Wood, Earl H.

In dogs studied without thoracotomy the detection of immediately appearing indicator in the left ventricle following its injection approximately 1 cm downstream from the aortic valve can be used as a reliable index of the presence or absence of aortic regurgitation. With this type of injection and under the conditions of these experiments, the ratio of the area encompassed by the immediately appearing portion of the dilution curve recorded from the left ventricle to the area encompassed by the primary portion of the curve recorded at the femoral artery bears a close positive correlation (correlation coefficient: 0.9) to the severity of aortic regurgitation as estimated by back perfusion of the valve at necropsy. Within limitations, the position of the tip of the sampling catheter in the left ventricle in dogs with aortic regurgitation is not an important determinant of the amount of immediately appearing indicator detected in this chamber. Furthermore, although the amount of indicator detected in the left ventricle following injections of very short duration is time-dependent, if the duration of the injection is extended to cover the full systolic or diastolic phase of one heartbeat or continues over one or more heartbeats, the exact timing of this injection in relation to the cardiac cycle is not an important determinant of this variable. It is concluded that under these conditions the positive correlation established between the amount of immediately appearing dye detected in the left ventricle (expressed as the regurgitant fraction) and the severity of aortic regurgitation determined by back perfusion at necropsy can be used as a valid means of estimating the severity of aoritic regurgitation in dogs. The applicability of this indicator-dilution method to the study of clinical aortic reguragitation merits further study.

In summarizing the diagnostic value of the indicator-dilution technics in the study and investigation of congenital heart disease, perhaps a simple listing of the advantages and disadvantages of the technics will suffice. The advantages are as follows: 1. The technics have a high degree of safety. In the more than 3000 cardiac patients in whom they have been utilized there have been no deaths and little or no morbidity. 2. The sensitivity of the technics for the detection and localization of right-to-left and left-to-right shunts is much superior to that of the conventional blood-gas methods. 3. Quantitative determinations of systemic and pulmonary blood flow and intrathoracic shunts by indicator-dilution methods can be made simultaneously over periods of less than 1 minute; when indicator dyes are used, these technics do not require manometrie analyses, accessory respiratory exchange studies, or active cooperation of the patient; and they are independent of foreign (anesthetic) gases in the blood. 4. The technics are versatile and can be extremely useful either for the detection and diagnosis of minimal, relatively simple defects or for the elucidation of the extremely complex types of congenital malformations. 5. The information obtained is of both a qualitative and a quantitative nature; that is, the nature and location of defects can be determined and, in addition, the magnitude and direction of flow through the defects in question as well as the levels of systemic and pulmonary blood flows can be estimated. The disadvantages of the technics are as follows: 1. Rather complicated and sensitive recording equipment is essential. 2. Considerable technical skill is required of the persons carrying out the procedure in the placement of the necessary needles and catheters and in the proper selection of the multiple possible injection and sampling sites reached by these instruments in order to obtain the information needed to establish the diagnosis in question. 3. Correct interpretation of the diagnostic significance of the dilution curves obtained requires careful study and considerable experience by the person responsible for this phase of the investigation. In other words, these technics entail relatively little discomfort and little danger for the patient, but require good equipment and, more important, considerable time, effort and skill on the part of the investigator and his assistants in order to obtain highly successful results.

The effects of passive tilting on the circulatory pressures and on the magnitude of shunts across chronic atrial septal defects were studied in 11 dogs with closed chests. These determinations were performed with the dogs in four positions: 45-degree head-up, 45-degree head-down, right lateral decubitus, and left lateral decubitus. Control values were obtained with the animals in the supine horizontal position. The 45-degree head-up tilt position was associated with an increase in the magnitude of the right-to-left shunt detected after injections of indicator into the inferior vena cava, while change from supine to the 45-degree head-down tilt resulted in a decrease or disappearance of a pre-existing right-to-left shunt detected after similar injections. These effects on the right-to-left shunt were not demonstrable after injections of indicator into the superior vena cava, either during the head-up or head-down tilt, in spite of identical alterations in circulatory pressures. A significant interanimal correlation between the magnitude of the changes in systemic or right atrial pressures and the magnitude of the changes in right-to-left shunts could not be demonstrated. Passive changes in posture from supine to 45-degree head-up or head-down positions were not associated with demonstrable systematic effects on the magnitude of the left-to-right shunts. It is postulated that the descent of the heart and diaphragm during the head-up tilt results in a closer anatomical relation between the ostium of the inferior vena cava and the defect in the atrial septum and consequently an increase in the preferential shunting of inferior caval blood through this defect. In these animals, changing the body position from the supine to the right lateral decubitus position was consistently associated with an increase in the magnitude of the left-to-right shunts across the atrial septal defect and an increase in pulmonary blood flow. Since these animals have preferential shunting of blood from the right lung in the left-to-right direction, it is believed that the increase in blood flow through the dependent right lung is responsible for the observed increase in the left-to-right shunt. The left lateral decubitus position was not associated with appreciable effects on pulmonary blood flow or left-to-right shunting of blood across the atrial septal defect.

