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Pulmonary venous wedge pressure closely reflects the pulmonary arterial mean pressure. Pulmonary arterial wedge pressure is not significantly different from the left atrial mean pressure. Neither the former nor the latter is an approximation of the pulmonary capillary pressure.

Data obtained in 94 tests in nine anesthetized, thoracotomized dogs were re-examined to determine relationships among oxygen uptake per minute ( qO 2 ), cardiac frequency ( F ), mean aortic pressure ( P ), stroke volume ( V ), and heart weight ( G ). Statistical analysis supports the concept that during each systole qO 2 /G is empirically related most closely to the peak mechanical tension ( T ) developed by a spherical heart, in accord with the relationship qO 2 /G-b / F = aT , in which a and b are derived constants for each heart. Peak tension is half the product of the mean pressure and the radius (estimated from the stroke volume). Myocardial oxygen uptake is viewed as developing through a mechanical positive feedback mechanism which triggers the shortening of contractile elements. The number of such elements triggered in each beat determines the peak tension, the oxygen requirement of that beat, the resulting energy release, and the work. The oxygen costs of tension increase with the size of the heart since tension must be applied throughout the entire surface of each shell of contractile elements. Mechanical efficiency varies with the square of the stroke radius; it is not directly affected by mean aortic pressure or cardiac frequency. Factors affecting the all-or-none phenomenon, mobilization of tension, myocardial efficiency, the law of the heart, myocardial oxygen uptake, and the coronary flow are discussed in light of these findings.

SUMMARY Carotid sinus baroreceptor nerve activity was examined in eight renal hypertensive dogs with a duration of hypertension of 5 to 82 days and a mean blood pressure of 148 ± 0.5 mm Hg. These results were compared with those from 13 normotensive dogs. Whole carotid sinus nerve activity and single fiber activity were recorded in the isolated carotid sinus preparation in response to controlled static and pulsatile pressures. Stimulusresponse curves were constructed. Single fiber adapted (5-second) threshold pressure under static pressure conditions increased from 104 ± 3.2 mm Hg in the normotensive dogs to 128 ± 3.9 mm Hg in the hypertensive dogs. Inflection pressure and plateau pressure also were higher in hypertension. Impulse frequency at the inflection pressure was reduced from 39 ± 2.3 impulses/sec in normotensive dogs to 32 ± 1.7 impulses/sec in hypertensive dogs and, at the plateau pressure, from 43 ± 2.1 impulses/sec to 34 ± 1.9 impulses/sec. Whole nerve recordings also showed significantly higher threshold, inflection, and plateau pressures in the hypertensive dog. In response to pulsatile pressure, the single fiber sensitivity decreased from 0.17 ± 0.01 to 0.11 ± 0.01 impulses/cycle per mm Hg. Continuous nerve activity under these conditions occurred at a mean pressure of 166 ±3. 5 mm Hg for the normotensive dogs, compared with 188 ± 0.4 mm Hg for the hypertensive dogs. Hypertensive dogs showed an impulse frequency identical to that of normotensive dogs at pressures equivalent to each dog's unanesthetized mean blood pressure. Baroreceptor resetting was not seen before 5 days of hypertension. It was possible to reverse the resetting by lowering the blood pressure in one dog which had been hypertensive for 47 days.

An in vitro preparation was used to examine the discharge of low pressure receptors in response to a pulsatile pressure stimulus. Both slowly adapting and rapidly adapting receptors were examined. After an increase in the mean level and/or dP/dt of a pulsatile pressure input, the discharge of slowly adapting receptors reached a steady state level within 2 minutes. An increase in mean pressure, with constant pulsatile amplitude and dP/dt, produced a sustained increase in the number of spikes/cycle, in the average number of spikes/second, and in the average frequency during the burst within the cycle. In slowly adapting receptors, an increase in dP/dt at a constant mean pressure and pulsatile amplitude produced a decrease in the number of spikes/cycle and an increase in both the average number of spikes/second and the frequency during the burst. Rapidly adapting receptors showed very different characteristics, responding to a pulsatile pressure input above threshold with an irregular discharge which averages approximately 1 spike/cycle at all mean pressure levels. Although rapidly adapting receptors transiently increased their discharge in response to an increase in mean pressure, within 60 seconds discharge returned to the pre-increase level. Rapidly adapting receptors continued to discharge with an average of 1 spike/cycle as dP/dt was increased. Since dP/dt was raised by increasing the frequency of the pulsatile input, the number of spikes/second increased. In the steady state, slowly adapting receptors transmit information reflecting mean pressure levels. They are also sensitive to the rate of change of pressure. Rapidly adapting receptors, on the other hand, are insensitive to mean pressure but do respond to the frequency of the stimulus.

