Regional blood flow and cardiac output during acute hypoxia in the anesthetized dog

The circulatory effects of breathing low concentrations of oxygen were studied in ten anesthetized dogs. Simultaneous measurements were made of cardiac output (indicator dilution technique) and blood flow to the head, kidney, and hind limb (electromagnetic flowmeters). Four experiments were performed with the addition of succinylcholine to inhibit the ventilatory response to hypoxia and maintain pCO2 constant. A rise in cardiac output and mean arterial pressure occurred which was significantly correlated to the decrease in arterial oxygen saturation. No threshold for these responses was found. Blood flow tended to increase during hypoxia in the regions studied but the responses were variable and only the change in renal blood flow had a significant correlation to arterial oxygen unsaturation. Systemic and regional vascular resistances during hypoxia varied both in direction and magnitude of change. The preponderant effects of hypoxia influence cardiac output more than peripheral vascular resistance.

Exercise blood flow changes in the human forearm during physical t raining

Forearm blood flow measurements were made with a venous occlusion plethysmograph on six competitive swimmers and six control subjects at rest and during a prescribed rhythmic exercise throughout a 13-week training and detraining program. The swimmers‗ mean exercise forearm blood flow increased progressively during the training program, reaching a maximum at the peak of training toward the end of the 5th week, after which it decreased to near posttraining levels during the period of detraining. The increase in the swimmer's exercise flow between the 1st week of training (4.02 ± 0.59 ml/100 ml min) and the 5th week of training (6.40 ± 0.56 ml/100 ml min) was statistically significant; their decrease in mean exercise blood flow during the 3rd week of detraining (4.70 ± 0.24 ml/100 ml min) was also statistically significant. There was no significant change in the swimmers' resting blood flow; nor were there significant changes in either the exercise or the resting blood flows of the control group. The technique for measuring blood flow in the forearm during rhythmic exercise is explained.

Submitted on December 11, 1962

Pressure-flow relationships in the perfused dog spleen

Isolated and nonisolated preparations of dog spleens were perfused at various rates by means of a blood pump inserted proximal to the splenic artery. Pressures were measured continuously and vascular resistances were calculated. In both preparations vascular resistance decreased as perfusion pressure gradient increased over a wide range (67–313 mm Hg). Pressures and resistances were consistently lower at each perfusion rate in the isolated preparations, presumably due to periarterial nerve transection. The data suggest that the fall in vascular resistance is primarily brought about by a passive opening of closed channels and a further distention of open vessels unrelated to neurogenic factors. There is no evidence that the smooth muscle of the perfused splenic vascular bed actively contracts as flow and pressure increase.

HYDRODYNAMICS OF AORTIC BLOOD FLOW

A thermistor was used to record the temperature at regular intervals across the aortic diameter of dogs during the continuous infusion of cold indicator solution at various upstream locations. The results showed that the indicator mixed across the greater part of the diameter of the aorta in the ascending portion and proximal half of the arch but maintained a more streamlined pattern in the distal portion of the arch and descending thoracic aorta. These observations were confirmed by cineangiography. When two thermistors were located in different positions in the aortic root, and when cold saline was injected suddenly in their vicinity, the temperatures recorded by the two thermistors equalized immediately following the onset of systole. Sound waves also were detected during the systolic interval in the ascending but not in the descending aorta.

These findings are consistent with the presence of disturbed, but not necessarily turbulent flow in the ascending aorta; and with predominantly streamlined, but not necessarily completely laminar flow in the descending portion. The factors producing disturbed flow in the aortic root and the transition of flow to a more streamlined pattern in the descending portion are discussed in hydrodynamic terms.

It is postulated that during systole the blood ejected into the aorta separates from a slower moving boundary layer near the walls and at the edges of the valve cusps. This separation produces disturbed flow and mixing in the entering jet. While the thin layer adjacent to the walls as well as stagnant areas behind the valves do not fully mix with the central stream, they represent a relatively small proportion of the total aortic blood flow. Additional mixing is produced by the prominent backflow in early diastole and by the branchings of the coronary and brachiocephalic arteries.

