ESTIMATION OF CARDIAC OUTPUT AND STROKE VOLUME FROM THERMAL EQUILIBRATION ANDHEARTBEAT RATES IN FISH

WILLIAM W. REYNOLDS & MARTHA E. CASTERLIN

Biology Department, Pennsylvania State University, Wilkes-Barre, Pennsylvania 18708 U .S.A .

Received April 30, 1977

Keywords : cardiac output, stroke volume, heartbeat, thermal equilibration, thermoregulation, fish electrocardiogram

Abstract

Cardiac output and stroke volume were estimated for a 200 glargemouth blackbass (Micropterus salmoides) by a modifiedwhole-body thermodilution method using the relation betweenthermal equilibration rates and heartbeat frequencies . The reci-procal of the thermal time constant, k (min'), was related to theheartbeat frequency, F (beats min - '), by the equation k=0.00146F + 0 .309 ; the slope is the weight-specific stroke volume (ml g - ')and the intercept is the weight-specific heat transfer constant(cal 'C-'min-'g-).Stroke volume was 0 .292 ml (0.00146 ml/gbody weight), yielding cardiac output values ranging from 44ml kg ' min - ' (at 3o beats min - ) to 158 ml kg ' min - ' (at io8beats min - '), or 4.4 to 15 .8% of body weight. Active (convective)heat transfer due to blood flow constituted an estimated 11 to34% (mean 22.5%) of total heat transfer, depending on heartbeatfrequency ; this variability constitutes physiological thermoregu-lation .

Introduction

Fishes are able to regulate their body temperatures be-haviorally by selecting preferred or optimal temperaturesin a heterothermal environment (Reynolds & Casterlin1976; Reynolds et al., 1976; Reynolds, 1977a) . Internaltissue equilibrium temperatures differ only slightly fromambient water temperatures (Reynolds et al., 1976;Miiller, 1976 ; Dean, 1976), except in scombrid fishes withspecialized rete mirabilia for thermal isolation of theactive red swimming muscles (Neill et al., 1976). How-ever, physiological (primarily cardiovascular) mecha-nisms can alter the rate at which thermal equilibrium isreached following a change in ambient temperature, and

Dr. W. Junk b . v . Publishers - The Hague, The Netherlands

Hydrobiologia vol. 57, 1, pag . 49-52 . 1978

thereby supplement behavioral mechanisms in the over-all thermoregulation of the organism .

Cardiovascular changes which modify heat flow to orfrom deep body tissues constitute an important com-ponent of physiological thermoregulation in endotherms(Adair, 1976) and ectotherms (Smith, 1976 ; Reynolds,1977b) alike. In fact, alteration of thermal equilibrationrates during heating and cooling is a definitive indica-tion of physiological thermoregulation (Smith, 1976) .Ectotherms are often reported to heat faster than theycool, although faster cooling may sometimes occur(Craig, 1973; Reynolds, 1977b) . There is clear evidence ofsuch limited physiological thermoregulation in at leastsome fishes, since thermal equilibration rates are alteredin correlation with changes in heartbeat and ventilatoryfrequencies during heating and cooling (Reynolds,1977b). The large thermal inertia of red muscle in scom-brids apparently obscures such relationships, leading Neillet al. (1976) to doubt the existence of true physiologicalthermoregulation in these specialized forms .

While Reynolds (1977b) demonstrated a correlationbetween thermal equilibration rate and heartbeat fre-quency in largemouth blackbass, Micropterus salmoidesLacepede, heart stroke volume and cardiac output werenot measured in that study, leaving a gap in the presumedrelationship between blood flow and convective heat flow .Recently, Spaargaren (1976) has described an ingeniousmethod for determining stroke volume and cardiac out-put in crustaceans, from simultaneous measurement ofheartbeat frequency and thermal equilibration rate . Thepresent communication describes the application of thismethod to a fish, the largemouth blackbass M. sal-moides .