Dahl Salt Sensitive (DS) rats rapidly develop high blood pressure when exposed to a high-salt diet. Recent studies suggest that DS rats have poorly functioning baroreceptor afferents and baroreflexes even when salt intake is restricted. This study examines baroreceptor pressure- and mechano-transduction in DS, Dahl Resistant (DR), and Sprague-Dawley (SD) rats during low- and high-salt conditions. Single unit, regularly discharging baroreceptors were studied using an in vitro aortic arch-aortic nerve preparation. Pressure thresholds and suprathreshold pressure sensitivities were determined from responses to slow ramps of pressure. Pressure-diameter relations measured in each rat were used to transform pressure threshold and pressure sensitivity values to their mechanical equivalents in terms of aortic wall strain. A total of 407 unit baroreceptors were studied from 49 rats. Tail systolic blood pressures were significantly higher only in DS during high salt. Pressure threshold was similar for all groups on low salt. Exposure to a high-salt diet increased the mean pressure threshold for all three groups. Pressure threshold for high-salt diet was highest in DS and lowest in DR. Pressure sensitivities were lowest in DS and highest in DR on low salt. High salt had no significant effect on pressure sensitivity. The differences in threshold apparent when expressed in terms of pressure were eliminated by conversion to their mechanical equivalents (strain threshold and strain sensitivity). The results suggest that baroreceptors in the two Dahl rat strains represent two extremes from normal baroreceptor function. DS tend to be less pressure responsive than normal (SD), and DR tend to be somewhat more responsive to pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

A probe consisting of a cotton thread fitted inside a 15-cm Teflon tube with an inner diameter of 0.8 mm and connected to a transducer was used to measure the interstitial pressure in the subcutaneous tissue in six anesthetized dogs. The measurements were reproducible with a standard deviation of ±0.64 mm Hg. In six dogs the mean pressure in the subcutaneous tissue of the thorax was --2.74 mm Hg (range --3.25 to --1.99 mm Hg). After hemorrhage and dextran (10%) infusion, the mean pressure decreased by 1.70 mm Hg. After infusion of lactated Ringer's solution, the pressure increased to the control values.

Hypothermic animals subjected to total venous inflow occlusion and open cardiotomy develop elevations in right atrial mean pressure and postmortem evidences of myocardial failure. Acetyl strophanthidin, given before inflow stasis, prevented myocardial failure and increased survival. Myocardial failure was attributed to anoxia resulting from decreased coronary blood flow and alterations in the oxygen content of the blood supplying the myocardium. Perfusion of the coronary system with small quantities of oxygenated blood obtained from donor animals permitted open cardiotomy with survival.

Blood pressures have been taken from the carotid and femoral artery in sheep following experimental coarctation of the aorta at about the level of the ligamentum arteriosum. Systolic, diastolic, mean, and pulse pressures have been recorded at intervals of from 3 to 6 months over a period of as long as 3 years in some cases. Unoperated sheep have been used as controls. The main changes are a rise in all the carotid pressures and in the femoral diastolic, a fall in the femoral systolic and pulse pressures, and no significant change in the femoral mean pressure. These changes in hemodynamics resulting from coarctation may be adequately explained by the mechanical effects of the coarctation. Over a period of 3 years there is a slight rise of blood pressures in both the normal and the coarctated sheep.