Central Body Temperature as a Guide to Optimal Heart Rate

Central Body Temperature as a Guide to Optimal Heart Rate JERRY C. GRIFFIN. KENNETH R. JUTZY. JOHN P. CLAUDE and JAMES W. KNUTTI From the Division of Cardiology, and School of Medicine and Center for Integrated Electronics in Medicine, Stanford University La frequence cardiaque determinee par la temperature centrale. Les stimuiateurs cardiaques a frequence fixe ne restaurent pas la fonction hemodynamique normale. Puisque un pourcentage eieve de patients porteurs d'un stimulateur cardiaque a des anomalies auriculaires. un autre moyen de "piloter" le pacemaker est presente: celui de ia temperature du sang dans I'oreiilette droite. Les sondes a enregistrement thermique mesurent la variation de temp^.rature pendant I'exercice et peuvent servir de guide pour determiner la frequence cardiaque optimaie. GRIFFIN, J.C, ETAL.: Central body temperature as a guide to optimal heart rate. Studies in man suggest thai fixed-rate artificial pacemakers do not return hemodynamic function to normal, since the principal mechanism for the increase in cardiac output with exercise, increased heart rate, is not restored. Special pacemakers are available that can detect atrial activity and pace the ventricles in coordination, but nearly half of the patients receiving artificial pacemakers have abnormal atrial function (atriai fibrillation, sick sinus syndrome}. This study examined the effects of exercise on the temperature of blood returning to the right atrium. Precision thermistors, placed in the right hearts of conscious dogs, recorded temperature increases of 1"C (range 0.4-1.5''C} during submaximal treadmill exercise. Temperature change correlated well with work load and changes in heart rate. [PACE, Vol. 6, March-April, Part II. 1983} temperature, artificial pacemaker,

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Introduction It is generally accepted that fixed-rate ventricular pacing does not restore normal hemodynamics since the only mechanism available to increase cardiac output is to increase stroke volume. The ability to increase output by this mechanism is more limited than by sinus tachycardia, even in patients with normal hearts, and may be severely limited in those with abnormal left ventricular function. Therefore, some index of peripheral requirement for increased cardiac output is needed in order to adjust artificial pacemaker rate in a more physiologic manner. One method that is well established is atrial synchronous pacing, accomplished by placing a catheter in the right atrium and pacing the venAddress for reprints: Jerry C, Griffin. M.D., Department of Medicine, Section of Cardiology. Baylor Cotlege of Medicine, 6535 Fannin M/S F-905, Houston, Texas 77030.

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tricle after each atrial beat that is sensed.' This approach, however, requires normal atrial and sinoatrial node function. Cardiac pacing was initially devised for the treatment of bradycardia resulting from complete heart block. More recently, other indications for cardiac pacing bave been established, particularly the therapy of bradycardia/tachycardia or sinus node dysfunction syndrome. At present, it is estimated that nearly half the patients receiving cardiac pacemakers have abnormal sinus node function, or atrial fibrillation,' For these patients, some non-electrographic index of appropriate cardiac rate is needed. Of those proposed, temperature is the most likely to be reliably measured by an internally implantable system. Therefore, if the very preliminary findings of this investigator are confirmed, a pacemaker can be constructed such that an exercise induced elevation of central blood temperature will result in an increase in

March-April 1983, Part II

PACE, Vol. 6

TEMPERATURE. A GUIDE TO HEART RATE

cardiac rate and improved hemodynamic function.*

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Four large mongrel dogs were implanted with thermistor probes in the right atrium (RA). right ventricle [RV], and main pulmonary artery (PA). These were tunneled from the thoracotomy incision to the back of the neck and left until three to four weeks after surgery. The probes were then exteriorized and the animals studied during treadmill exercise. Exercise was performed in four-minute stages al zero grade and 1. 2, 3, 4, and 5 miles per hour. This is estimated to have increased oxygen consumption three- to four-fold," Multiple runs were made on separate days in three animals to estimate reproducibility.

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WORKLOAD (mph) Figure 1. Temperature change is plotted against treadmill exercise work load in miles per hour. Each line represents a separate dog. Coefficient of regression fiinearj for individual dogs ranged from 0.96 to 0.99.

Results The change in central blood temperature with exercise was significant and easily measured, 1,0 + .35''C (mean ± SD), at maximum exercise. The instrumentation used for temperature measurement was accurate to ±,1''C. No consistent difference in temperature was found between the various measurement sites: RA, RV, or PA. Both temperature and heart rate increased with increasing work load (2 way ANOVA and linear regression, both p < .0001 with a significant inter-animal variation both p = .005). The relationship of temperature change to work load was linear with high correlation coefficients, ranging from 0.96 to 0.99 for each of the four animals (Fig. 1). The relationship of core blood temperature change to change in heart rate was best expressed as power function (Fig. 2). The increase in central blood temperature generally paralleled that of the increase in heart rate during exercise with two exceptions: temperature increased either more slowly or perhaps not at all at low work loads and the return of central blood temperature to base line was slightly slower than that of heart rate (Fig. 3]. Based upon very limited data, the measurements appear reproducible in a given animal from day to day.

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Discussion Other investigators have recognized the need for a variable rate pacemaker which would be responsive to some physiologic factor other than atrial activity. The most advanced such device is the pH-triggered pacemaker.^ This pacemaker has a pH-sensing element on the electrode which detects changes in venous blood pH and alters pacemaker firing rate over a range of 65 to 110 pulses per minute. In addition, preliminary reports have appeared describing a similar application of central venous oxygen saturation and respiratory rate. The problem with the physiologic variables described above is that they are difficult to mea-

Figure 2. The change in temperature (1 resistance unit equaJs V'C} is plotted against heart rate for all animals. The relationship is best described as a power function,

March-April 1983, Part II

r = 0.8i, Y = .013 X2.03.

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Treadmill Exercise Level (mph) 1 2 3 4 5

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