Distribution Of Blood Flow In Lungs Research Articles

The influence of gravity on regional lung blood flow in humans: SPECT in the upright and head-down posture.

Previous studies in humans have shown that gravity has little influence on the distribution of lung blood flow while changing posture from supine to prone. This study aimed to evaluate the maximal influence of posture by comparison of regional lung blood flow in the upright and head-down posture in 8 healthy volunteers, using a tilt table. Regional lung blood flow was marked by intravenous injection of macroaggregates of human albumin labeled with 99mTc or 113mIn, in the upright and head-down posture, respectively, during tidal breathing. Both radiotracers remain fixed in the lung after administration. The distribution of radioactivity was mapped using quantitative single photon emission computed tomography (SPECT) corrected for attenuation and scatter. All images were obtained supine during tidal breathing. A shift from upright to the head-down posture caused a clear redistribution of blood flow from basal to apical regions. We conclude that posture plays a role for the distribution of lung blood flow in upright humans, and that the influence of posture, and thereby gravity, is much greater in the upright and head-down posture than in horizontal postures. However, the results of the study demonstrate that lung structure is the main determinant of regional blood flow and gravity is a secondary contributor to the distribution of lung blood flow in the upright and head-down positions.NEW & NOTEWORTHY Using a dual-isotope quantitative SPECT method, we demonstrated that although a shift in posture redistributes blood flow in the direction of gravity, the results are also consistent with lung structure being a greater determinant of regional blood flow than gravity. To our knowledge, this is the first study to use modern imaging methods to quantify the shift in regional lung blood flow in humans at a change between the upright and head-down postures.

Physiological imaging of the lung: single-photon-emission computed tomography (SPECT)

Emission tomography provides three-dimensional, quantitative images of the distribution of radiotracers used to mark physiological, metabolic, or pathological processes. Quantitative single photon emission computed tomography (SPECT) requires correction for the image-degrading effects due to photon attenuation and scatter. Phantom experiments have shown that radioactive concentrations can be assessed within some percentage of the true value when relevant corrections are applied. SPECT is widely spread, and radiotracers are available that are easy to use and comparably inexpensive. Compared with other methods, SPECT suffers from a lower spatial resolution, and the time required for image acquisition is longer than for some alternative methods. In contrast to some other methods, SPECT allows simultaneous imaging of more than one process, e.g., both regional blood flow and ventilation, for the whole lung. SPECT has been used to explore the influence of posture and clinical interventions on the spatial distribution of lung blood flow and ventilation. Lung blood flow is typically imaged using macroaggregates of albumin. Both radioactive gases and particulate aerosols labeled with radioactivity have been used for imaging of regional ventilation. However, all radiotracers are not equally suited for quantitative measurements; all have specific advantages and limitations. With SPECT, both blood flow and ventilation can be marked with radiotracers that remain fixed in the lung tissue, which allows tracer administration during conditions different from those at image registration. All SPECT methods have specific features that result from the used radiotracer, the manner in which it is administered, and how images are registered and analyzed.

Marked differences between prone and supine sheep in effect of PEEP on perfusion distribution in zone II lung

The classic four-zone model of lung blood flow distribution has been questioned. We asked whether the effect of positive end-expiratory pressure (PEEP) is different between the prone and supine position for lung tissue in the same zonal condition. Anesthetized and mechanically ventilated prone (n = 6) and supine (n = 5) sheep were studied at 0, 10, and 20 cm H2O PEEP. Perfusion was measured with intravenous infusion of radiolabeled 15-microm microspheres. The right lung was dried at total lung capacity and diced into pieces (approximately 1.5 cm3), keeping track of the spatial location of each piece. Radioactivity per unit weight was determined and normalized to the mean value for each condition and animal. In the supine posture, perfusion to nondependent lung regions decreased with little relative perfusion in nondependent horizontal lung planes at 10 and 20 cm H2O PEEP. In the prone position, the effect of PEEP was markedly different with substantial perfusion remaining in nondependent lung regions and even increasing in these regions with 20 cm H2O PEEP. Vertical blood flow gradients in zone II lung were large in supine, but surprisingly absent in prone, animals. Isogravitational perfusion heterogeneity was smaller in prone than in supine animals at all PEEP levels. Redistribution of pulmonary perfusion by PEEP ventilation in supine was largely as predicted by the zonal model in marked contrast to the findings in prone. The differences between postures in blood flow distribution within zone II strongly indicate that factors in addition to pulmonary arterial, venous, and alveolar pressure play important roles in determining perfusion distribution in the in situ lung. We suggest that regional variation in lung volume through the effect on vascular resistance is one such factor and that chest wall conformation and thoracic contents determine regional lung volume.

