Noninvasive Measurement of Cardiac Function during Exercise, Using Resaturation Curves* Lucy S. Goodenday, M.D.;• • Brian Ayotte, M.D.;t Erik S. Carlsson, M.D.;; and Malcolm B. Mcilroy, M.D.§

1be reataratioD curve, a noninvasive indicator-dilution test using an ear oldmeter to detect rates of change in arterial oxygen saturation during breathing of various concentrations of oxygen, was used to assess cardiac perfonnance in normal subjects and in 108 patients with cardJac: valvular disease. Measurements made during exercise indnded the time constant of resaturation (T) and beat-to-beat changes in arterial oxygen saturation (the left heart clearance fraction). At maldmum rates of voluntary work, patients bad a significantly reduced clear-

ance fraction and longer T than normal subjects. Clearance fraction and T improved in patients after aortic valve replacement; deterioration occurred in T and clearance fraction over time in patients treated medically, as compared to normal subjects who showed UUie change during a ten-year period. Cleanmce fraction and T correlated with hemodynamic data obtained during cardiac catheterization. 1be resaturation curve provides an objective measure of cardiac impairment that can be readUy repeated during foUow-up of patients with heart disease.

testing has become a routine procedure in Exercise most cardiac laboratories; however, the reliabil-

become a routine part of the clinical evaluation of patients in this laboratory. A total of 158 studies have been performed in 108 patients with cardiac valve disease, and serial studies at intervals up to ten years have been performed in seven normal subjects and in 22 patients with clinically unchanged heart disease. We describe here the results obtained with this test and demonstrate its usefulness in long-term follow-up of patients with cardiac disease.

ity of the information gained from such tests varies with the motivation of the patient to continue and with the familiarity of the patient with the test. 1 There is frequently a need for an objective, noninvasive, readily repeatable test that will measure not only the patient's disability (which may be what he will or won't do rather than what he can do), but also the degree of cardiac impainnent. The resaturation curve, a noninvasive index of cardiac function and a variation of indicator-dilution curves,2 was first described in 1966.3 It was then shown to give objective and repeatable values during exercise. In more than 1,000 tests in patients with various cardiac disorders since that time, the resaturation curve has proved completely safe; it has •From the Cardiovascular Research Institute and the Departments of Medicine and Radiology, University of California, San Francisco, and the Veterans Administration Hospital, San Francisco. This study was supported in part by National Institutes oE Health grants HL-06285 and 5Tl2-05740-01. ••Fonner Assistant Clinical Professor of Medicine and Chief, Cardiac Clinic, Veterans Administration Hospital; presently Assistant Professor of Medicine, University of Michigan School of Medicine, and Chief, Cardiology Division, Veterans Administration Hospital, Ann Arbor, Mich. tFormer Cardiac Resident, Cardiovascular Research Institute· presently Associate Professor of Medicine, School of Medicine, University of Manitoba, St. Boniface, Manitoba. iProfessor of Radiology. §Professor of Medicine. Manuscript received August 14, 1975; revision accepted May 18. Reprint requests: Editorial Desk, 1315 Muffrtt Hospital, Cardiooa8cular Research Institute, University of California, San Francisco 94143

732 GOODENDAY ET AL

MATERIALS AND METIIODS

The Clinical Test An ear oximeter (Waters-Conley XEOOA) was used in most studies. In some later studies another oximeter (Waters 0-500) was employed. Other necessary equipment included that found in most pulmonary function laboratories: a cycle ergometer, a breathing valve, and a physiologic recorder with an electrocardiograph and DC amplifiers. The respiratory valve must have a stopcock enabling the investigator to switch inspired gas from room air to low concentrations of oxygen and to 100 percent oxygen. Monitoring of the oxygen pressure or nitrogen pressure at the mouthpiece is helpful to remind the physician not to leave the patient breathing a gas mixture low in oxygen content. Although we measured oxygen consumption to check the work rate indicated by the cycle ergometer, this is not necessary if a calibrated ergometer is employed. Resaturation curves3 are recorded during a standard exercise test.4 The subject exercises on a cycle ergometer for a fixed period (usually six minutes) at a fixed workload while breathing room air. A short period of mild hypoxia is then induced by having the subject breathe a gas mixture low in oxygen content for one to three breaths; the intent is to bring the alveolar oxygen tension ( P A02) to between 50 and 60

CHEST, 70: 6, DECEMBER, 1976

5 Hconds

ECG

ARTERIAL~ OXYGEN

SATU~ATION

FIGURE

1. Resaturation curves obtained in a normal subject during moderate exercise.

mm Hg for five to ten seconds. The patient then breathes 100 percent oxygen, which abruptly raises the PA02 to about 350 mm Hg and causes effectively instantaneous resaturation of hemoglobin in the blood in the pulmonary capillaries. The changes in arterial oxygen saturation ( Sa02) that result from the changes in inspired oxygen tension are recorded with an ear oximeter. The sequence is repeated at higher workloads until the patient's tolerance is reached. Analysis of the Test

Analysis of resaturation curves involves measuring the time required for Sa~ to rise 63 percent of the distance to its final equilibrium value from an arbitrary initial starting point. This is the time constant ( r) of resaturation ( indicator clearance). A sample curve is shown in Figure 1, in which r is indicated. Like other indicator-dilution curves, the resaturation curve (in which reduced hemoglobin is the indicator) may be replotted on semilogarithmic paper, and a straight line may

be drawn through the points. The equation describing the curve is (1)

where C is the concentration of reduced hemoglobin in arbitrary units, Co is the initial concentration, t is the elapsed time in seconds, e is the base of the natural logarithm, and r is the time constant of the curve. r equals V/Q where V is the volume of the dominant mixing chamber between injection and sampling sites and Q is the cardiac output.li Q is the product of stroke volume and heart rate, and r can be expressed as follows : r

= V/Q = SV

v

(2)

X HR

where SV equals stroke volume and HR equals heart rate. Rearranging this equation, a dimensionless ratio, SV/V, can be calculatied: SV= 1 (3) V rXHR

Table 1--Compari.on of Reaaturarion Time Corutant (r) and Cleararu:e FrGt:rion at Similar Work Rate. in Normal Subject• and Patient. aoith CartliGt: J'al11e IJiNae • r , seconds

Work Rate (Vo., ml/min)

Normal Subjects (No.)

Patients with aortic valvular disease 279 ± 74 514 ±59 719 ±56 912 ± 60 1,101 ± 52 1,274 ± 58

3.9 2.2 1.9 1.7 1.6 1.4

Patients with mitral stenosis 734 ± 219t

1.9 ± 0.7 (20)

± 0.9 ± 0.3 ± 0.7 ± 0.5 ± 0.5 ± 0.4

(31) (20) (20) (20) (20) (20)

Clearance Fraction, percent

Patients (No.)

p

Noninvasive measurement of cardiac function during exercise, using resaturation curves.

Noninvasive Measurement of Cardiac Function during Exercise, Using Resaturation Curves* Lucy S. Goodenday, M.D.;• • Brian Ayotte, M.D.;t Erik S. Carls...
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