DRUG ACTION ON PULMONARY CIRCULATION OF UNANESTHETIZED DOGS

A preparation is described which overcomes most of the difficulties previously encountered in the assessment of vasomotor responses of the pulmonary vessels to pharmacologic agents. The use of electromagnetic flow transducers on the main and left pulmonary arteries allows for the observation of immediate local effects following unilateral drug administration before general systemic effects occur. The opposite lung serves as a control, since it is subjected to the same changes of left artrial pressure, pulmonary arterial pressure, and pulmonary blood flow as the test lung. Our studies have demonstrated a direct pulmonary vasoconstrictor response to 5-hydroxytryptamine and acetylcholine in both the conscious and anesthetized dog. Epinephrine was also shown to have a direct pulmonary vasoconstrictor activity, but this could be elicited in only three of the eight dogs studied. Isoproterenol, methoxamine, aminophylline, phenylephrine, γ-amino butyric acid and pentobarbital had no apparent direct pulmonary vasomotor effect.

INTRINSIC REGULATION OF SKELETAL MUSCLE BLOOD FLOW

Isolated, innervated and acutely or chronically denervated resting skeletal muscle of the dog exhibited high intrinsic vascular tone. Sustained alteration of perfusion pressure or venous pressure evoked autoregulatory responses; vascular resistance increased at elevated driving pressures and decreased at reduced driving pressures. Comparable changes of resistance were observed after sustained alteration of blood flow during pump perfusion. Autoregulation was consistently observed during muscle contractions with constant pressure or constant flow perfusion. In resting and contracting skeletal muscle a low venous blood oxygen saturation (VO 2 ) was characteristic of preparations exhibiting a high degree of autoregulation. In innervated resting muscle autoregulation was concealed by reflex or local stimuli and its disappearance was associated with a decreased vascular resistance and an increased VO 2 . In resting chronically denervated muscle or in contracting muscle, whether innervated or denervated, such stimuli failed to affect the autoregulatory response. A schema has been proposed to define the "states" of skeletal muscle blood flow and some of the factors responsible for the presence or absence of autoregulation.

MYOCARDIAL REACTIVE HYPEREMIA IN THE UNANESTHETIZED DOG

Studies of myocardial reactive hyperemia in dogs 5–24 days after the implantation of electromagnetic flowmeters on coronary artery branches reveal that the duration of occlusion and the control flow rate are independent determinants of the volume of reactive hyperemia flow. Flow debt is almost always overpaid, the average figure being 500 ± 200%. Phasic coronary flow curves show an increase in both systolic and diastolic flow during reactive hyperemia, the increase in systolic flow being due in part to decreased myocardial contractility. Myocardial reactive hyperemia is thought to be due to the accumulation of vasodilator metabolites during the period of coronary occlusion.

MEASUREMENT OF CARDIAC OUTPUT BY THERMODILUTION WITH CONSTANT RATE INJECTION OF INDICATOR

A mathematical formula has been suggested for measurement of cardiac output by the thermodilution technique using constant rate indicator injection. The theoretical assumptions have been checked in model experiments and in experiments on dogs. The best reproducibility was attained when the indicator was injected into the right atrium, near the tricuspid valve, and the dilution was detected in the pulmonary artery. Recirculation of the cool indicator is negligible and takes place only after the attainment of the curve plateau. No systematic difference was found in simultaneous measurements made by the Fick principle and the injection technique. The correlation coefficient, r , was 0.96.

BUFFER DISTRIBUTION IN ISOLATED PERFUSED RAT HEART MEASURED BY RUBIDIUM 86

The pattern of distribution of Rb86 in sections of the isolated perfused heart was determined as a measure of uniformity of coronary artery flow. Results suggest that coronary flow was uniform after nonrecirculated perfusion for 15 min. Similar results were obtained in hearts perfused for 30 and for 60 min. The distribution of Rb86 was comparable in hearts perfused for 30 min with perfusate circulated through either coarse porosity, sintered-glass filters or 5-µ Millipore filters. A nonuniform distribution of isotope was demonstrated with ligation of branches of coronary arteries and with the introduction of air. The results suggest that hearts perfused by recirculation of buffer through coarse porosity, sintered-glass filters have a uniform distribution of perfusate throughout the heart.