Distribuição do fluxo sangüíneo pulmonar na bronquiolite viral aguda

To assess lung perfusion patterns in inpatients with acute viral bronchiolitis using quantitative 99mTc-MAA scintigraphy so as to establish an association with clinical and radiological findings. A comparative, prospective case series study with a focus on diagnosis was carried out in a population of patients with acute viral bronchiolitis admitted to Hospital de Clínicas de Porto Alegre. Inclusion criteria were age between 1 and 24 months and first sudden-onset wheezing episode, suggesting bronchiolitis. The patients in the study were submitted to clinical, radiological and 99mTc-MAA lung perfusion evaluation during the first 24 hours of admission. Statistical analysis employed t test, taking into consideration a significance level of 0.05. The regional distribution of lung blood flow in the 38 patients studied was more pronounced in the upper section of the left lung in relation to the right lung (P<0.001). The distribution gradients of blood flow between the upper and middle and upper and lower sections were higher in the left lung (P<0.001). The distribution gradients of blood flow between the middle and lower sections were higher in the right lung, but without significance. The distribution gradient of the pulmonary flow in the anteroposterior axis was >1.0 in the upper and middle sections of both lungs. In the lower section it was >1.0 only in the right lung (P<0.007). There was no association between lung blood flow distribution patterns and clinical or radiological findings. There was no evidence of an association between lung blood flow distribution and ventilation-perfusion ratio in hospitalized infants with acute viral bronchiolitis; only a tendency to redirect lung blood flow towards upper lung sections was observed.

Effect of sodium nitroprusside and diethylcarbamazine on hypoxic pulmonary vasoconstriction and regional distribution of pulmonary blood flow in experimental pneumonia.

The interaction between the effects of indomethacin and sodium nitroprusside or diethylcarbamazine infusion on the efficacy of hypoxic pulmonary vasoconstriction (HPV) and regional distribution of lung blood flow was studied in 15 pentobarbital-anesthetized dogs with acute pneumonia caused by Pseudomonas aeruginosa. After induction of pneumonia central hemodynamics, gas exchange, and regional distribution of lung blood flow (radionuclide-labeled microsphere method) were measured during ventilation of both lungs with oxygen and again with one lung ventilated with nitrogen. The dogs were then randomly assigned to one of three treatment groups: Group I (n = 5) received indomethacin alone (2 mg/kg); Group I-D received indomethacin and diethylcarbamazine (50 mg/kg over 20 min followed by 1 mg/kg/min for the rest of the experiment); Group I-N (n = 5) received indomethacin with sodium nitroprusside to achieve a 20- to 30-mm Hg reduction in mean blood pressure. All measurements were then repeated during both oxygen ventilation and one-lung nitrogen ventilation. In all three groups there was no effect of nitrogen inhalation on distribution of lung blood flow prior to drug treatment, indicating absence of HPV. After treatment, in Group I, perfusion of the pneumonic lung fell from 0.27 +/- 0.08 to 0.10 +/- 0.03 (p < 0.05) of total lung blood flow, and nitrogen ventilation of the left lung reduced perfusion to that region from 0.23 +/- 0.02 to 0.13 +/- 0.02 (p < 0.05), indicating restoration of HPV. In Groups I-D and I-N, HPV was persistently absent or markedly attenuated after treatment, but the percentage of the cardiac output perfusing the pneumonia region fell by an amount similar to that in Group I (0.26 +/- 0.07 to 0.11 +/- 0.04 in Group I-D and 0.35 +/- 0.03 to 0.21 +/- 0.06 in Group I-N, both p < 0.05). Because these two chemically unrelated pulmonary vasodilators effectively blocked HPV restoration but had no effect on vasoconstriction in the pneumonia region after indomethacin, it is concluded that regional lung blood flow redistribution in pneumonia is mediated by a mechanism other than HPV.

