Pressure-volume characteristics of aortas of harbor and Weddell seals.

The mechanical properties of the radially enlarged proximal segment of the aorta of diving marine mammals was studied on 15 excised aortas of harbor seals and five aortas of Weddell seals. This was done by recording static pressure-volume relationships for the whole thoracic aorta, the aortic bulb, and the descending thoracic aorta and passive length-tension measurements of aortic strips. Aortic bulb volume distensibility was found to be much greater than that of the descending thoracic aorta or of an equivalent aortic segment of terrestrial mammals. The consequences were that the total potential energy and volume that may be stored within the aortic bulb is very large, with a capacity for storage of the stroke work of more than two normal heart beats and a volume of more than three times normal stroke volume. The aortic bulb has an average radius and wall thickness twice that of the descending aorta, but at any level of distension the wall stress (g/cm2) is the same throughout. The static mechanical properties of aortic strips from the bulb and descending thoracic aortas were not markedly different, so that the differences in the pressure-volume relationships are explained by differences in geometry of the two sections. The expanded aortic bulb functions through energy and volume storage actions and through uncoupling actions to maintain arterial pressures and stroke volume at near predive levels during a dive.

Geometric similarity of aorta, venae cavae, and certain of their branches in mammals.

The diameters of the aorta and venae cavae at various points throughout their lengths, the diameters of their major branches, and the lengths of various aortic and vena caval segments were measured in plastic corrosion casts of the arterial and venous systems of the normal adult mouse, rat, rabbit, dog, goat, horse, and cow, extending over a body weight range of 38,000-fold (arterial) and 1,100-fold (venous). It is shown that the diameters and lengths of these vessels are described by power-law equations relating the particular diameter or length to body weight (BW) raised to a particular diameter or length to body weight (BW) raised to a particular power, i.e., diameter = a BWb. Equations for the diameters and lengths of the vessels are given for slightly distended vessels and for vessels distended in the physiological range.

Functional consequences of expanded aortic bulb: a model study.

An investigation of the mechanical effects and physiological functions of the dilated ascending aorta of diving mammals was undertaken with mathematic modeling methods. A mathematical model of a prototype (canine) arterial system was constructed and was evaluated by comparing model-predicted pressure and flow wave forms at four vascular locations with published accounts of experimental measurements. The prototype model was modified to serve as a model of the diving mammal arterial system by changing peripheral vascular parameters and by changing the dimensions of the ascending aorta section of the model. This modified model gave a very good simulation of pressure and flow behavior in diving mammal arteries during a dive. Various distribution patterns of compliance addition to the prototype aortic pattern were evaluated as to the effect of these patterns on aortic input properties. It was concluded that the geometric distribution pattern found in diving mammal arteries was optimal with respect to reducing aortic impedance and peak systolic pressure development and thus favored the function of the left ventricle. This mechanical function could represent an important part of the total picture of adaptation to prolonged ischemia.

Responses of domestic fowl to repeated +Gz acceleration.

The responses of domestic fowl to repeated exposures of 4 min to +6Gz (8 times daily, 5 d weekly) are reported. Survivorship curves for the test group of 48 birds divided into three response categories: mode I, highly susceptible, with all individuals dying on the first day; mode II, more tolerant, with mortality occurring within the first 20 d of treatment; and mode III, highly tolerant, with mortality occurring only after 20 d of treatment. Observations of lymphocyte frequency, an index of systemic stress, and postmortem observations indicate that this heterogeniety has a biological basis.

Tolerance of domestic fowl to high sustained +Gz.

A system is described for the acceleration treatment of domestic fowl. A reasonable endpoint for acceleration tolerance is provided by a bradycardia which occurs fairly close to the lethal limit. In a group of 61 male Rhode Island Red chickens exposed to 6 Gz, the mean tolerance (+/- S.D.) was 11.1 +/- 10.6 min. Among individuals, the acceleration tolerance is inversely related to both body size and age, and positively to pre-treatment heart rate.

Influence of the ambient acceleration field upon acute acceleration tolerance in chickens.

It has been demonstrated both in the Russian and in the American space programs that there is a progressive deconditioning of the circulatory system with exposure to weightlessness. Astronauts and cosmonauts returned to Earth have a significantly reduced orthostatic tolerance which will persist for a few days. We have measured the acceleration tolerance of domestic fowl, acutely exposed to a 6 g+z field, as the time over which a normal heart rate can be maintained. This period of circulatory accommodation ends abruptly with a marked bradycardia, which with continued treatment is terminal. For animals which have been previously chronically accelerated (e.g., physiologically adapted to a 2.5 g field) the acute acceleration tolerance is greatly increased. It appears that the influence of the ambient acceleration field in conditioning the responsiveness of the circulatory system is a general phenomenon.

Analysis of myocardial and total body integral extraction ratios of rubidium-86.

Coronary blood flow (QM) measurement with radiorubidium (Rb) assumes that Rb distributes to the myocardium in proportion to flow. This assumption is correct if the integral myocardial Rb extraction ratio (ERM) equals total body extraction (ERTB). A right-heart-bypass preparation was employed to test the hypothesis that ERM = ERTB and to examine the determinants of Rb extraction. Dogs were anesthetized with pentobarbital, and arterial, coronary venous, and total body venous Rb concentrations were continuously measured for 4 min after injection. We found that ERM (0.56 +/- 0.01) was significantly less than ERTB (0.70 +/- 0.01), P less than 0.01 (n = 29) and concluded that Rb did not distribute in proportion to flow. We do not recommend this method for clinical use. ERM is flow dependent and ERRB is a function of the total cardiac output and the distribution of cardiac output. Before employing Rb in animal experiments, it is recommended that a preliminary study be performed comparing flow measured with Rb to an independent measure of blood flow.

Circulatory effects of halothane and halothane-nitrous oxide anesthesia in the dog: spontaneous ventilation.

The cardiovascular effects of equipotent (minimum alveolar concentration; MAC) doses of halothane versus halothane plus 25% N2O (H25N2O) in spontaneously breathing dogs do not differe except that nitrous oxide increased mean arterial pressure (AP) and decreased arterial oxygen partial pressure (PAO2). When 75% nitrous oxide was added to halothane anesthesia, AP, mean pulmonary artery pressure (PAP), heart rate (HR), cardiac output (CO), stroke volume (SV), total peripheral resistance (TPR), and left ventricular work (LVW) increased and PAO2 and hemoglobin saturation decreased. Arterial oxygen tensions below 80 torr were common at moderate and deep anesthetic levels of halothane plus 75% N2O (H75N2O). The specific contribution of N2O, hypoxemia, hypercapnia, or temporal recovery (or a combination of these) in producing cardiovascular stimulation were not determined.