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Material and methods

A 200 g M. salmoides was subjected to sudden changes inambient water temperature (OT) of 2°C (this small inter-val minimized the hysteresis exhibited with 10 to 20°Cheating and cooling intervals-cf. Reynolds, 1977b), invarious portions of the range 12 to 32'C. (Temperaturewas used as a convenient means of modulating heart ratesand consequently thermal equilibration rates) . Eighteenexperiments were conducted, nine heating and ninecooling. Heart rate was monitored by electrocardio-graphy as described by Reynolds (1977b) . A thermo-couple probe (time constant less than 0 .5 sec) was insertedapproximately 2.5 cm into the intestine via the anus,while another measured the ambient water temperature .The difference between gut and ambient water tempera-tures was monitored to within ± o . 1°C on a digital ther-mometer, and simultaneously recorded alongside theEKG trace and time base on a chart recorder .

The thermal time constant r, which for a linear first-order system is the time required for the temperature ofan object to travel 63% of the way to the applied tempera-ture (Milsum, 1966), was employed as a measure of ther-mal equilibration rate . This constant, expressed in min-utes, is independent of the magnitude of the AT interval(Smith, 1976). Some investigators (e .g., Spaargaren,1974; Reynolds, 1977b ; Spigarelli et al., 1977) have em-ployed the half-time (t 1 /2 ) for temperature equilibration,which is equal to 0.693 r (Spaargaren, 1974). Others (e .g .,Stevens & Fry, 1970, 1974 ; Neill et al., 1976) have ex-pressed thermal equilibration rates in terms of degreesC/min, which is disadvantageous (Smith, 1976) becauseunder step-function conditions of heating and cooling therate is constantly changing (cf . Reynolds, 1977b), so thatthe heat exchange rate is not directly comparable duringdifferent portions of the equilibration process . Also, therate of temperature change in degrees C/min depends onthe size of the AT interval .

The total heat exchange of a live aquatic animal can beseparated into a `passive' (S paargaren, 1976) or conductive(Beitinger et al., 1977) component (which would alsocharacterize a dead animal) and an `active' (Spaargaren,1976) or convective (Beitinger et al., 1977) component dueto blood flow in living animals. According to Crawshaw(1976), approximately half the overall heat exchange of afish with its environment occurs through the gills andthe other half occurs through the general body surface.Spaargaren (1976) gives the equation

k=V . F+Pw

w50

where k = 1/7, V = stroke volume in ml per beat, w =weight of the animal in g, F = heartbeat frequency in beatsper minute, p is a heat transfer constant expressed in cal

C- ' min - ', the slope V of the linear relation (cf . Fig . i) isw

the weight-specific stroke volume of the heart in ml per g

of body weight, and the y-intercept P is the weight-wspecific heat transfer constant . The y-intercept, at anextrapolated heartbeat frequency of o (as in a dead ani-mal) represents the passive or conductive component ofthermal exchange, while the magnitude of the active orconvective component depends upon the slope andheartbeat frequency (i.e ., cardiac output) .

Results

A linear least-squares regression of k on F for 18 datapoints yielded the equation

k = 0.00146 F + 0 .309as diagrammed in Fig . i . Thus the calculated weight-specific constant of (passive or conductive) heat transfer(y-intercept) for this fish is 0 .309 cal C_' min-'g- ', andthe weight-specific stroke volume (slope) is 0 .00146 mlg-' . For this 200 g fish, the calculated stroke volume V is0.292 ml. Over the observed range of heartbeat frequen-cies (F = 30 to 1o8 beats min) for the fish, the cardiacoutput (minute volume, V = F V) is 8 .8 to 31 .5 ml, or 44to 158 ml kg- ' min' . This is 4.4 to 15 .8% of body weight .

00

20

40

60

80

100

120F, beats-min -1

Fig. 1 . The relation of the thermal time constant r (time to reach63% of new equilibrium temperature), and its reciprocal k, toheartbeat frequency F in a 200 g largemouth blackbass, Microp-terus salmoides . The linear regression equation (fitted by leastsquares) is K = 0 .00146 F + 0.309 . The slope is the weight-specificstroke volume (0.00146 ml/g), and the intercept is the weight-specific heat transfer constant (0 .309 cal IC- ' min - ' g-1) .

