Optimizing myocardial hypothermia: I. Temperature probe design and clinical inferences

Comparisons of methods of myocardial hypothermia for cardiac transplantation

BACKGROUND

Myocardial hypothermia of less than 10 degrees C is an essential component of preservation of donor hearts before implantation. Here we report temperature changes and comparison of methods for maintenance of myocardial hypothermia during implantation.

METHODS

Twenty patients were prospectively randomized into two equal groups. In one cohort myocardial hypothermia was maintained by the "Stanford method" of continuous lavage of the pericardium and left atrium with refrigerated saline solution. In the other a cooling jacket was used without saline lavage. Temperatures at multiple sites were measured at 30-second intervals from initiation of cardiac suturing until aortic cross-clamp removal. Comparisons were made between groups at each temperature site.

RESULTS

The cooling jacket group temperatures were significantly lower at the left ventricular epicardium and endocardium than those of the Stanford method group.

CONCLUSIONS

During cardiac implantation maintenance of myocardial hypothermia with a cooling jacket resulted in significantly deeper and more consistent hypothermia of the left ventricle than pericardial and left atrial lavage with refrigerated saline solution. Blood loss from aspirated saline lavage and perfusate dilution by the saline solution were eliminated.

Multichannel myocardial temperature probe

We present a new system to measure myocardial temperature at multiple points in myocardium. This system consists of a probe with multiple thermocouples, a data collector, and a personal computer. We have demonstrated a change in distribution of temperature in myocardium during hypothermic cardioplegic arrest.

Coronary bypass with ejection fraction of 0.20 or less using centigrade cardioplegia: long-term follow-up

Forty-two patients with an ejection fraction of 0.20 or less underwent coronary artery bypass grafting from 1986 to 1990 using a method of myocardial protection we term "centigrade cardioplegia," combining single-dose, cold, crystalloid cardioplegia, systemic hypothermia, and local hypothermia. Thirty-day mortality was 4.8% (2/42). Perioperative morbidity included two myocardial infarctions (4.8%) and one stroke (2.4%), which fully resolved. Postoperative left ventricular function improved (left ventricular ejection fraction, 0.157 +/- 0.028 to 0.226 +/- 0.085; p < 0.0002), as did New York Heart Association class (3.4 +/- 0.73 to 1.8 +/- 0.63; p < 0.0001) and Canadian class (3.3 +/- 0.81 to 0.61 +/- 0.92). Survival, 88% at 1 year, declined to 68% at 3 years and 34% at 6 years. This high-risk group had very acceptable short-term results, indicating adequate intraoperative myocardial protection. Four clinical variables were associated with long-term survival: (1) chief complaint of pain only (p = 0.05), (2) history of unstable angina (p = 0.04), (3) Canadian class less than IV (p = 0.05), and (4) New York Heart Association class less than IV (p = 0.05). Reduced survival, although not statistically significant (p = 0.07), was noted for right ventricular ejection fraction of 0.30 or less. These factors may help predict which patients with severe left ventricular dysfunction will benefit from revascularization.

Optimizing myocardial hypothermia: II. Cooling jacket modifications and clinical results

After induction of myocardial hypothermia by cold cardioplegic solution, myocardial rewarming occurs at 0.5 degrees to 1.0 degrees C/min. In addition to preventing myocardial rewarming from systemic and pulmonary venous return, continuous cooling of the myocardial surface must be provided. Modifications of a previously reported cooling jacket are described. These modifications include decreased width and thickness of the metal skeleton for easier application and increased malleability, respectively. Also, the double-row flow channel markedly minimizes obstruction of flow secondary to kinking and allows inlet and outlet lines to attach at adjacent points of the jacket thus minimizing obstruction of the operative field. The effectiveness of the jacket in 36 patients undergoing valve replacement and 19 patients having pulmonary thromboendarterectomy was evaluated by measurement of myocardial temperatures at multiple sites throughout aortic cross-clamping. Temperatures at all sites were maintained at 12 degrees C or less. Temperatures measured in phrenic nerve pedicles ranged from 25 degrees to 27 degrees C. During cooling, heat removal by the jacket was 330 calories/min. During maintenance of myocardial hypothermia, heat flow was 190 calories/min. Modifications of a cooling jacket facilitate usability and an array of sizes enhances applicability.