J Thorac Cardiovasc Surg 1999;118:568
© 1999 Mosby, Inc.
Letters to the Editor
Director, Cardiovascular Research Laboratory
Department of Surgery
New York University Medical Center
550 First Ave
Suite 9V, Skirball Building
New York City, NY 10016
To the Editor:
The brief communication "Fourier Analysis of the Intra-aortic Balloon Pump" (J Thorac Cardiovasc Surg 1999;117: 827-8) correctly demonstrates that frequency domain analysis is a powerful tool for analysis of nonlinear systems, such as the arterial bed. However, there is a misunderstanding concerning impedance matching. Arterial blood flow impedance is the relative resistance of blood flow to a given pressure differential. Typically, one analyzes instantaneous blood pressure and flow at a point in the vasculature (ascending aorta) and calculates (using frequency domain techniques) the characteristic input impedance. This is commonly displayed as amplitude and phase shifts across a frequency spectrum. The amplitude of the impedance at a given frequency represents the relative resistance to blood flow at that frequency. 1
The statement "Matching the heart rate to the fundamental frequencies obtained from frequency domain analysis of the arterial waveforms would have major benefits for patients because of the impedance matching that would result" is incorrect. Matching the heart rate to frequencies in which the characteristic input impedance is at a minimum would result in a more efficient blood flow (ie, require less energy). However, that determination is a result of computing the "input impedance," not the frequency spectra of the pressure waveform.
Indeed, the data presented in this communication demonstrate the tautologous result of analyzing the data. The author states: "Optimal heart rate with the IABP in 1:1 mode was 77 beats/min, in 1:2 mode was 69, 101, and 135 beats/min, and in 1:3 mode 66, 89, 111, and 134 beats/min." When one measures the actual patient heart rates in the tracings, the results are, respectively, 77, 69, and 66 beats/min. The analysis performed in this report only demonstrates that the patients heart rate was different at the various times the recordings were performed. This is intrinsically true, since spectral analysis of a pressure tracing measures the primary and harmonic frequencies of periodic events (ie, the patients heart rate). It is impossible to quantify arterial impedance without measuring blood flow.
The goals of input impedance matching may be worthwhile to enhance blood flow. However, there are several functions that the intra-aortic balloon pump performs, and these should not be compromised in this effort. These functions include ventricular afterload reduction and increasing diastolic blood pressure to enhance myocardial blood flow. Scientific studies are required to help us demonstrate the advantages of careful input impedance analysis of the arterial bed.
|HOME||HELP||FEEDBACK||SUBSCRIPTIONS||ARCHIVE||SEARCH||TABLE OF CONTENTS|
|ANN THORAC SURG||ASIAN CARDIOVASC THORAC ANN||EUR J CARDIOTHORAC SURG|
|J THORAC CARDIOVASC SURG||ICVTS||ALL CTSNet JOURNALS|