Comparison of the effects of halothane on skinned myocardial fibers from newborn and adult rabbit: II. Effects on sarcoplasmic reticulum

Haemodynamic depression by halothane is age-related in paediatric patients

The hypothesis that young infants are more sensitive to the haemodynamic depressant effects of halothane compared with older children was tested. One hundred and sixty unpremedicated, ASA physical status I or II paediatric patients without cardiac or pulmonary disease were divided into five age groups: term neonates, 1-6 months, 6-24 months, 2-6 years and 6-12 years. Anaesthetic induction was achieved with halothane in oxygen and air via mask. Vecuronium 0.1 mg.kg-1 was administered intravenously. During normocapnic manual ventilation by mask, endtidal halothane concentration was maintained at either 2xage-specific MAC (Method I) or 1.7% (Method II) in 20 patients in each age group for 10 min. In both Method I and Method II, systolic and mean blood pressure of term neonates and infants aged 1-6 months decreased significantly (P < 0.01) compared with other age groups. The results of this study demonstrate that neonates and young infants are more susceptible to haemodynamic depression during halothane anaesthesia than are older children, confirming clinical experience.

Comparison of cross-bridge cycling kinetics in neonatal vs. adult rat ventricular muscle

The developmental shift in contractile protein isoform expression in the rodent heart likely affects actin-myosin cross-bridge interactions. We compared the Ca2+ sensitivity for force generation and cross-bridge cycling kinetics in neonatal (postnatal days 0-3) and adult (day 84) rats. The force-pCa relationship was determined in Triton-X skinned muscle bundles activated at pCa 9.0 to 4.0. In strips maximally activated at pCa 4.0, the following parameters of cross-bridge cycling were measured: (1) rate of force redevelopment following rapid shortening and restretching (ktr); and (2) isometric stiffness at maximal activation and in rigor. The fraction of attached cross-bridges (alpha fs) and apparent rate constants for cross-bridge attachment (fapp) and detachment (gapp) were derived assuming a two-state model for cross-bridge cycling. Compared to the adult, the force-pCa curve for neonatal cardiac muscle was significantly shifted to the left. Neonatal cardiac muscle also displayed significantly smaller alpha fs, slower ktr and fapp; however, gapp was not significantly different between age groups. These data indicate that weaker force production in neonatal cardiac muscle involves, at least in part, less efficient cross-bridge cycling kinetics.

Anesthetic modulation of myocardial ischemia and reperfusion injury in pigs: comparison between halothane and sevoflurane

PURPOSE

Halothane offers protection against the reperfusion injury of the myocardium. This study compared sevoflurane with halothane in its potential to modulate the effects of acute severe ischemia and reperfusion on the myocardium.

METHODS

Experiments were conducted on 25 pigs. Anesthesia consisted of thiopental, vecuronium and fentanyl. The lungs were mechanically ventilated with oxygen and nitrogen. Animals were randomly allocated to receive either I MAC halothane or sevoflurane. A control group received fentanyl and pentobarbital. Regional myocardial function was measured with sonomicrometers. The left anterior descending coronary artery was occluded for 15 min followed by 60 min reperfusion.

RESULTS

Neither halothane nor sevoflurane protected the heart against the effects of acute and severe regional myocardial ischemia. During reperfusion, 89% of the animals receiving sevoflurane suffered from ventricular fibrillation compared with 30% in the halothane group (P < 0.005). Five minutes into the reperfusion period the animals subjected to halothane anesthesia demonstrated an 88% recovery in regional myocardial systolic function while in the sevoflurane group the recovery was 40% of pre-ischemic control (P < 0.05).

CONCLUSION

Halothane is associated with less reperfusion arrhythmias and, in addition, recovery of regional myocardial function during reperfusion was more rapid in the presence of halothane than with sevoflurane.

