Differential depression of myocardial contractility by halothane and isoflurane in vitro

Interaction of verapamil and halogenated inhalation anesthetics on hypoxic pulmonary vasoconstriction

Calcium channel blockers and halogenated inhalation anesthetics reduce hypoxic pulmonary vasoconstriction (HPV) when administered separately to isolated rat lungs. This study was undertaken to investigate the effect of combining the calcium channel blocker verapamil with halothane or isoflurane. HPV was elicited in three groups of experiments. First, we studied the effect of halothane 1.3 MAC and varying concentrations of verapamil. Halothane reduced HPV as a mean by 34.7%, and a dose-dependent reduction was seen with verapamil. The depressant effect of the combination of halothane and verapamil was significantly greater than when the drugs were administered alone. We further investigated in separate groups the effects of varying concentrations of halothane and isoflurane, administered both separately and in combination with a constant dose of verapamil (1.02 nmol). Both anesthetics depressed HPV in a dose-dependent fashion. Verapamil reduced HPV as a mean by 34.2% and 39.3% in the halothane and isoflurane groups, respectively. The inhibition caused by combining verapamil with an anesthetic was significantly greater than when administered separately. We conclude that verapamil in combination with halothane or isoflurane has an additive dampening effect on HPV.

Myocardial contractility, blood flow, and oxygen consumption in healthy dogs during anesthesia with isoflurane or enflurane

Left ventricular contractility (Vmax), myocardial blood flow (MBF), and oxygen consumption (O2C) were determined together with systemic hemodynamic parameters in a total of 21 mongrel dogs. Baseline recordings were obtained under basal anesthetic conditions with a narcotic (piritramid, IV). In the control group (n = 7), recordings were obtained during a three-hour observation period with infusion of piritramid. In experimental groups measurements were repeated with equi-anesthetic concentrations of isoflurane (0.7 and 1.4 vol%; n = 8) and enflurane (1.1 and 2.2 vol%; n = 6). Dose-dependent reductions of arterial pressure, cardiac output (CO) and peripheral vascular resistance were observed with isoflurane and enflurane. CO at the higher anesthetic level was depressed significantly more with enflurane. This difference was obviously due to a more severe depression of myocardial contractility with enflurane; Vmax was decreased by 18% and 26% with enflurane, but only by 10% and 17% with isoflurane (P less than 0.01). MBF and the fraction of CO received by the heart were increased above their baseline values with both concentrations of isoflurane. In contrast, the fraction of CO remained constant with enflurane while MBF decreased. O2C was reduced due to decreases of afterload and left ventricular contractility. The reduction was greater with enflurane than with isoflurane. All parameters remained unchanged in the control group. The results of this study indicate that the most striking difference in the actions of isoflurane and enflurane on cardiac parameters is on myocardial vascular resistance; MBF is increased with isoflurane, but is decreased with enflurane although myocardial perfusion pressure is reduced by almost identical amounts.(ABSTRACT TRUNCATED AT 250 WORDS)

[Is isoflurane a calcium antagonist?]

The vasodilating and myocardial depressant effects of isoflurane have been allocated to calcium channel blockade. The present study aimed to test this hypothesis by assessing the effect of isoflurane on the contractile response to potassium stimulation of vascular smooth muscle. Seventy two left anterior descending and circumflex coronary artery rings were removed in twelve dogs and mounted in organ chambers filled with Krebs-Ringer bicarbonate solution and aerated with 95% O2-5% CO2. Rings were pretreated with either 3.8% isoflurane (2.5 MAC in the dog) or 10(-8) mol.l-1 nifedipine, a calcium entry blocker. They were stimulated by addition of 10 to 150 mmol.l-1 potassium chloride. At 70 mmol.l-1 K+, the tension generated by the untreated rings was 119 +/- 4.25% of control, while in the isoflurane treated group the tension was 99 +/- 2.4% of control. In the opposite, the tension was 25 +/- 7.11% in the nifedipine treated rings. Likewise, when isoflurane was added to rings preconstricted with 40 mmol.l-1 potassium chloride, no relaxation occurred, while nifedipine produced relaxation. Isoflurane, unlike nifedipine, had a weak effect on ring tension. The calcium-entry blockade effect of isoflurane appeared weak, dose-dependent and virtually absent at clinical concentrations. Therefore, the vasodilation seen with clinical concentrations of isoflurane is mediated by mechanisms other than calcium-entry blockade.

