Malcolm B. McIlroy

Detecting the threshold of anaerobic metabolism in cardiac patients during exercise

Abstract The measurement of the respiratory gas exchange ratio (R) during a standard exercise test is used to detect the onset of anaerobic metabolism during exercise, which results from failure of the cardiovascular system to supply the oxygen requirements of the tissues. The method described uses end-tidal gas concentrations to calculate R while the exercise test is taking place. Blood sampling is unnecessary, and the results can be determined during the test, thus avoiding exhaustive exercise. The method provides an objective measurement of one of the factors influencing exercise tolerance.

Abnormalities of Renal Function and Circulatory Reflexes in Primary Aldosteronism

The renal response to intravenous administration of isotonic saline solution and circulatory reflex reactivity was studied in nine patients with primary aldosteronism before and after surgical removal of an adrenal adenoma. In preoperative tests, infusion of saline was followed by abnormally rapid renal excretion of sodium and water, such as occurs in patients with autonomic insufficiency. Circulatory reflex function was also abnormal in all patients; the most severe impairment was shown by two patients with hypokalemia and weakness of long duration. Postoperatively, both renal and circulatory reflex responses returned to normal or near normal. In preoperative tests on three patients, potassium repletion effected a similar correction of both renal and circulatory reflex abnormalities. The cause of these abnormalities in patients with primary aldosteronism is not known. Potassium depletion may be a factor, althougha general disturbance of autonomic cardiovascular function cannot be excluded.

The circulatory effects of squatting

Abstract Squatting from the standing position increases arterial blood pressure, cardiac output, and “central blood volume” in normal subjects. Squatting in the lying position or in water has no effect. Squatting exerts its action on the circulation, not by kinking arteries and veins to the legs, but by bringing the heart closer to the level of the feet. Patients with Fallots tetralogy have an increase in arterial oxygen saturation when they squat, whether at rest or after exercise. The rise in saturation is associated with a reduction in the right-to-left shunt. We suggest that the reason for the increase in saturation in patients with Fallots tetralogy is anatomical and related to a shunt from the right ventricle to the aorta.

Exercise performance of the survivors of hyaline membrane disease

We measured the cardiopulmonary response of 62 school-aged survivors of hyaline membrane disease to the stress of submaximal steady-state exercise on a cycle ergometer. The children represented a 48% follow-up of infants with HMD who-survived in our hospital in the years 1968 to 1974. They had incidences of wheezing, pulmonary infections, and abnormal chest roentgenograms after discharge similar to those in groups reported by other investigators. The first group required only supplemental oxygen or continuous positive airway pressure; the second required oxygen and CPAP plus mechanical ventilation with intermittent positive pressure; and the third, half of whom had been treated with positive pressure, developed pulmonary air leaks. Children in all groups had normal heart rates and oxygen consumptions for their heights and work rates. Ventilation was normal in all but three subjects. These three patients had had air leaks and one had severe bronchopulmonary dysplasia, requiring 36 months of oxygen therapy; the other two were only 5 years of age. All three developed hypercarbia during moderate exercise. There was no difference in the tidal volumes, respiratory rates, dead space, or the distribution of ventilation between the three groups and the normal subjects. The alveolar-to-skin oxygen tension gradient decreased in all groups and the normal subjects during light exercise, implying that the match of ventilation to perfusion improved. Most survivors of HMD appear to have normal cardiopulmonary function by school age.

Non-invasive ultrasonic cardiac output measurement in intensive care unit

A non-invasive method for measuring cardiac output utilizing M-mode echography and pulsed Doppler ultrasound is described. Measurements were obtained in 26 of 29 randomly selected, mechanically ventilated patients. These values were compared with simultaneously measured cardiac outputs by thermodilution. There was a statistically significant linear relationship between Cardiac Output measured by Doppler (DCO) and Thermodilution (TDCO): DCO = 0.86 TDCO + 0.29 l/min (r = 0.96, n = 26, SEE = 0.45 l/min) over the range of 1.75-8.5 l/min. DCO had the additional advantage of measuring peak flow velocity and maximal blood flow acceleration during early systole, indices of left ventricular pumping ability. Ultrasonic monitoring of cardiac output may be an important supplement to invasive methods in critical care.

