Johan Petersson

Posture primarily affects lung tissue distribution with minor effect on blood flow and ventilation

We used quantitative single photon emission computed tomography to estimate the proportion of the observed redistribution of blood flow and ventilation that is due to lung tissue shift with a change in posture. Seven healthy volunteers were studied awake, breathing spontaneously. Regional blood flow and ventilation were marked using radiotracers that remain fixed in the lung after administration. The radiotracers were administered in prone or supine at separate occasions, at both occasions followed by imaging in both postures. Images showed greater blood flow and ventilation to regions dependent at the time of imaging, regardless of posture at radiotracer administration. The results suggest that a shift in lung parenchyma has a major influence on the imaged distributions. We conclude that a change from the supine to the prone posture primarily causes a change in the vertical distribution of lung tissue. The effect on the vertical distribution of blood flow and ventilation within the lung parenchyma is much less.

Lung Ventilation and Perfusion in Prone and Supine Postures with Reference to Anesthetized and Mechanically Ventilated Healthy Volunteers

Background:The literature on ventilation (V) and lung perfusion (Q) distributions during general anesthesia and controlled mechanical ventilation in supine and prone position is contradictory. The authors aimed to investigate whether V, Q, and ventilation to perfusion ratio (V/Q ratio) matching in anesthetized and mechanically ventilated volunteers are gravity dependent irrespective of posture. Methods:Seven healthy volunteers were studied at two different occasions during general anesthesia and controlled mechanical ventilation. One occasion studied ventral to dorsal V and Q distributions in the supine posture and the other in the prone posture. Imaging was performed in supine posture at both occasions. A dual radiotracer technique and single photon emission computed tomography were used. V and Q were simultaneously tagged with 99mTc-Technegas (Tetley Manufacturing Ltd., Sydney, Australia) and 113mIn-labeled macroaggregates of human albumin (TechneScan LyoMAA, Mallinckrodt Medica, Petten, The Netherlands), respectively. Results:No differences in V between postures were observed. Q differed between postures, being more uniform over different lung regions in prone posture and dependent in supine posture. The contribution of the vertical direction to the total V/Q ratio heterogeneity was larger in supine (31.4%) than in prone (16.4%) (P = 0.0639, two-tailed, paired t test) posture. Conclusions:During mechanical ventilation, prone posture favors a more evenly distributed Q between lung regions. V distribution is independent of posture. This results in a tendency toward lower V/Q gradients in the ventral to dorsal direction in prone compared with supine posture.

Regional lung blood flow and ventilation in upright humans studied with quantitative SPECT

We used quantitative Single Photon Emission Computed Tomography (SPECT) to study the effect of the upright posture on regional lung blood flow and ventilation. Nine (upright) plus seven (prone and supine) healthy volunteers were studied awake, breathing spontaneously. Regional blood flow and ventilation were marked in sitting upright, supine and prone postures using (113m)In-labeled macroaggregates and inhaled Technegas ((99m)Tc); both remain fixed in the lung after administration. All images were obtained while supine. In comparison with horizontal postures, both blood flow and ventilation were greater in caudal regions when upright. The redistribution was greater for blood flow than for ventilation, resulting in decreasing ventilation-to-perfusion ratios down the lung when upright. We conclude that gravity redistributes regional blood flow and ventilation in the upright posture, while the influence is much less in the supine and prone postures.

Gas exchange and ventilation–perfusion relationships in the lung

This review provides an overview of the relationship between ventilation/perfusion ratios and gas exchange in the lung, emphasising basic concepts and relating them to clinical scenarios. For each gas exchanging unit, the alveolar and effluent blood partial pressures of oxygen and carbon dioxide (PO2 and PCO2) are determined by the ratio of alveolar ventilation to blood flow (V′A/Q′) for each unit. Shunt and low V′A/Q′ regions are two examples of V′A/Q′ mismatch and are the most frequent causes of hypoxaemia. Diffusion limitation, hypoventilation and low inspired PO2 cause hypoxaemia, even in the absence of V′A/Q′ mismatch. In contrast to other causes, hypoxaemia due to shunt responds poorly to supplemental oxygen. Gas exchanging units with little or no blood flow (high V′A/Q′ regions) result in alveolar dead space and increased wasted ventilation, i.e. less efficient carbon dioxide removal. Because of the respiratory drive to maintain a normal arterial PCO2, the most frequent result of wasted ventilation is increased minute ventilation and work of breathing, not hypercapnia. Calculations of alveolar–arterial oxygen tension difference, venous admixture and wasted ventilation provide quantitative estimates of the effect of V′A/Q′ mismatch on gas exchange. The types of V′A/Q′ mismatch causing impaired gas exchange vary characteristically with different lung diseases. A review of ventilation–perfusion relationships and gas exchange, basic concepts and their relation to clinical cases http://ow.ly/wMUwq

Positive End-expiratory Pressure Redistributes Regional Blood Flow and Ventilation Differently in Supine and Prone Humans

Background:Animal studies have demonstrated an interaction between posture and the effect of positive end-expiratory pressure (PEEP) on regional ventilation and lung blood flow. The aim of this study was to explore this interaction in humans. Methods:Regional lung blood flow and ventilation were compared between mechanical ventilation with and without PEEP in the supine and prone postures. Six normal subjects were studied in each posture. Regional lung blood flow was marked with 113mIn-labeled macroaggregates and ventilation with Technegas (99mTc). Radiotracer distributions were mapped using quantitative single-photon emission computed tomography. Results:In supine subjects, PEEP caused a similar redistribution of both ventilation and blood flow toward dependent (dorsal) lung regions, resulting in little change in the V/Q correlation. In contrast, in prone subjects, the redistribution toward dependent (ventral) regions was much greater for blood flow than for ventilation, causing increased V/Q mismatch. Without PEEP, the vertical ventilation-to-perfusion gradient was less in prone postures than in supine, but with PEEP, the gradient was similar. Conclusions:During mechanical ventilation of healthy volunteers, the addition of PEEP, 10 cm H2O, causes redistribution of both lung blood flow and ventilation, and the effect is different between the supine and prone postures. Our results suggest that the addition of PEEP in prone might be less beneficial than in supine and that optimal use of the prone posture requires reevaluation of the applied PEEP.

