarction, some recent work questions this, for example the recent study by Ziegelstein [Am J Cardiol 2001 Jan 1;87(1):7-10] which showed in a population of 173 728 patients that magnesium use (in only 5.1%) was associated with increased mortality. (One cannot impute a causal relationship). The study above suggests that Mg has no role in prophylaxis of perioperative supraventricular tachyarrhythmias, but of course magnesium fanatics will be quick to point out that higher doses should perhaps have been given, invoking studies such as that of Maslow et al, a year earlier [J Cardiothorac Vasc Anesth 2000 Oct;14(5):524-30]. In Maslow's study, however, 35% of those not receiving magnesium were frankly hypomagnesaemic, and the study was retrospective. Other studies also attest to the uselessness of magnesium [Ann Thorac Surg 2000 Jan;69(1):126-9]. We can only hope that such studies will decrease the propensity of surgeons to use weird multidrug regimens such as that described by Ott et al [J Card Surg 1999 Nov-Dec; 14(6): 437-43] who combine T3, magnesium, T4, metoprolol, procainamide, digoxin, steroids and diuretics (!!), with a 7.5% death rate in their patients who developed atrial fibrillation. Interestingly enough, with MIDCAB (minimally invasive direct coronary artery bypass) multivariate analysis shows that it is not the procedure, but the clinical characteristics of the patient population which appears to determine the lower incidence of post-operative atrial fibrillation seen with this operation [J Am Coll Cardiol 2000 Nov 15;36(6):1884-8]. MIDCAB patients tend to be younger with less severe coronary artery disease. There are other predictors of post-operative AF apart from age, male sex, and severity of coronary artery disease. One such predictor is signal-averaged P-wave duration of over 155ms (odds ratio 5.37!) [Circulation 2000 Mar 28;101(12):1403-8]. The bottom line is surely to keep it simple, use beta blockade wherever possible, but especially in those at high risk, and throw away the rest (especially the digoxin)! The second article is a good example of how one should not extrapolate, if one can possibly avoid this vice! For if we consider their study carefully, we note that the final heart rate in the halothane group was 65 ± 11, that in the isoflurane group, 93 ± 8 beats per minute. One could just as easily argue that despite its small "QTc prolonging effect" isoflurane is the better agent, as the initiation of arrhythmias such as torsades seems to be more likely at slower rates, and halothane is more likely to be arrhythmogenic! One form of prevention of torsades is to either infuse isoprenaline (isoproterenol) or to overdrive pace such patients, in order to achieve tachycardia. (Another is, heh, Magnesium)! In the absence of substantial studies of the use of volatiles in patients with prolonged QTc syndrome, one should however avoid all such speculation! Ed       Up Date of First Publication : 2001-05-09 Date of Last Update : 2006/10/23 Web page author: Click here