The effect of reduced myocardial cyclic AMP content on the response to milrinone in the isolated guinea pig heart.

BACKGROUND: Cardiac beta receptor down-regulation is associated with a reduction of tissue cyclic adenosine monophosphate (AMP) content. Milrinone exerts its effects by inhibiting the metabolism of existing cyclic AMP. The purpose of this study was to evaluate the effect of reduced myocardial cyclic AMP content on the pharmacologic action of milrinone. METHODS: A reduction of myocardial cyclic AMP content was produced by creating catecholamine depletion in the hearts of adult guinea pigs with intraperitoneal reserpine. Control animals received the reserpine vehicle. Isolated heart perfusion was maintained with modified Krebs buffer, and hearts were paced at 270 beats/min. A latex balloon and transducer-tipped catheter were inserted into the left ventricle. Isovolemic work was maintained at a constant balloon volume. Hearts from control and reserpine treated animals were perfused for 20 minutes with buffer containing either no milrinone, 1.7 x 10(-6), or 1.0 x 10(-4) mol/L milrinone (n = 12 for each dose). Maximal positive and negative dP/dt were assessed. The hearts were then frozen and cyclic AMP was measured. RESULTS: Cyclic AMP content was significantly lower in the reserpine-treated hearts at each milrinone concentration (0.33 +/- 0.01 vs 0.46 +/- 0.01; 0.33 +/- 0.01 vs 0.53 +/- 0.01; 0.30 +/- 0.01 vs 0.61 +/- 0.02 pmol/mg wet weight, p < 0.05). Milrinone significantly increased positive and negative dP/dtmax (p < 0.05), but no difference was observed between control and reserpine-treated hearts. CONCLUSIONS: Endogenous catecholamine depletion reduces myocardial cyclic AMP content but does not attenuate the response to milrinone in the isolated heart.

Thermal response in acute porcine malignant hyperthermia.

This study was designed to evaluate how vital organ and skin-surface temperatures correlate with other clinical signs of a malignant hyperthermia (MH) episode. Six susceptible swine were anesthetized with thiopental and nitrous oxide and kept normothermic (approximately equal to 38 degrees C). After a 30-min control period, halothane (1 minimum alveolar anesthetic concentration) was administered, followed in 5 min by a bolus of succinylcholine (2 mg/kg intravenously). Monitoring included: 1) ETCO2; 2)PaO2, PaCO2, pHa; 3) cardiovascular function; 4) core temperatures (esophagus, pulmonary artery, and rectum); 5) organ temperatures (brain, kidney, liver, and four skeletal muscles); and 6) skin temperatures (forehead, neck, and axilla). Within 10 min after exposure to halothane and succinylcholine, all animals developed fulminant MH. Kidney, liver, and brain temperatures increased more rapidly than pulmonary artery temperature with the onset of MH. Temperatures significantly increased in the visceral organs prior to the detection of contractures within skeletal muscles. The masseter, longissimus dorsi, quadriceps, deltoid, and extensor digiti II intramuscular temperatures were 1-2 degrees C less than pulmonary artery and esophageal temperatures during the episodes, whereas those of the kidney, liver, and brain were the same or slightly greater. When it occurs, core hyperthermia during acute MH results largely from heat produced in central organs, not in skeletal muscle per se. In these swine, changes in axilla skin surface temperatures correlated well with core temperature trends, whereas those of the neck and forehead did not. Unless a skin-surface probe can be placed in close proximity to a major vessel, cutaneous temperatures should not be substituted for measurements at an appropriate core site.

Prior hypothermia attenuates malignant hyperthermia in susceptible swine.

This study was designed to determine the extent by which mild or moderate hyperthermia attenuates the triggering of malignant hypothermia (MH) induced by the combined administration of halothane and succinylcholine. Sixteen susceptible swine were initially anesthetized with nontriggering drugs and then either kept normothermic (approximately equal to 38 degrees C, n = 6) or cooled to induce mild (approximately equal to 35 degrees C, n = 6), or moderate (approximately equal to 33 degrees C, n = 4) hypothermia. Next, after a 30-min control period, the normothermic and mildly hypothermic animals were administered 1 minimum alveolar anesthetic concentration (MAC) halothane followed by a bolus dose of succinylcholine (2 mg/kg). Within 10 min all normothermic animals developed fulminant MH, whereas the onset of MH was slowed or was absent in the mildly hypothermic group. To test whether moderate hypothermia could more effectively minimize the signs of a MH episode, this group of animals was exposed to 1.5 MAC halothane followed 10 min later by a 3-mg/kg bolus of succinylcholine. MH was not induced and anesthesia was then changed to nontriggering drugs (ketamine and pancuronium). The animals were then aggressively rewarmed to 38 degrees C: a slight increase in the ETCO2 was detected, but MH episodes did not spontaneously occur. Subsequently, the readministration of halothane and succinylcholine rapidly provoked fulminant MH. We concluded that the induction of mild hypothermia impairs triggering and reduces the progression of MH induced by the combined administration of halothane and succinylcholine, whereas moderate hypothermia was completely protective and thus could be considered for prophylaxis.

