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TECHNOLOGY, COMPUTING, AND SIMULATION

The Effect of Peripheral Resistance on Impedance Cardiography Measurements in the Anesthetized Dog

Lester A. H. Critchley, MD, FFARCSI^*, Zhi Y. Peng, MB BS, MD^*, Benny S. Fok, BSc^*, and Anthony E. James, BVSc, MSc

*Department of Anaesthesia and Intensive Care and The Laboratory Animal Services Centre, The Chinese University of Hong Kong, Shatin, Hong Kong, China

Address correspondence and reprint requests to Lester A. H. Critchley, MD, FFARCSI, Department of Anesthesia and Intensive Care, The Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, Hong Kong, China. Address e-mail to hcritchley{at}cuhk.edu.hk.

In the vasodilated and septic patient, the impedance method of measuring cardiac output (CO) may underestimate the true value. In this study, we sought to determine whether impedance CO (CO_IC) measurements are influenced by total peripheral resistance (TPR). In eight anesthetized and ventilated dogs, a high-precision flowprobe was placed on the ascending aorta, and direct CO was measured (CO flowprobe (CO_FP)). Mean arterial blood pressure was measured from the femoral artery. Simultaneous CO_IC measurements were made. TPR (mean arterial blood pressure x 80/CO_FP) was varied over 1–2 h by using infusions of phenylephrine and adrenaline and inhaled halothane. The bias between methods of CO measurement (CO_IC – CO_FP) was calculated and compared with TPR by using correlation and regression analysis. A total of 547 pairs of CO measurements were collected from the 8 dogs as TPR was varied. CO_FP changed by a mean of 190% (range, 89%–425%), and TPR changed by a mean of 266% (range, 94%–580%) during the experiment. The impedance method underestimated CO when TPR was low and overestimated CO when TPR was high. There was a logarithmic relationship between the CO bias and TPR. Correlation coefficients (r) between the CO bias and TPR ranged from 0.46 to 0.89 (P < 0.0001). The bias changed by 0.62 ± 1.8 L/min, or by 34%, every time TPR halved or doubled. This finding explains the poor agreement between CO_IC and other methods of CO measurement found in validation studies involving critically ill patients.

This article has been cited by other articles:

				Z.-Y. Peng, L. A. H. Critchley, and B. S. P. Fok An investigation to show the effect of lung fluid on impedance cardiac output in the anaesthetized dog Br. J. Anaesth., October 1, 2005; 95(4): 458 - 464. [Abstract] [Full Text] [PDF]

Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins with the assistance of Stanford University Libraries' HighWire Press®. Copyright 2006 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999