Effect of hemodilution on ventilatory fluctuations of pulmonary capillary blood volume

By diluting the hematocrit (Ha) in the rabbit's circulation without changing its blood volume, we found that the ventilatory-induced fluctuation (delta rho) in the density of aortic blood and Ha (which was in the range of 8-39%) are related by this linear regression: delta rho = 0.63 g/l (-0.009 + Ha). In this hemodilution experiment, the rabbits were ventilated by an intermittent positive pressure of 6 mmHg at a frequency of 30-35 cycles/min. Based on the Fahraeus effect for capillary blood flows and the dispersion of the density indicator in the rabbit's central circulation, we computed from the fluctuation of the measured density within a ventilation cycle the fluctuation of pulmonary capillary blood volume and found it to be 4.1 +/- 0.4% of the capillary blood volume for all hematocrits. Since the same fluctuation in the airway pressure was used to induce the volumetric fluctuation, its independence of Ha indicates that the hemodilution has no effect on the viscoelasticity of pulmonary capillaries.     

Effect of blood flow on gas transport in a pulmonary capillary

The effects of blood velocity on gas transport within the alveolar region of lungs, and on the lung diffusing capacity DL have for many years been regarded as negligible. The present work reports on a preliminary, two-dimensional investigation of CO convection-diffusion phenomenon within a pulmonary capillary. Numerical simulations were performed using realistic clinical and morphological parameter values, with discrete circular red blood cells (RBCs) moving with plasma in a single capillary. Steady-state simulations with stationary blood (RBCs and plasma) were performed to validate the model by comparison with published data. Results for RBCs moving at speeds varying from 1.0 mm/s to 10 mm/s, and for capillary hematocrit (Ht) from 5% to 55%, revealed an increase of up to 60% in DL, as compared to the stationary blood case. The increase in DL is more pronounced at low Ht (less than 25%) and high RBC speed and it seems to be caused primarily by the presence of plasma. The results also indicate that capillary blood convection affects DL not only by improving the plasma mixing in the capillary bed but also by replenishing the capillary with fresh (zero concentration) plasma, providing an additional reservoir for the consumption of CO. Our findings cast doubt on the current belief that an increase in the lung diffusing capacity of humans (for instance, during exercising), with fixed hematocrit, can only be accomplished by an increase in the lung volume effectively active in the respiration process.     

Effect of lung inflation on lung blood volume and pulmonary venous flow

Phasic changes in lung blood volume (LBV) during the respiratory cycle may play an important role in the genesis of the respiratory wave in arterial pressure, or pulsus paradoxus. To better understand the effects of lung inflation on LBV, we studied the effect of changes in transpulmonary pressure (delta Ptp) on pulmonary venous flow (Qv) in eight isolated canine lungs with constant inflow. Inflation when the zone 2 condition was predominant resulted in transient decreases in Qv associated with increases in LBV. In contrast, inflation when the zone 3 condition was predominant resulted in transient increases in Qv associated with decreases in LBV. These findings are consistent with a model of the pulmonary vasculature that consists of alveolar and extra-alveolar vessels. Blood may be expelled from alveolar vessels but is retained in extra-alveolar vessels with each inflation. The net effect on LBV and thus on Qv is dependent on the zone conditions that predominate during inflation, with alveolar or extra-alveolar effects being greater when the zone 3 or zone 2 conditions predominate, respectively. Lung inflation may therefore result in either transiently augmented or diminished Qv. Phasic changes in left ventricular preload may therefore depend on the zone conditions of the lungs during the respiratory cycle. This may be an important modulator of respiratory variations in cardiac output and blood pressure.     

Pulmonary diffusing capacity and pulmonary capillary blood volume during parabolic flights

Vaïda, Pierre, Christian Kays, Daniel Rivière,Pierre Téchoueyres, and Jean-Luc Lachaud.Pulmonary diffusing capacity and pulmonary capillary blood volumeduring parabolic flights. J. Appl.Physiol. 82(4): 1091-1097, 1997.Data from theSpacelab Life Sciences-1 (SLS-1) mission have shown sustained butmoderate increase in pulmonary diffusing capacity(DL). Because of the occupational constraints of the mission, data were only obtained after24 h of exposure to microgravity. Parabolic flights are often used tostudy some effects of microgravity, and we measured changes inDL occurring at the very onsetof weightlessness. Measurements ofDL, membrane diffusing capacity,and pulmonary capillary blood volume were made in 10 male subjectsduring the 20-s 0-G phases of parabolic flights performed by the"zero-G" Caravelle aircraft. Using the standardized single-breathtechnique, we measuredDL for CO andnitric oxide simultaneously. We found significant increases inDL for CO (62%),in membrane diffusing capacity for CO (47%), inDL for nitric oxide (47%), andin pulmonary capillary blood volume (71%). We conclude that majorchanges in the alveolar membrane gas transfers and in the pulmonarycapillary bed occur at the very onset of microgravity. Because thesechanges are much greater than those reported during sustainedmicrogravity, the effects of rapid transition from hypergravity tomicrogravity during parabolic flights remain questionable.