Comparison of methods of recording and analysis of Doppler blood velocity signals in normal subjects

Doppler blood velocity signals were recorded from the aorta and pulmonary artery in normal adults, children and premature infants, using three different pulsed, range-gated instruments. The tracings analyzed by three independent methods of spectral analysis with axially aimed transducers showed velocity patterns with a narrow range of frequencies (blunt velocity profile) with a constant acceleration of blood flow in early systole. A similar velocity pattern was seen in the premature infants aorta in which the transducer beam was larger than the vessel insonated. We conclude that the normal velocity pattern in the central circulation is close to blunt, and that tracings obtained with transducers that insonate varying proportions of the vessel give similar signals.

Ear Oximeter Calibration Using Alveolar Po2Measurements

An indirect method of calibrating an ear oximeter using alveolar Po2measurements has been tested in 10 normal subjects. An oximeter calibration curve was constructed for each individual subject by comparing the output of an ear oximeter with arterial oxygen saturation calculated from alveolar Po2and PCO2during 10 to 20 seconds of hypoxia produced by breathing low oxygen mixtures in the supine position. Hypoxia is terminated by having the subject breathe oxygen, and it is assumed that arterial saturation reaches 100% and that discrepancies between end-expired and arterial gas tensions due to ventilation-perfusion imbalance, impaired diffusion, and right-to-left shunts are negligible under the conditions of calibration. The accuracy of the indirect calibration method was tested by comparing it with direct calibration using arterial blood samples. It is concluded that the indirect calibration method gives a reasonably accurate measure of arterial oxygen saturation in the range of 25–100%.

Quantitative Assessment of Peripheral Arterial Obstruction in Raynaud's Phenomenon: Development of a Predictive Model of Obstructive Arterial Cross-Sectional Area and Validation with a Doppler Blood Flow Study

The objective of this study was to develop a method for the analytical assessment of arterial obstruction in conditions of Raynauds phenomenon capable of providing diag nostic criteria. Numerous attempts have been made to determine and quantify arterial obstruction in terms of Doppler ultrasound measurements of arterial blood velocity. Absent from these methods is a formulation that allows an assessment of arterial obstruc tion based on the obstructed area as derived from direct measurement. The authors used spectral analysis of velocity signals from a pulsed, range-gated Doppler ultrasonic instru ment to make quantitative measurements of arterial blood flow velocity in hands of normal subjects and persons with Raynauds phenomenon. They measured the peak and mean velocity during the cardiac cycle and the time integral of the velocity signal over the cardiac cycle. These measurements for two distinct hemodynamic states induced by temperature changes allowed them to calculate the fractional change in arterial cross- sectional area produced by the change in temperature through the application of a hydraulic model of digital arterial circulation. They found an equation expressing frac tional obstructed area expressed as: dA/A = 2(dD - τdν - ντ)/(D + νdτ), where D is the time integral of the velocity signal; τ is the blood flow interval, ν is the blood velocity; and dD, dτ and dv are the differences in D, τ, and ν at two different hemodynamic states produced by two different temperature states. Their findings suggest that over a temper ature range of 35°-25°C, normal subjects experience 0.05/°C reduction in cross-sectional area while Raynaud subjects experience a reduction of 5.8%/°C. The results, based on findings in 13 subjects, suggest that Doppler ultrasound can differentiate persons with Raynauds phenomenon from normal subjects. Additionally, the hydraulic model appears to offer the potential of assessing relative stenotic area in other arterial obstructive diseases.

Echographic application of the Gorlin formula for assessment of aortic stenosis: Correlation with cardiac catheterization in pediatric patients

We have investigated the use of M-mode echocardiography for the diagnosis of aortic valve stenosis by application of the Gorlin formula. The hemodynamic parameters which enter this formula are shown to derive wholly from noninvasive measurements. The predicted valve areas correspond with those derived by classical catheterization studies at a level of r = 0.89, SE = 0.11 cm2, n = 10. These results suggest that the Gorlin formula for aortic stenosis may have application in a noninvasive context.

Doppler ultrasonic investigation of Raynaud's phenomenon: Effect of temperature on blood velocity

We have used spectral analysis of signals from a pulsed, range-gated Doppler ultrasonic instrument to make quantitative measurements of arterial blood flow velocity in the hands of normal subjects and persons with Raynauds phenomenon. We measured the peak velocity during the cardiac cycle and the time integral of the velocity signal over the cardiac cycle. This latter parameter gives a sensitive indication of the degree of vasoconstriction in response to cold. Our preliminary results, based on findings in 13 subjects, suggest that Doppler ultrasound can differentiate persons with Raynauds phenomenon from normal subjects.