536 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



throus^h an indwellins; cannula extending posteriorly 

 from the left atrium to the outside. A slightly longer 

 catheter on a special miniature differential pressure 

 gauge (2) is threaded through the lumen of the 

 cannula, past the mitral valve orifice and into the 

 left ventricular cavity. Pressure from a balloon in the 

 pleural space near the left ventricular wall is impressed 

 upon the back of the gauge, which then responds to 

 effective left ventricular pressure (fig. lA). 



Left ventricular outflow is continously monitored 

 by means of a pulsed ultrasonic flowmeter (2, 12, 13, 

 16, 18). In principle, the time required for a burst 

 of 3-megacycle vibrations to pass a fixed distance 

 diagonally across the root of the aorta is the same in 

 either direction when the blood is stationary (fig. iB). 

 If blood is moving out of the ventricle the sound 

 travels faster downstream than upstream and this 

 difference in transit time is directly proportional to the 

 mean velocity of flow across the aorta. If the dimen- 

 sions of the aorta are held fixed by a rigid plastic 

 cylinder supporting the ultrasonic crystals, the mean 

 velocity is directly related to the instantaneous 

 volume flow and the gauge can be directly calibrated 

 in these terms. 



Changes in cardiac dimensions have been recorded 

 in terms of the diameter, length, and circumference 

 of the left ventricle (2). Currently left ventricular 

 diameter is monitored 1000 times a second as the 

 transit time of ultrasonic bursts between two barium 

 titanate crystals mounted on opposite sides of the 

 chamber (fig. iC). Since these sound waves travel 

 1.5 mm per ixsec through the ventricular wall and 

 blood, this transit time can be readily converted into 

 distance (2, 35). Unfortunately, the complex config- 

 uration of the ventricular chamber, coupled with the 

 fact that its different dimensions do not change by the 

 same amounts, appears to preclude accurate computa- 

 tion of changes in ventricular volume from the changes 

 in any one dimension (17). However, changes in 

 diameter may be regarded as representing the action 

 of a sample of myocardium which reduces that 

 dimension during systole. 



Other variables can be derived by means of elec- 

 tronic analogue computers. When applied to an 

 appropriate differentiating circuit, the signals from 

 the pressure transducer are transformed into deflec- 

 tions whicli indicate continuously the slope (rate of 

 change) of the ventricular pressure curve. Thus, the 

 apex of the upward deflection represents the steepest 



slope during the isovolumetric pressure rise, and the 

 trough of the downward deflection represents the 

 steepest slope of the pressure drop at the end of 

 systole. 



Applying the signal from the aortic flowmeter to an 

 integrating circuit corresponds to adding the volume 

 flow for each successive instant during systole, so that 

 the peak deflection attained at the end of systole 

 represents the total flow during the stroke. By means 

 of another differentiating circuit, the rate of change 

 of diameter can be derived from the sonocardiometer 

 records. These deflections are a continuous representa- 

 tion of the changing slope of the diameter curve, 

 inxerted for comparison with the flow record. Since 

 the rate of change of volume is a definition of flow, 

 the record showing the rate of change of diameter is 

 an expression of the extent to which the changing 

 diameter is equivalent to a corresponding change in 

 ventricular volume. Comparisons between the rate of 

 change of diameter and directly registered aortic 

 flow provide an opportunity to test the internal con- 

 sistency of the observations. 



The product of the rate of change of \olume (flow) 

 and the pressure is a definition of power (the rate of 

 doing work). If the rate of change of diameter is 

 continuously multiplied by the effective left \-entric- 

 ular pressure, the resulting record illustrates a 

 function of the "power" developed by the sample of 

 myocardium which produces the change in diameter. 

 Tiie area under the power curve, derived by an 

 integrating circuit, is an expression of the "work" 

 performed during a cardiac cycle by that sample of 

 myocardium. The height of the step during each 

 successive cycle during specific inter\als (e.g., 5 

 sec) represents the accumulated work per unit time 

 and accounts for both stroke "work" and heart rate. 

 A more accurate indication of the "power" and 

 "work" developed by the entire ventricle is obtained 

 by directly multiplying the instantaneous aortic flow 

 by the effective \entricular pressure. The heart rate 

 is continuously registered by a rate meter triggered 

 by the rising phase of each successive pressure pulse. 

 The steps representing the stroke flow can be added 

 successively during consecutive 2-sec intervals to 

 provide an indication of the cardiac output per unit 

 time. This value takes into account both stroke 

 volume and heart rate. 



In various combinations these primary and deri\ed 

 parameters have been recorded during many experi- 

 mental and spontaneous adjustments in the cardiac 

 function of intact unanesthetized dogs (36). 