Protective effect of prone posture against hypergravity-induced arterial hypoxaemia in humans

Patients with acute respiratory distress syndrome have increased lung tissue weight and therefore an increased hydrostatic pressure gradient down the lung. Also, they have a better arterial oxygenation in prone (face down) than in supine (face up) posture. We hypothesized that this effect of the direction of gravity also existed in healthy humans, when increased hydrostatic gradients were induced by hypergravity. Ten healthy subjects were studied in a human centrifuge while exposed to 1 or 5 G in anterio‐posterior (supine) or posterio‐anterior (prone) direction. We measured blood gases using remote‐controlled sampling and gas exchange by mass spectrometry. Hypergravity led to marked impairments of arterial oxygenation in both postures and more so in supine posture. At 5 G, the arterial oxygen saturation was 84.6 ± 1.2 % (mean ±s.e.m.) in supine and 89.7 ± 1.4 % in prone posture (P < 0.001 for supine vs. prone). Ventilation and alveolar PO2 were increased at 5 G and did not differ between postures. The alveolar‐to‐arterial PO2 difference increased at 5 G to 8.0 ± 0.2 kPa and 6.6 ± 0.3 kPa in supine and prone postures (P= 0.003). Arterial oxygenation was less impaired in prone during hypergravity due to a better‐preserved alveolo‐arterial oxygen transport. We speculate that mammals have developed a cardiopulmonary structure that favours function with the gravitational vector in the posterio‐anterior direction.

Imaging regional PaO2 and gas exchange

Several methods allow regional gas exchange to be inferred from imaging of regional ventilation and perfusion (V/Q) ratios. Each method measures slightly different aspects of gas exchange and has inherent advantages and drawbacks that are reviewed. Single photon emission computed tomography can provide regional measure of ventilation and perfusion from which regional V/Q ratios can be derived. PET methods using inhaled or intravenously administered nitrogen-13 provide imaging of both regional blood flow, shunt, and ventilation. Electric impedance tomography has recently been refined to allow simultaneous measurements of both regional ventilation and blood flow. MRI methods utilizing hyperpolarized helium-3 or xenon-129 are currently being refined and have been used to estimate local PaO(2) in both humans and animals. Microsphere methods are included in this review as they provide measurements of regional ventilation and perfusion in animals. One of their advantages is their greater spatial resolution than most imaging methods and the ability to use them as gold standards against which new imaging methods can be tested. In general, the reviewed methods differ in characteristics such as spatial resolution, possibility of repeated measurements, radiation exposure, availability, expensiveness, and their current stage of development.

Inhalation anesthesia increases V/Q regional heterogeneity during spontaneous breathing in healthy subjects.

Background:The underlying mechanism for the increased alveolar-arterial oxygen tension difference resulting from almost all forms of general anesthesia is unknown. We hypothesized that inhalation anesthesia influences the intrapulmonary distribution of ventilation (V) and perfusion (Q), leading to less advantageous V/Q matching. Methods:Ten healthy volunteers were studied in supine position on two separate occasions, once awake and once during mild anesthesia (sevoflurane inhalation) with maintained spontaneous breathing. On both occasions, the distribution of V and Q were simultaneously imaged using single photon emission computed tomography. V was tagged with [99mTc]-labeled carbon particle aerosol and Q with [113mIn]-labeled macroaggregates of human albumin. Atelectasis formation during anesthesia was prevented using low concentrations of oxygen in inhaled air. Results:Mean V and Q distributions in the ventral-to-dorsal direction, measured in 20 equally spaced volumes of interest and in three regions of interest of equal volume, did not differ between conditions. Anesthesia, when compared with the awake state, significantly decreased the total heterogeneity of the Q distribution (P = 0.002, effect size 1.16) but did not alter V (P = 0.37, effect size 0.41). The corresponding V/Q total heterogeneity was higher under anesthesia (P = 0.002, effect size 2.64). Compared to the awake state, the V/Q frequency distribution under anesthesia became wider (P = 0.009, 1.76 effect size) with a tendency toward low V/Q ratios. Conclusion:Inhalation anesthesia alone affects Q but not V, suggesting that anesthesia has a direct effect on the active regulatory mechanism coordinating Q with V, leading to less favorable V/Q matching.

Regional lung ventilation in humans during hypergravity studied with quantitative SPECT.

Recently we challenged the view that arterial desaturation during hypergravity is caused by redistribution of blood flow to dependent lung regions by demonstrating a paradoxical redistribution of blood flow towards non-dependent regions. We have now quantified regional ventilation in 10 healthy supine volunteers at normal and three times normal gravity (1G and 3G). Regional ventilation was measured with Technegas ((99m)Tc) and quantitative single photon emission computed tomography (SPECT). Hypergravity caused arterial desaturation, mean decrease 8%, p<0.05 vs. 1G. The ratio for mean ventilation per voxel for non-dependent and dependent lung regions was 0.81±0.12 during 1G and 1.63±0.35 during 3G (mean±SD), p<0.0001. Thus, regional ventilation was shifted from dependent to non-dependent regions. We suggest that arterial desaturation during hypergravity is caused by quantitatively different redistributions of blood flow and ventilation. To our knowledge, this is the first study presenting high-resolution measurements of regional ventilation in humans breathing normally during hypergravity.