VISCOSITY AND CLOTTING OF BLOOD IN VENOUS THROMBOSIS AND CORONARY OCCLUSIONS

Viscosity of blood was measured in the absence of anticoagulants by means of an improved cone-in-cone viscometer. Data obtained in a study of a series of patients suffering from venous thrombosis and coronary occlusion were compared with data from a control group of healthy subjects. The probability that the large difference in viscosity observed between normal and abnormal blood is due to chance is approximately 1 in 1,000,000. The viscosity of blood remains constant, at any rate of shear, during the first two to four minutes after removal of blood. Clotting starts after various time periods, depending not only on the intrinsic properties of the particular blood sample, but also on the velocity gradient at which such clotting takes place.

The following general trends were established : at 36°C the onset of clotting is much more rapid at high rates of shear than at low rates of shear. It appears, however, that at lowered temperatures this trend is reversed.

The causes of aggregation of red cells, and the role of this aggregation in clotting, are discussed. It is emphasized that blood is thixotropic, i.e., that its viscosity is both time and rate-of-shear dependent. Thixotropy of blood is due, mainly, to a reversible aggregation of red cells. This suggests the existence of intermittent sedimentation of red cell aggregates during flow in blood vessels. Owing to the existence of a velocity profile, in which the axial region is under zero or near-zero rate of shear, such aggregates should be present especially in large veins and arteries. It appears that an increased thixotropy and viscosity are symptomatic of some thrombotic states.

FUNCTIONAL CARDIAC DETERIORATION DURING DEVELOPMENT OF HEMORRHAGIC CIRCULATORY DEFICIENCY

A massive infusion test has been devised to evaluate the functional capacity of the heart.

In five dogs extensive surgical procedures followed by episodes of arterial hypotension at 30 mm Hg and lasting 90 minutes caused severe cardiac incapacity, which was demonstrated by failure of rate of cardiac work to increase throughout the infusion together with a large rise in left atrial pressure. This incapacity was not evident in tests made immediately after the hypotensive episode. The damage became more and more severe in tests performed 90 and 150 minutes after the end of the hypotension even though mean arterial pressure was maintained at 100 mm Hg. Operated control dogs did not show cardiac deterioration in tests timed as above. The rate of cardiac work continued to increase throughout the infusion with a relatively small rise of left atrial pressure.

In a series of five dogs, flow to the coronary arteries and to those supplying the head and forelimbs was maintained by normal blood pressure while the rest of the animal was made hypotensive for 90 minutes. The arterial pressure in the whole animal was then returned to an average level of 100 mm Hg. Tests made at intervals after the hypotension showed equivocal evidence of cardiac deterioration.

In another series of four dogs, prepared as above, the partial hypotension lasted 150 minutes before the arterial pressure was returned to normal. Tests at intervals after hypotension showed cardiac damage in all the animals and severe damage in three. Evidence is presented indicating that the peripheral vascular bed had deteriorated functionally as a result of 150 minutes of partial hypotension.

LOCAL BLOOD FLOW IN HUMAN LEG MUSCLE MEASURED BY A TRANSIENT RESPONSE THERMOELECTRIC METHOD

The initial transient response of a Gibbs type thermoelectric probe embedded in human resting leg muscle was used for absolute quantitative measurement of local blood flow per unit tissue volume (local perfusion). The probe consisted of two thermistor-containing needles, one of which was heated by a constant electrical power input. The temperatures of both thermistors were recorded continuously on a two-channel, fast-response recorder. Upon sudden occlusion of the blood flow to the leg, each temperature vs. time record exhibited a change of slope. The change in slope of the temperature difference, divided by the temperature difference, (degrees/minute degree) was identified with the local perfusion (milliliters/minute milliliter) existing just before occlusion. The local perfusions determined agreed in range and mean with literature values of average perfusion by venous occlusion plethysmography. The nature of the local blood flow measured by the present method is discussed relative to that by other methods.