The active or convective component of heat transfervaried from approximately i i% at 3o beats min -'to 34%at io8 beats min- ' (x = 22.5% at 69 beats min- ) .

Discussions and conclusions

The range of cardiac output values calculated for the bass(44 to 158 ml kg- ' min - ) compares reasonably well withpreviously published values for fish, ranging from 5 toloo ml kg- ' min - ' (Randall, 1970). It appears that underconditions of changing ambient temperature, cardiacoutput and thermal equilibration rate vary directly withheartbeat frequency, implying a fairly constant strokevolume. This is supported by (and is supportive of) the re-sults of Stevens et al. (as reported by Randall, 1968, 1970)for the lingcod Ophiodon elongatus, in which increasingtemperature caused a marked increase in cardiac outputdue almost entirely to an increase in heartbeat frequency,while stroke volume remained constant (cf. also Grod-zinski, 1955 ; Labat et al., 1961; Laurent, 1962 ; Wilber,1961). This is in marked contrast to the cardiac responseto hypoxia, wherein cardiac output remains fairly con-stant curing hypoxic bradycardia as the fish shifts from ahigh-frequency, low-stroke-volume state to a low-fre-quency, high-stroke volume state (Randall, 1968) .

In fish, some of the heat exchange with the environ-ment occurs through the general body surface, and somethrough the gills (Crawshaw, 1976) . The portion of heattransfer through the gills is affected by perfusion and ven-tilation rates (Crawshaw, 1976 ; Reynolds, I 977b). Theseare directly correlated since, in at least some species, thereis often i : i synchrony of heartbeat and ventilatory fre-quencies during heating, cooling or hypoxia (Tsukuda,1961; Randall, 1968; Roberts, 1973, 1975 ; Reynolds,1977b). Heat flow through the body surface depends onmovement of the ambient water relative to the fish, andtherefore on activity of the fish (Dean, 1976; Miiller,1976)which in turn affects cardiac and ventilatory rates (Ran-dall, 1968) . Thus, although cardiac frequency and minutevolume are not the sole determinants of heat flow, theyare correlated with other factors that participate in ther-mal exchange .

From comparison of thermal equilibration rates oflive and dead fish, Beitinger et al. (1977) estimated thatabout 20% of thermal exchange is due to blood flow (theconvective or active component) . In the present study, theactive component was estimated to vary from 11 to 34%of the total thermal exchange, depending on heartbeat

frequency. The mean value of 22 .5% compares very wellwith that of Beitinger et al. (1977) . Most importantly,modulation of the thermal equilibration rate by heartrate is clearly demonstrated, a definitive indication(Smith, 1976) of physiological thermoregulation . Thislimited physiological response, which foreshadows im-portant physiological thermoregulatory mechanismsfound in higher vertebrates, may be of value to fish by ex-tending the time available for behavioral thermoregula-tory responses following an abrupt ambient temperaturechange .

Summary

I . Cardiac output and stroke volume were estimated fora 200 g largemouth blackbass (Micropterus salmoides)by a modified whole-body thermodilution techniqueusing the relation between thermal equilibration ratesand heartbeat frequencies .

2. The reciprocal of the thermal time constant, k(min - '), was related to the heartbeat frequency, F (beatsmin'), by the equation k = 0 .00146F + 0.309; the slope isthe weight-specific stroke volume (ml g - ') and the inter-cept is the weight-specific heat transfer constant (cal 'C - 'min - ' g -1 ) .

3. Stroke volume was 0.292 ml (0.00146 ml/g bodyweight), yielding cardiac output values ranging from 44ml kg-'min' (at 3o beats min - ') to 158 ml kg -1 min- '(at ,o8 beats min - ), or 4 .4 to 15.8% of body weight .

4. Active (convective) heat transfer due to blood flowconstituted an estimated I I to 34% (mean 22 .5%) of totalheat transfer, depending on heartbeat frequency ; thisvariability constitutes physiological thermoregulation .

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