Mechanism of decreased forward stroke volume in children and swine with ventricular septal defect and failure to thrive

Children with ventricular septal defect (VSD) often demonstrate failure to thrive (FTT). Such patients usually have reduced systemic cardiac output which has been postulated as a cause for their growth retardation. This study was conducted to ascertain the mechanism of the reduced cardiac output in children with VSD and FTT and also in a porcine model of VSD. Forward stroke volume was reduced in VSD-FTT children, 31 +/- 8 ml/m2, compared to normal children, 49 +/- 15 ml/m2 (P less than 0.05), but was not reduced in children with VSD and normal growth and development (41 +/- 16 ml/m2). Forward stroke volume was also reduced in swine with VSD compared to controls. Contractility assessed by mean velocity of circumferential shortening (Vcf) corrected for afterload was similar in normals and VSD-FTT children. Contractile performance was also similar in normal and VSD swine. Afterload assessed as systolic stress was similar in FTT-VSD children and normal subjects. Preload assessed as end-diastolic stress was increased in the VSD-FTT group. End-diastolic volume was not larger in the VSD-FTT group. We conclude that the reduced stroke volume seen in VSD-FTT children and VSD-swine was not due to reduced contractility, increased afterload or reduced preload. The reduced stroke volume may have been due to failure of end-diastolic volume to increase adequately.

Myocardial recovery from the cardiac depressant effects of enflurane and halothane

Recovery from the cardiac depressant effects of enflurane and halothane was examined in the dog heart-lung preparation (HLP) and in right ventricular muscle isolated from guinea pig hearts. In the HLP. recovery was studied under two conditions: (1) After a two-hour exposure to anesthetic concentrations increasing from 0.36 to 1.2 MAC, and (2) after a one-hour exposure to a single concentration that raised the left atrial pressure (LAP) to 9 to 10 mmHg. Under either condition, +dP/dtmax. was significantly less depressed with enflurane and returned to preanesthetic control levels, while recovery with halothane remained significantly below control. Following the longer exposure. recovery of the LAP and left ventricular function curves (LVFC) was significantly less with halothane; however, this difference was not observed after the shorter exposure period. In electrically paced, isometrically contracting right ventricular strips exposed for one hour to 2.25 vol% enflurane (a concentration that reduced contractility by 45%), force development returned within 60 minutes to values above preanesthetic control values. After an identical depression for one hour with halothane (0.80 vol%), force development recovered to values less than those observed following enflurane. These data indicate that the recovery from anesthetic-induced negative inotropic effects in isolated cardiac preparations is better with enflurane than halothane.

Effects of isoflurane on myocardial metabolism during postischaemic reperfusion in the rat

In experiments on isolated rat heart lung preparation, the effects of isoflurane on myocardial metabolism during postischaemic reperfusion were evaluated with intramyocardial high energy phosphates, lactate and glycogen. Hearts were perfused for 10 min initially and made globally ischaemic for 8 min in ischaemic groups. Afterwards, they were reperfused for 12 min. Isoflurane was administered from 5 min after the start of perfusion to the end of reperfusion. There was no significant difference in myocardial lactate levels between ischaemic isoflurane and control groups. However, the ATP level in the hearts in the ischaemic isoflurane group was significantly higher than that in the ischaemic control group (17.96 +/- 1.31 vs 15.50 +/- 0.87; P less than 0.005). The administration of isoflurane to the isolated rat heart during pre- and post-ischaemia enhanced metabolic recovery in the postischaemic state.