Dose-response relationship of clonidine in tetracaine spinal anesthesia.

The study was undertaken to define a dose-response relationship for clonidine in prolonging canine tetracaine spinal anesthesia. Using a randomized blind cross-over design, six mongrel dogs were given subarachnoid injections (1 ml) of the following solutions over an 8-week period: tetracaine 4 mg (control), or tetracaine 4 mg with clonidine in doses of 10, 25, 50, 100, 150, 200, and 300 micrograms. With clonidine doses equal to or exceeding 50 micrograms/ml, motor and sensory blockade were significantly (P less than 0.01) prolonged, when compared to the control times. Analysis of data by second order polynomial regression analysis produced a relationship defined by Y = 23.241 + 1.104(x) - 0.0023(x2) with r2 = 0.92 and P less than 0.001 for sensory blockade and Y = 38.7072 + 1.64425(x) - 0.004125(x2) with r2 = 0.90 and P less than 0.005 for motor blockade. From these curves, a plateau in clonidine dose-response for both sensory blockade and motor blockade occurred at 150 micrograms. The increase in duration of spinal anesthesia with clonidine may be related to a direct post-synaptic alpha 2 adrenoceptor arteriolar effect, a spinal cord pre- or post-synaptic alpha 2 antinociceptive action or supraspinal alpha 2 modulation of nociception. No animals showed evidence of neurologic dysfunction during the study. The authors conclude that a well-defined dose-response relationship exists for clonidine in canine tetracaine spinal anesthesia.

Effects of clonidine on narcotic requirements and hemodynamic response during induction of fentanyl anesthesia and endotracheal intubation.

The effects of clonidine, a centrally acting alpha 2-adrenergic receptor agonist, on depth of fentanyl anesthesia and on cardiovascular response to laryngoscopy and intubation were studied. Twenty-four patients undergoing aortocoronary bypass surgery (ACBS) with a history of arterial hypertension, coronary artery disease (NYHA class 3-4), and well-preserved left ventricular function were assigned randomly to either Group 1 (n = 12), who received standard premedication, or Group 2 (n = 12), who received clonidine 5 micrograms X kg-1 po in addition to standard premedication 90 min before estimated induction time. Depth of anesthesia was assessed by on-line aperiodic computerized analysis of the electroencephalogram (Lifescan EEG Monitor). Fentanyl was administered in 250-micrograms increments to shift the EEG to the 0.5-3-Hz frequency range (delta activity) in all subjects. In both groups, the anesthetic regimen effectively prevented hyperdynamic cardiovascular responses to laryngoscopy and intubation. No significant differences in measured or derived hemodynamic variables were observed between the two groups during the awake control period, except for stroke volume index (SVI), which was significantly greater in Group 1, 44 +/- 9 ml X beat-1 X m-2 compared with Group 2, 35 +/- 3.3 ml X beat-1 X m-2 (P less than 0.05). By contrast, fentanyl requirements in Group 2 were significantly reduced by 45% when compared with Group 1, i.e., from 110 +/- 23 to 61 +/- 19 micrograms X kg-1 (P less than 0.001). The authors conclude that at a similar anesthetic depth, as assessed by the EEG shift into the lower frequency range (0.5-3 Hz), a markedly reduced fentanyl dose effectively prevented the hyperdynamic cardiovascular response to laryngoscopy and intubation in the group of patients premedicated with clonidine. This is likely explained by the known synergistic inhibitory action of opiates and alpha 2-adrenoceptor agonists on central sympathetic outflow.

Prostaglandins and anaesthesia.

Since their discovery, the eicosanoids have been found to have a role in an increasing number of clinically important areas. In anaesthesia and intensive care, the study of the eicosanoids in the pathophysiology and treatment of various clinical problems is very recent. It is hoped that this review, along with the reference articles, will provide a basis on which to add some of the new information which is rapidly becoming available.