Regional alveolar gas composition and lung function in sheep

The influence of regional alveolar oxygen and carbon dioxide tensions on the distribution of lung blood flow and gas exchange was studied in unanaesthetised sheep. Right apical lobe (RAL) hypoxia, induced by administering nitrogen or nitrogen/oxygen mixtures to the lobe, stimulated a prompt, graded and well sustained reduction in lobar blood flow. Maximum hypoxia was accompanied by an approximate 65% reduction in perfusion, a significant fall in RAL carbon dioxide tension and output, a reversal of lobar oxygen flux and an average 13 Torr fall in arterial oxygen tension. The reduction in perfusion and gas exchange persisted in the face of elevated systemic oxygen tensions produced by giving pure oxygen instead of air to the remainder of the lung (RL). Mild RAL hypercapnia potentiated the hypoxia-induced change in perfusion and gas exchange. During lobar hypoxia RL blood flow and gas exchange increased to maintain total pulmonary gas exchange at an essentially constant level. RAL hyperoxia did not significantly alter the distribution of perfusion or gas exchange.

Factors affecting regional pulmonary blood flow in left heart valvular disease

The base to apex distribution of blood flow per unit of lung volume was assessed by pulmonary scintigraphy in 99 patients with rheumatic heart disease. The prevalent valvular lesion was mitral stenosis in 56, mitral regurgitation in 14, aortic stenosis in 11 and aortic insufficiency in 18. Each patient underwent hemodynamic assessment by cardiac catheterization within a few days of scintigraphic study. Some hemodynamic parameters were then correlated with the ratio between upper and lower third of lung scan (U:L ratio) taken as a numerical index of the base to apex distribution of lung blood flow. The U:L ratio had a positive correlation coefficient with the mean pulmonary wedge pressure (MPWP) but the dispersion of individual data was wide. Dividing the patients into groups according to the level of MPWP and to the location of U:L ratio either above or below the regression line, variance analysis showed that a highly significant part of this dispersion can be accounted for by pulmonary vascular resistance (PVR). The same conclusion was reached using different statistical approaches. These findings suggest that inversion of pulmonary blood flow is not only associated with elevation of MPWP and increase of vascular resistance in the dependent lung zones but also with a rise in the general PVR. Furthermore, the cardiothoracic ratio, as index of the heart size relative to the chest cage and, therefore, of the parenchymal compression in the lower lung zones, also proved statistically and at the individual level responsible at least in part for this dispersion. Considering these multiple influences on the regional distribution of lung blood flow in these cardiac patients, a very close correlation between U:L ratio and MPWP is not to be expected. Consequently, an estimate of MPWP from U:L ratio is open to great uncertainties. Furthermore, our data show that, at the same level of MPWP, U:L ratio is higher when PVR is higher. This observation means that PVR increases and pulmonary perfusion inversion develops to some extent independently from the actual value of MPWP. We then suggest that water filtration in the lung interstitium may be ruled out as a factor responsible per se for these changes and can only be retained as a mechanism initiating them. We postulate that functional and pathologic changes in the lung vessels and interstitium may develop on the basis of a different individual response to a given stimulus.

Effects of changes in topographical distribution of lung blood flow on gas exchange.

The left lung of a dog was suspended in a Lucite box, ventilated with negative pressure, and perfused with venous blood from another dog at 37 C. By varying the pulmonary artery pressure, it was possible to perfuse all the lung or leave increasing proportions of the upper and middle lobes unperfused. The mean Pco2 difference between end-tidal gas and pulmonary venous blood was 1.7 mm Hg (se mean 0.6) when all the lung was perfused, but steadily increased up to 17 mm Hg as more and more of the lung was unperfused. Alveolar dead space/alveolar tidal volume increased linearly with the proportion of lung unperfused. By contrast, the venous admixture component remained small as increasing amounts of lung were unperfused. The results were compared with calculations made on a theoretical model of the lung and it was concluded that the uneven distribution of blood flow caused by hydrostatic pressure differences down the lung may seriously interfere with CO2 exchange when the pulmonary artery pressure is low, but that this type of uneven distribution affects O2 exchange much less. isolated lung; hydrostatic effect; alveolar-arterial differences; physiologic dead space; alveolar dead space; shunt effect; venous admixture; increased acceleration; hypoxemia; hemorrhage; positive-pressure breathing Submitted on September 10, 1964