[Calcium inhibitors and anesthesia]

Calcium blockers (CB) are routinely used. This could lead to possible interference with anaesthetic drugs. CB prevent calcium from entering the cell by inhibiting the slow voltage-dependent calcium channels. They act mostly on heart and smooth muscle. Of all the possible indications, the three that are confirmed are coronary heart disease, arterial hypertension and supraventricular rhythm disturbances. Most of the work published and the cases reported concerns interactions between CB and halogenated anaesthetic agents; the latter's actions on the heart depend on cellular calcium exchange. Also, the cardiovascular effects of these anaesthetics are similar to that of CB. Experimentally, halothane and enflurane have direct cardiac inhibitory effects similar to verapamil and diltiazem, whereas isoflurane's properties seem closer to the dihydropyridines (nifedipine and nicardipine). Giving verapamil or diltiazem increases the number of sino-atrial and atrio-ventricular blocks when using a halogenated agent. Clinically, interpreting the effects of CB during anaesthetic induction is difficult because of the pathology (coronary heart disease, cardiac failure), the other drugs (beta-blockers and nitrates) and the type of anaesthesia (emergency or elective). Interactions can give rise to anything from a severe cardiovascular collapse, requiring catecholamines, to a mild fall in blood pressure which responds well to plasma expansion, or even no effect on blood pressure. Rebound is seen on stopping CB in patients with coronary heart disease or arterial hypertension; stopping them before surgery does not therefore seem justified. However, extreme care must be taken when using halogenated agents for patients under treatment with CB and/or beta-blockers. A wary anaesthetist will be able to adapt the technique to the patient. It has been suggested that CB could be used to treat preoperatively myocardial ischaemia (diltiazem), hypertensive crises (nifedipine, nicardipine) and ventricular rhythm disturbances (verapamil); this must be done with caution, the patient being closely monitored (haemodynamic and electrocardiographic monitoring). Postoperatively, intranasal nifedipine, continuous intravenous nicardipine or diltiazem have been used to treat increases in arterial blood pressure during recovery and to adapt the cardiovascular system to the increased metabolic needs. Here again, close patient monitoring is essential. In any case, treatment with CB which has been stopped should be started up again as soon as possible.

Differential inotropic effects of halothane and isoflurane in dog ventricular tissues

The differential effects of halothane (0.25-0.75%) and isoflurane (0.5-1.25%) on the electromechanical activity of canine ventricular tissues were compared in vitro. In Purkinje fibres, halothane but not isoflurane could induce an initial increase of contractile force which was not blocked by diltiazem or propranolol. In ventricular muscles, halothane decreased the resting state contraction more markedly than isoflurane. The results suggest that halothane induces a greater negative inotropy than isoflurane through a differential alteration of intracellular Ca2+ stores.

[Hemodynamic effects of intranasal nifedipine during clamping of the abdominal aorta]

The haemodynamic changes due to cross-clamping of the abdominal aorta below the renal arteries were studied in ten patients. Anaesthesia was induced with thiopentone and maintained with fentanyl and vecuronium and inhalation of 60% nitrous oxide in oxygen. At the fifth minute, clamping increased mean arterial pressure (Pa) by 11%, systemic vascular resistance (Rsa) by 26% and decreased cardiac output (CO) by 20%. Nifedipine was administered intranasally at this time. Heart rate remained unchanged; mean pulmonary arterial and mean pulmonary wedge pressures were slightly decreased. Pa and Rsa fell to significantly lower levels between the fifth and fifteenth minutes (24 and 43% respectively). Although CO increased by 28%, this was not significant. The administration of intranasal nifedipine during anaesthesia was well tolerated. This study demonstrated that intranasal nifedipine prevented adverse haemodynamic effects of cross-clamping of the aorta below the renal arteries.