Myocardial protection: what the surgeon does

Normothermic versus hypothermic hyperkalemic cardioplegia: effects on myocyte contractility

BACKGROUND

This study was designed to determine the effects of prolonged hyperkalemic cardioplegic arrest under normothermic or hypothermic conditions with respect to left ventricular myocyte contractile performance and beta-adrenergic responsiveness.

METHODS

Isolated left ventricular porcine myocytes were randomly assigned to one of three groups: (group 1) normothermic control, (group 2) hypothermic cardioplegic arrest, or (group 3) normothermic cardioplegic arrest. Myocyte contractility was evaluated by high-speed video microscopy at baseline and after beta-adrenergic stimulation with isoproterenol (25 nmol/L).

RESULTS

Myocyte velocity of shortening was decreased after both hypothermic and normothermic cardioplegic arrest (68 +/- 2 and 69 +/- 2 microns/s, respectively) compared with normothermic control values (96 +/- 2 microns/s; p < 0.05). This relative reduction in baseline contractile function was equivalent in both cardioplegia groups (p = 0.5356). With beta-adrenergic stimulation, myocyte velocity of shortening was 186 +/- 4 microns/s in the hypothermic and 176 +/- 3 microns/s in the normothermic cardioplegia groups (p = 0.0563). However, myocyte contractility with beta-adrenergic stimulation was reduced in both cardioplegia groups compared with normothermic controls (205 +/- 4 microns/s; p < 0.05, respectively).

CONCLUSIONS

Hyperkalemic cardioplegic arrest under either normothermic or hypothermic conditions resulted in an equivalent reduction in baseline myocyte contractile function with reperfusion/rewarming. Hypothermic cardioplegic arrest may have provided mild protective effects on beta-adrenergic responsiveness. Nevertheless, these results suggest that an important contributory factor for diminished myocyte contractility after simulated cardioplegic arrest was prolonged exposure to a hyperkalemic environment.

Timing of tracheal extubation in adult cardiac surgery patients

In this article, we examine 14 studies conducted from 1974 to 1994 on "early" endotracheal extubation (0 to 12 hours postoperatively) in adult cardiac surgery patients. Aspects reviewed include: criteria for patient selection; criteria for extubation; analyses of feasibility and safety; effects of anesthetic technique; and patient morbidity. Advantages and disadvantages of early or "fast-track" extubation are discussed as are directions for future research. Selection criteria varied among studies; patients were most commonly excluded because of severe, preexisting pulmonary disease or ventricular dysfunction. Based on the studies examined, however, at least 70% to 80% of adult patients would meet selection criteria. Three universal criteria were applied in all studies: (1) patient is awake and responsive; (2) adequate gas exchange while breathing spontaneously; and (3) cardiovascular stability. To facilitate early extubation in appropriately selected patients, the choice of anesthetic technique and postoperative sedation technique appears to be important. Anesthetic techniques based on inhalational anesthetic agents, supplemented by moderate doses of narcotics, are more appropriate than high-dose narcotic anesthesia for early extubation protocols. Postoperative sedation with propofol, which has a rapid offset of action, may be particularly advantageous. Every published investigation has concluded that early extubation is safe, feasible, and desirable. Morbidity and mortality have not been shown to be affected by early extubation. Anesthetic technique and the patient's medical condition are the two major factors to consider in accomplishing early extubation.