Comparative myocardial depression of sevoflurane, isoflurane, and halothane in cultured neonatal rat ventricular myocytes

In this study, we compared the direct myocardial depressant effects of sevoflurane, isoflurane, and halothane and determined whether an L-type Ca2+ channel agonist, Bay K 8644, could attenuate the myocardial depression induced by these anesthetics in cultured neonatal rat ventricular myocytes. Ventricular myocytes were obtained from neonatal rats by enzymatic digestion with collagenase and then cultured for 6-7 days. The myocytes were stabilized in serum-free medium, and the spontaneous beating rate and contractile amplitude were measured by using a fiberoptic sensor. Each anesthetic decreased the beating rate and amplitude in a concentration-dependent manner (1%-4% vol/vol) (P < 0.001), with halothane decreasing the beating rate and amplitude the most (P < 0.01). Isoflurane caused larger decreases in the beating rate than sevoflurane at 3% and 4% (P < 0.05). Potency for suppression of contractile amplitude was in the order of halothane > > isoflurane > sevoflurane. However, the myocardial depressant effects of the anesthetics were not different when their concentrations were corrected for minimum alveolar anesthetic concentration values. Bay K 8644 significantly prevented the anesthetic-depressed amplitude (P < 0.05). We conclude that sevoflurane, isoflurane, and halothane have direct myocardial depressant effects on cultured neonatal rat ventricular myocytes and that the reduction of sarcolemmal L-type Ca2+ channel current levels mediates the myocardial depression observed in these immature hearts.

IMPLICATIONS

Sevoflurane, isoflurane, and halothane have a direct cardiodepressant effect on cardiac excitation-contraction coupling in the immature heart, which is mediated by an interaction with the L-type Ca2+ channel.

Isoflurane and halothane impair both systolic and diastolic function in the newborn pig

PURPOSE

Volatile anaesthetics have considerable effects on diastolic relaxation in the adult myocardium. We hypothesized that isoflurane (1) and halothane (H) may have even greater effects on diastolic function in the newborn, as the newborn heart has increased passive stiffness and altered calcium handling relative to the adult. Using a newborn pig model, we compared I and H at three clinically relevant concentrations with respect to both systolic and diastolic function.

METHODS

Sixteen newborn pigs were randomized for study at control (background fentanyl 100 micrograms.kg-1.hr-1 and 0.5, 1.0 and 1.5 MAC of I (n = 8) or H (n = 8). Temperature, arterial blood gases, and LVEDP were controlled. Left ventricular pressure (LVP) was monitored continuously and LV anterior-posterior dimension was determined by using sonomicrometry crystals. Systolic function was assessed by peak positive dP/dT (dP/dTmax) and the slope of the end-systolic pressure-dimension (ESP-D) relationship. Diastolic relaxation was given by peak negative dP/dT (-dP/dTmax) and the time constant for ventricular relaxation (tau). Left ventricular stiffness was calculated from the slope of the end-diastolic pressure-dimension (EDP-D) relationship.

RESULTS

At equal MAC concentrations, I and H were identical in effect for every variable studied. Systolic function was depressed at all anaesthetic concentrations. Control dP/dTmax (I:1815 +/- 561 (SD) mmHg.sec-1, H:1841 +/- 509) decreased to 832 +/- 341 with 1.5 MAC I and 691 +/- 127 with 1.5 MAC H (P < 0.05 vs control). ESP-D slope decreased from 62 +/- 31 mmHg.mm-1 at control to 15 +/- 11 with 1.5 MAC I and from 79 +/- 16 to 37 +/- 15 with 1.5 MAC H (P < 0.05 vs control). Diastolic function was affected only at higher MAC anaesthesia. Control tau increased from 18.0 +/- 6 msec to 29.1 +/- 7.5 with 1.5 MAC I and from 20.8 +/- 5.9 to 30.0 +/- 11.3 with 1.5 MAC H (P < 0.05). EDP-D slope was increased at both 1 and 1.5 MAC anaesthesia. EDP-D slope increased from 0.16 +/- 0.24 mmHg.mm-1 at control to 0.58 +/- 0.46 with I MAC I and from 0.16 +/- 11 to 0.50 +/- 0.35 with 1 MAC H. The -dP/dTmax decreased at every MAC level of anaesthesia.

CONCLUSION

These combined systolic and diastolic effects help to explain the increased sensitivity of the newborn myocardium to volatile anaesthetics.

Effects of cyclopiazonic acid, an inhibitor of the Ca++ ATPase of sarcoplasmic reticulum, on Ca++ transport, contraction and relaxation in cardiac muscle

Cyclopiazonic acid is a potent inhibitor of cardiac sarcoplasmic reticulum Ca++ ATPase. It scarely affects inotropism but significantly impairs lusitropism suggesting a greater role for cardiac sarcoplasmic reticulum in the control of cardiac relaxation than in the control of cardiac contraction.