Large changes in stroke volume and peripheral vascular resistance were induced by varying the heart rate and by intra-aortically infusing acetylcholine or angiotensin II in six dogs with heart block and electromagnetic flowmeters chronically implanted around their ascending aortas. Changes in stroke volume, aortic and atrial pressures, and systemic resistance were monitored continuously for 3-6 hours under morphine-pentobarbital anesthesia. The characteristics of diastolic pressure decay at heart rates ranging from 60 beats/min to 200 beats/min and during transient periods of asystole were studied, especially with reference to the distortions caused by reflected pressure waves. The diastolic phase of pressure pulses recorded over a segment of the thoracic aorta several centimeters long centered about 4 cm cephalad to the dorsal insertion of the diaphragm could be closely approximated by a straight line on a semilogarithmic scale. Under the conditions of these experiments, changes in the slope of that line and of its reciprocal, the time constant, correlated well with concomitant variations in peripheral vascular resistance. This relationship appears to be of practical value for continuous monitoring of systemic resistance directly from the diastolic segments of pressure pulses recorded from the lower thoracic aorta.

Preferential right-to-left shunting of inferior vena caval blood and left-to-right shunting of right-lung blood has been demonstrated to occur in experimental atrial septal defects in dogs as in the congenital defects in human beings. Existence of these preferential shunts proves that uniform mixing of blood does not occur in either atrium.

The possibility of measuring pulmonary blood flow in patients with central arteriovenous shunts utilizing the initial portion of indicator-dilution curves was explored in 16 patients by comparing these values for pulmonary blood flow with determinations by the direct Fick method. A good correlation between Fick and indicator-dilution values was obtained, and no systematic differences were demonstrated. The standard deviation of the differences between Fick and dye values was in the order of 20 to 25 per cent.

Central (aortic or subclavian), brachial, radial and femoral pressure pulses were recorded simultaneously in 12 healthy subjects during conditions of rest, exercise and 70 degree head-up tilt. Peripheral systolic pressure at rest uniformly exceeded the central systolic pressure generated by the same heartbeat. The average radial pulse pressure was 146, 146 and 165 per cent of central pulse pressure during rest, exercise and tilt while radial mean pressures were 94, 93 and 98 per cent of central mean pressures respectively. Summation of the incident pulse wave with reflected waves from the periphery and resonance effects in the peripheral arterial systems may produce these changes in pressure and contour.

In 12 patients with atrial septal defect or anomalous pulmonary venous connection, pulmonary vein wedge mean pressure uniformly exceeded mean pressure in the left atrium or pulmonary vein and with one exception was less than mean pressure in the pulmonary artery. There was a positive correlation between pulmonary arteriolar resistance and the difference in pressure between pulmonary artery and pulmonary vein wedge position. The oxygen saturation of blood, withdrawn from the pulmonary vein wedge position, uniformly exceeded that of left atrial and usually also that of pulmonary vein blood.

Relationships between artefacts in recorded intracardiac pulses and the dynamic response and susceptibility of catheter-manometer systems to artefacts caused by motion of the catheter were investigated. Sine-wave motions of the catheter, resembling motion frequently seen when the catheter is in the pulmonary artery, produce sine-wave pressure variations with peak-to-peak amplitudes of 10 mm. Hg. Square-wave motions (impacts) produce high-frequency pressure variations, greatest (10 to 200 mm. Hg) when directed along the axis and much less when perpendicular to the axis of the catheter. High-fidelity records of pressure by conventional cardiac catheter-manometer systems are most unlikely when such catheters are threaded through the beating heart.