     

Ventilatory changes of pulmonary capillary blood volume assessed by arterial density

By use of an improved density measuring system, we found that the gravimetric density of arterial blood of dogs fluctuates at the same frequency as the spontaneous or mechanical ventilation. Similar density fluctuations were observed in the blood leaving isolated, perfused lobes of dogs that were ventilated cyclicly. Employing an analysis that balanced the erythrocyte and plasma flows through distensible capillaries containing blood with a tube hematocrit lower than the hematocrit in large blood vessels, we derived a relationship to estimate from the density fluctuation the change in pulmonary capillary blood volume (Vc). For mechanical ventilation, the maximum change in density over one ventilation cycle increased from 0.084 +/- 0.01 to 0.47 +/- 0.05 (SE) g/l as the frequency decreased from 29 to 6 cycles/min. These density changes were estimated to be the result of an 1-16% change in Vc. A larger tidal volume for the mechanical ventilation led to a larger density fluctuation. The maximum density change of spontaneous respiration of 6 cycles/min was one-sixth of the mechanical case, indicating a much smaller change in Vc during spontaneous respiration. When the airway flow resistance was increased for spontaneous respiration, larger density fluctuations were observed.     

Loss of alveolar membrane diffusing capacity and pulmonary capillary blood volume in pulmonary arterial hypertension

Background

Reduced gas transfer in patients with pulmonary arterial hypertension (PAH) is traditionally attributed to remodeling and progressive loss of pulmonary arterial vasculature that results in decreased capillary blood volume available for gas exchange.

Methods

We tested this hypothesis by determination of lung diffusing capacity (DL) and its components, the alveolar capillary membrane diffusing capacity (D_m) and lung capillary blood volume (V_c) in 28 individuals with PAH in comparison to 41 healthy individuals, and in 19 PAH patients over time. Using single breath simultaneous measure of diffusion of carbon monoxide (DL_CO) and nitric oxide (DL_NO), DL and D_m were respectively determined, and V_c calculated. D_m and V_c were evaluated over time in relation to standard clinical indicators of disease severity, including brain natriuretic peptide (BNP), 6-minute walk distance (6MWD) and right ventricular systolic pressure (RVSP) by echocardiography.

Results

Both DL_CO and DL_NO were reduced in PAH as compared to controls and the lower DL in PAH was due to loss of both D_m and V_c (all p < 0.01). While DL_CO of PAH patients did not change over time, DL_NO decreased by 24 ml/min/mmHg/year (p = 0.01). Consequently, D_m decreased and V_c tended to increase over time, which led to deterioration of the D_m/V_c ratio, a measure of alveolar-capillary membrane functional efficiency without changes in clinical markers.

Conclusions

The findings indicate that lower than normal gas transfer in PAH is due to loss of both D_m and V_c, but that deterioration of D_m/V_c over time is related to worsening membrane diffusion.     

Effect of acetazolamide on cerebral blood flow and capillary patency.

This study investigated the effects 2 h after administration of acetazolamide on cerebral blood flow and the pattern of cerebral capillary perfusion. Arterial blood pressure, heart rate, arterial blood gases, and pH were recorded in two groups of rats along with either regional cerebral blood flow or the percentage of capillary volume per cubic millimeter and number per square millimeter perfused as determined in cortical, thalamic, pontine, and medullary regions of the brain. Blood pressure, heart rate, and arterial PCO2 were not significantly different between the rats receiving acetazolamide (100 mg/kg) and the controls. Arterial blood pH was significantly lower in the acetazolamide rats. Blood flow increased significantly in the cortical (+ 102%), thalamic (+ 89%), and pontine (+ 88%) regions receiving acetazolamide. In control rats, approximately 60% of the capillaries were perfused in all of the examined regions. The percentage of capillaries per square millimeter perfused was significantly greater in the cortical (+ 52%), thalamic (+ 49%), and pontine (+ 47%) regions of acetazolamide rats compared with controls. In the medulla the increases in blood flow and percentage of capillaries perfused were not significant. Thus in the regions that acetazolamide increased cerebral blood flow, it also increased the percentage of capillaries perfused.