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Direct myocardial effects of halothane and isoflurane. Comparison between adult and infant rabbits
B W Palmisano1, R W Mehner, D F Stowe
1Department of Anesthesiology, Medical College of Wisconsin, Milwaukee 53226.
Insights
Newborns show distinct sensitivities to anesthetics compared to adults. While less affected in heart rate, infant hearts are more vulnerable to contraction-relaxation and conduction issues with halothane.
Area of Science:
- Cardiovascular Physiology
- Anesthesiology
- Neonatal Medicine
Background:
- Infants may exhibit heightened sensitivity to myocardial depression induced by potent inhalation anesthetics.
- Existing in vivo studies in infants often involve confounding factors, limiting the understanding of direct anesthetic effects.
- This study aims to isolate and compare the direct myocardial actions of halothane and isoflurane in newborns versus adults.
Purpose of the Study:
- To determine if newborn rabbits are more sensitive than adult rabbits to the direct electrophysiologic, contractile, and metabolic effects of halothane and isoflurane.
- To investigate age-related differences in myocardial response to specific anesthetic agents.
Main Methods:
- Isolated rabbit hearts (37 infant, 36 adult) were perfused at constant pressure and exposed to three doses of halothane and isoflurane.
- Evaluated parameters included heart rate, atrioventricular conduction, left ventricular function (pressures, dP/dt, relaxation time constant), coronary flow, and oxygen metabolism.
- Direct myocardial effects were assessed by comparing measurements between infant and adult groups across different anesthetic concentrations.
Main Results:
- Halothane demonstrated greater potency in depressing cardiac function and highlighting developmental differences compared to isoflurane.
- Infant hearts showed significantly prolonged atrioventricular conduction time and isovolumic relaxation time constant with halothane versus adult hearts.
- Infant hearts were more sensitive to halothane-induced depression of developed pressure, maximum dP/dt, and diastolic pressure elevation; heart rate depression was less pronounced in infants for both agents.
- No developmental differences were observed in the effects of either anesthetic on coronary flow or oxygen metabolism.
Conclusions:
- Myocardial physiology in newborns results in decreased sensitivity to heart rate depression by halothane and isoflurane.
- Newborns exhibit increased sensitivity to halothane-induced depression of contraction-relaxation and atrioventricular conduction.
- Anesthetic management in neonates requires consideration of these age-specific myocardial sensitivities.
Background:
Infants may be more sensitive than adults to myocardial depression by potent inhalation anesthetics. Most studies of cardiovascular effects of inhalation agents in infants are performed in vivo with multiple factors producing the observed effects. The purpose of this study was to determine if newborns are more sensitive than adults to the direct actions of halothane and isoflurane on global electrophysiologic, contractile and metabolic functions of the heart.
Methods:
Direct myocardial effects of the agents were determined using isolated rabbit hearts perfused at constant pressure. Three doses of halothane and isoflurane were administered to 37 infant (3-8 days old) and 36 adult rabbit hearts. Heart rate and rhythm, atrioventricular conduction time, left ventricular function (systolic, diastolic, and developed pressures; maximum and minimum values of the differential wave (dP/dt); and time constant of isovolumic LV relaxation), coronary flow and O2 consumption, and fractional O2 extraction were measured and compared between age and anesthetic groups.
Results:
Halothane was a more potent depressant of cardiac function than isoflurane and developmental differences were more evident with this agent. The most striking developmental differences in anesthetic effects were the significantly greater prolongation of atrioventricular conduction time and the time constant of isovolumic LV relaxation by halothane in infant compared with adult hearts. Infant hearts were also more sensitive to depression of left ventricular developed pressure and maximum value of the differential wave and to elevation of diastolic pressure by halothane. For both agents heart rate was less depressed in infants than in adults. There were no developmental differences in anesthetic effects on coronary flow and O2 metabolism.
Conclusions:
Developmental changes in myocardial physiology make the newborn less sensitive to direct depression of heart rate by halothane and isoflurane, but more sensitive to depression of contraction-relaxation and atrioventricular conduction by halothane.