Mechanics of the normal heart.

Even though studies on isolated papillary muscles and cardiomyocytes can be applied to the mechanics of a beating heart, it is not always easy for physicians to relate these findings to clinical medicine. Thus, it is important to extend the studies to intact heart either in simulations or in animal models and even better to validate the results with human subjects. Advances in engineering and computer technology have allowed us to bridge the gap between physiology and mechanics. Cardiomyocyte stress/strain relates to muscle energy expenditure, which dictates oxygen and substrate utilization. Appreciation of this sequential relationship by clinicians will facilitate the logical development and assessment of therapies. Theory of finite element analysis (FEA) can predict cardiac mechanics under normal and pathologic conditions. Imaging studies provide an avenue to relate these predictions indirectly to experimental studies. In this fashion, we can understand the mechanical basis for the micro- and macroanatomical twisting motion of the beating heart. The purposes of this manuscript are: (1) to examine the terms that are traditionally used to describe mechanical stresses and strain within the ventricle, (2) to explore the three-dimensional organization of cardiomyocytes that influences global ventricular function, (3) to apply mechanical measures to both single cardiomyofibrils and the intact ventricle (4) to evaluate mathematical and computer models used to characterize cardiac mechanics, and (5) to outline the clinical methods available to measure ventricular function and relate findings from FEA to pathologic conditions.

Quantification of surgical resident stress "on call".

BACKGROUND: We hypothesized that surgical resident stress involves both psychologic and physiologic components that manifest as changes in heart rate (HR) and circulating white blood cell (WBC) count. The purposes of this series of experiments were to monitor HR as a measure of stress "on call"; to monitor WBC count (1,000 cells/microL) during "on call" periods as a measure of stress; and to relate maximum HR and WBC count "on call" to surgical resident training level. STUDY DESIGN: HR was continuously documented by Holter monitor for 24hours "on call" in interns (n = 6), junior residents (n = 5), and senior residents (n = 5). Interns (n = 4), junior residents (n = 4), and senior residents (n = 4) during periods devoid of clinical responsibilities served as controls. WBC counts were obtained from residents "off" and "on call" for interns (n = 5) and junior residents (n = 5). RESULTS: Mean HR "on call" increased in all resident groups as compared with controls: intern mean HR increased from 71 +/- 3 to 87 +/- 2 beats per minute (bpm) (p = 0.003), junior resident mean HR increased from 74 +/- 3 to 88 +/- 4 bpm (p = 0.03), and senior resident mean HR increased from 69 +/- 2 to 80 +/- 2 bpm (p = 0.004). Intern maximum control HR was 119 +/- 3 and increased to 149 +/- 6 bpm (p = 0.005). The increase in maximum HR (control versus "on call") did not reach significance in junior residents (123 +/- 5 to 136 +/- 6 bpm, p = 0.14) and senior residents (115 +/- 6 to 116 +/- 3 bpm, p = 0.9). WBC count in interns increased from control values of 5.2 +/- 0.6 x 1,000 cells/microL to 7.5 +/- 0.9 x 1,000 cells/microL"on call" (p = 0.005). The WBC change in juniors was not significant (control: 6.8 +/- 0.7 x 1,000 cells/microL, "on call": 7.1 +/- 0.7 x 1,000 cells/microL; p = 0.37). CONCLUSIONS: When heart rate is used as an indicator of combined physiologic and psychologic stress, surgical residents achieve stress levels of tachycardia "on call." Surgical residents also exhibit an increase in circulating WBC count "on call." Both the degree of tachycardia and the increase in WBC count are inversely related to the level of training. Senior residents cope better with stress "on call" than junior residents and interns.