The Influence of Preload Measured as Diastolic Mural Force on Myocardial Contractility Indices

Abstract

There have been reports in the recent literature citing the influence of ventricular preload on indices of myocardial contractility. Most of these reports used left ventricular end-diastolic pressure (LVEDP) as an indicator of preload (1-6), but few have used an indicator which incorporated a geometry factor. Since ventricular radius contributes to the force in the ventricular wall and, therefore to the load on the myocardial fiber, directly measured mural force (MF = PrR2) should provide a more accurate indication of preload changes than pressure alone. Therefore, the purpose of this investigation was to examine the influence of preload changes as measured by either M F or LVEDP on five simultaneously recorded indices of contractility. These indices were peak rate of pressure rise of left ventricular pressure (dP/dt) (1 , 2), maximum physiological velocity of the contractile elements (Vpm) (3-6), time from onset of contraction to VPM (t-Vpm) (3, 4), the ratio of dP/dt to instantaneous pressure at 5 mmHg developed pressure [(dP/dt)/K.5] (7), and the ratio of dP/dt to common peak isovolumic developed pressure [ (dP/dt)/CPIP] (1, 4).Method. Eighteen mongrel dogs weighing from 18 to 31 kg (mean = 21.4 kg) were anesthetized with 20 mg/kg pentobarbital sodium. After institution of positive pressure respiration with room air, each animal received a supplemental dose of barbital sodium (150 mg/kg) intraperitoneally to maintain a steady state of anesthesia. Polyethylene catheters were inserted in a femoral vein and artery. The tip of the arterial catheter was positioned in the ascending This study supported by USPHS Grant No. HL-14545. 2 USPHS Trainee HL-05972 Present Address: Clinical Pharmacology Program, Woodruff Memorial Building, Atlanta, Georgia 30322.aorta for the measurement of aortic blood pressure with a Statham P23Dd pressure transducer. Through a ventral midline incision in the neck, both vagii were isolated and severed. A left lateral thoracotomy was performed and the pericardium widely incised. A short (15 cm), rigid, plastic cannula was inserted into the left ventricle through the apex for measurement of left ventricular pressure (LVP) with a Statham P23Dd pressure transducer. The first derivative of LVP (dP/dt) was determined using a Hewlett-Packard 8805B Carrier Amplifier, Option 7 Differentiator. All measured parameters were recorded on a Hewlett-Packard 7878A Physiological Monitoring System. Additionally, the LVP and dP/dt were recorded and stored on magnetic tape (Hewlett-Packard 3955 Magnetic Recording System).A divide circuit (MC 1594/1494 Monolithic Four-Quadrant Multiplier; Motorola Semiconductor Products) was used to calculate the instantaneous quotient (dP/dt)/ LVP. From this recording, Vpm and t-Vpm were determined using the Voigt model for cardiac muscle so that the quotient would not tend toward infinity at low pressures. The LVP, dP/dt and (dP/dt)/K.5 were then calculated directly from this trace using developed pressure rather than total pressure. The value for CPIP was determined to be 30 ...

Assessment of Myocardial Function in the Horse 2. Experimental Findings in Resting Horses

“…In man the normal range is wide, from 841 to 1696 mm Hg/sec (Gleason and Braunwald, 1962). Similar values have been found in other species (eg, Robie and Newman, 1975;Buss and Bisgard, 1976b).…”

“…t-Vpm is said to be less influenced in other species by preload than Vpm (Mehmel et al, 1970;Robie and Newman, 1975). However, in Case 565 t-Vpm did vary with left ventricular end-diastolic pressure being shorter when end-diastolic pressure was high.…”

“…In man and some experimental animals a constant has been calculated which, when applied to plots of (dp/dt)/P against P converts the numerical value of (dp/dt)/P into "muscle lengths per sec" (Robie and Newman, 1975). No such constant is available for the horse and, therefore, plots were made using the values of (dp/dt)/P which then have units of sec-l.…”

“…It is from these curves that the index of (dp/dt)/CPIP can be derived for the left ventricle. A common developed pressure of 30 mm Hg was used as described by Robie and Newman (1975). The values are listed in Table 11.…”

Assessment of Myocardial Function in the Horse 1. Theoretical and Technical Considerations

“…Rapid volume overload, caused by infusions of dextran, increased some of the indices (dp/dt max, (dp/dt)/P) indicating the effect of preload (Mahler, Ross ,O'Rourke and Covell, 1975;Rafflenbeul, Butterich, Muller and Krayenbuhl, 1975;Robie and Newman, 1975). The ratio of (dp/dt)/K5, where K is a constant and dp/dt is measured at a developed pressure of 5 mm Hg, and the maximum measured velocity of contractile element shortening, Vpm, were not influenced by rapid volume overload (Robie and Newman, 1975). The conclusion of the above workers was that none of the indices could be used alone to assess contractility and were possibly only useful in cases where large changes are induced.…”