Effect of catheter size on pressures recorded in small pulmonary veins in dog lung.

Controversy continues about the contribution of the veins to pulmonary vascular resistance. From data obtained in studies using intravascular catheters, it appears that a major fraction (up to 44%) of the total pulmonary vascular pressure drop resides in larger (greater than 1.0 mm diam) veins, whereas micropuncture data and various models give much less pressure drop. Theoretically, artifactual pressure drops can be obtained if an intravascular catheter partly obstructs the vessel. We made measurements of pressure in the same lung vein with two different-sized catheters (1.2 and 0.6 mm OD, respectively). In paired experiments the larger catheter always measured a higher pressure than the smaller one, except close to the large lobar vein outlet. In some of the experiments we measured the diameter of the vessel containing the indwelling catheter by freezing the lung and then serial-sectioned the frozen lung. From these data we could infer that the range of vein diameter in the which the smaller catheter measured a lower pressure was 1.5-4 mm. We conclude that the larger catheter overestimated the pressure because of greater obstruction. The pressures obtained with the smaller catheter suggest that little (less than 10%) of the total pulmonary vascular resistance resides in veins larger than approximately 1 mm diam under zone 3 baseline conditions.     

Isolated dog coronary arteries response to glomerulopressin

Coronary arteries were excised from pentobarbital anesthetized normal dogs. The isolated coronary arteries were placed in an oxigenated KRB bathing solution maintained at 37 degrees C and pH 7.4 changes in the basal tone were measured. The addition of glomerulopressin to the bathing solution produced a decrease of 26 +/- 1.5 mg of the basal tone. This decrease was prolonged for more than 40 minutes. Several inhibitors of prostaglandins synthesis were added to the bath such as corticosterone (2 X 10(-5) M), indomethacin (6 X 10(-6) M), acetylsalicylic acid (1.8 X 10(-4) M) and tranylcypromine (4 X 10(-4) M). All these inhibitors blocked the action of glomerulopressin. We conclude that glomerulopressin relaxes the coronary arteries and that this relaxation may be mediated through the novobiosynthesis of prostaglandins.     

Distensibility and pressure-flow relationship of the pulmonary circulation. II. Multibranched model

The contribution of distensibility and recruitment to the distinctive behavior of the pulmonary circulation is not known. To examine this question we developed a multibranched model in which an arterial vascular bed bifurcates sequentially up to 8 parallel channels that converge and reunite at the venous side to end in the left atrium. Eight resistors representing the capillary bed separate the arterial and venous beds. The elastic behavior of capillaries and extra-alveolar vessels was modeled after Fung and Sobin (Circ. Res. 30: 451-490, 1972) and Smith and Mitzner (J. Appl. Physiol. 48: 450-467, 1980), respectively. Forces acting on each component are modified and calculated individually, thus enabling the user to explore the effects of parallel and longitudinal heterogeneities in applied forces (e.g., gravity, vasomotor tone). Model predictions indicate that the contribution of distensibility to nonlinearities in the pressure-flow (P-F) and atrial-pulmonary arterial pressure (Pla-Ppa) relationships is substantial, whereas gravity-related recruitment contributes very little to these relationships. In addition, Pla-Ppa relationships, obtained at a constant flow, have no discriminating ability in identifying the presence or absence of a waterfall along the circulation. The P-F relationship is routinely shifted in a parallel fashion, within the physiological flow range, whenever extra forces (e.g., lung volume, tone) are applied uniformly at one or more branching levels, regardless of whether a waterfall is created. For a given applied force, the magnitude of parallel shift varies with proportion of the circulation subjected to the added force and with Pla.     

Distensibility and pressure-flow relationship of the pulmonary circulation. I. Single-vessel model

To ascertain the relative contributions of vascular distensibility and nonhomogeneous behavior within the pulmonary circulation to the distinctive nonlinear relationship between inflow pressure (Pin) and flow [pressure-flow (P-F) relationship] and between Pin and outflow pressure (Pout) at constant flow (Pin-Pout relationship), we developed a multibranched model in which the elastic behavior of, and forces acting on, individual branches can be varied independently. The response of the multibranched model is described in the companion article (J. Appl. Physiol. 68: 1514-1527, 1990). Here we describe the methods used and the responses of single components of the larger model. Perivascular pressure is modeled as a function of intravascular and transpulmonary pressures (Pv and Ptp, respectively) and vessel length as a function of lung volume. These and the relationship between vascular area (A) and transmural pressure (Ptm) were modeled primarily from the dog data of Smith and Mitzner (J. Appl. Physiol. 48: 450-467, 1980). Vasomotor tone is modeled as a radial collapsing pressure (Pt) in the same plane as Ptm. In view of lack of information about the relationship between Pt and A for a given active state, different patterns were assumed that span a wide range of possible relationships. The P-F and Pin-Pout relationships of single vessels were very similar to those reported for the entire intact circulation. Of note, the slope of the Pin-Pout relationship in the low Pout range (0-5 Torr) was very low (less than 0.25) and increased gradually with Pout toward unity. Vasomotor tone caused an apparent parallel shift in the P-F relationship in the physiological flow range of the dog (2-8 l/min) regardless of the pattern used to model the Pt vs. A relationship; different patterns affected the P-F relationship only over the low flow range before the parallel shift was established.