Aaron Lewicke

Measures of cardiac contractility variability during ischemia

Acute myocardial infarction (AMI) is one of the leading causes of death in the US. While heart rate variability (HRV) has been widely studied for ischemia detection, the changes in cardiac contractility variability have not yet been explored. This paper presents novel variability analysis using 23 linear and non-linear measures during myocardial ischemia. Multiple physiologic measures of cardiac contractility and heart rate variability are analyzed and compared before and after acute coronary artery occlusion in a swine model. Change in the spread of the Poincare plot of RR intervals was highly negatively correlated with the change in contractility reflective of ischemia (r = 3D-0.92, p<0.05). The change in approximate entropy of the S1 heart sound intensity was also highly correlated (r = 3D0.96, p<0.05) with the change in contractility due to ischemia. These preliminary results show the potential utility of nonlinear measures of variability to detect changes in the autonomic tone due to ischemia. These parameters, if measured continuously, may be used for early detection of AMI events in patients with implantable devices. Further research in a larger clinical study is warranted to confirm these findings.

Establishing Equivalence of Exercise Intensities

Introduction: Research trials that involve testing of physiological response to exercise and that include tests from more than one institution, or tests across long time periods, may be required to include tests from different ergometer types, mainly treadmill or leg cycle ergometer. The purpose of this study was to establish equivalence of metabolic de- mand when comparing treadmill to cycle ergometer protocols. Methods: Published equations were used to derive external power output performed on a cycle ergometer and treadmill to match external power output performance across the range of typical body weights for subjects. Results: When comparing a submaximal walking versus 10-watt incremental cycling protocol, the percent difference in metabolic demand ranged from -35% to 39% across the range of body weights from 50 kg to 150 kg. For the modified Bruce treadmill protocol, watts on the cycle had to be increased to match metabolic demand as body weight of the subject increases. Conclusion: To match estimated external power output performed on a treadmill, external power output on a leg cycle er- gometer must be increased as body weight increases.