Hypoxic pulmonary vasoconstriction does not contribute to pulmonary blood flow heterogeneity in normoxia in normal supine humans

Arai, Tatsuya; Henderson, A. Cortney; Dubowitz, David J.; Levin, David L.; Friedman, Paul J.; Buxton, Richard B.; Prisk, G. Kim; Hopkins, Susan R.

doi:10.1152/japplphysiol.90759.2008

Cited by 53 publications

(61 citation statements)

References 48 publications

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“…In this case, generalised vasoconstriction raises pulmonary arterial pressure without improving V9A/Q9 matching. Animal and human experiments indicate that HPV is of little importance for V9A/Q9 matching in normal lungs [53,58,59]. By contrast, HPV can be of critical importance in patients with disease causing regional alveolar hypoxia [53], improving PaO 2 by up to 20 mmHg (2.7 kPa) in patients with COPD or ARDS [54].…”

Section: V9a/q9 Mismatch and Vascular Tonementioning

confidence: 99%

Gas exchange and ventilation–perfusion relationships in the lung

2014

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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 (PO 2 and PCO 2 ) are determined by the ratio of alveolar ventilation to blood flow (V9A/Q9) for each unit. Shunt and low V9A/Q9 regions are two examples of V9A/Q9 mismatch and are the most frequent causes of hypoxaemia. Diffusion limitation, hypoventilation and low inspired PO 2 cause hypoxaemia, even in the absence of V9A/Q9 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 V9A/Q9 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 PCO 2 , 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 V9A/Q9 mismatch on gas exchange. The types of V9A/Q9 mismatch causing impaired gas exchange vary characteristically with different lung diseases. @ERSpublications A review of ventilation-perfusion relationships and gas exchange, basic concepts and their relation to clinical cases

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Section: V9a/q9 Mismatch and Vascular Tonementioning

confidence: 99%

Gas exchange and ventilation–perfusion relationships in the lung

2014

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“…Imaging sequence parameters were as follows: TI ϭ 550 -950 ms (based on subject's heart rate), TE ϭ 21.3 ms, field of view ϭ 40 cm, slice thickness ϭ 15 mm. The collected image matrix size was 256 ϫ 128 (reconstructed by scanner to 256 ϫ 256), giving voxels of 0.156 ϫ 0.156 ϫ 1.5 cm, or ϳ0.037 cm 3 . The HASTE imaging sequence had an inter-TE of 4.5 ms and 72 lines of phase encoding, resulting in a data acquisition time of 324 ms.…”

Section: Measuring Pulmonary Blood Flow Using Aslmentioning

confidence: 99%

“…We have extensively utilized a MRI technique that allows for absolute quantification of regional pulmonary blood flow (6) using arterial spin labeling (ASL) (3,6,20,24). Recently, we developed a MRI technique to measure regional specific ventilation (i.e., the local volume of delivered fresh gas divided by local gas volume at FRC) (46) using inhalation of 100% oxygen (4, 10) as a contrast agent.…”

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confidence: 99%

The gravitational distribution of ventilation-perfusion ratio is more uniform in prone than supine posture in the normal human lung

Henderson

Sá

Theilmann

et al. 2013

Journal of Applied Physiology

113

131

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Henderson AC, Sá RC, Theilmann RJ, Buxton RB, Prisk GK, Hopkins SR. The gravitational distribution of ventilation-perfusion ratio is more uniform in prone than supine posture in the normal human lung. J Appl Physiol 115: 313-324, 2013. First published April 25, 2013 doi:10.1152/japplphysiol.01531.2012.-The gravitational gradient of intrapleural pressure is suggested to be less in prone posture than supine. Thus the gravitational distribution of ventilation is expected to be more uniform prone, potentially affecting regional ventilation-perfusion (V A/Q ) ratio. Using a novel functional lung magnetic resonance imaging technique to measure regional V A/Q ratio, the gravitational gradients in proton density, ventilation, perfusion, and V A/Q ratio were measured in prone and supine posture. Data were acquired in seven healthy subjects in a single sagittal slice of the right lung at functional residual capacity. Regional specific ventilation images quantified using specific ventilation imaging and proton density images obtained using a fast gradient-echo sequence were registered and smoothed to calculate regional alveolar ventilation. Perfusion was measured using arterial spin labeling. Ventilation (ml·min Ϫ1 ·ml Ϫ1 ) images were combined on a voxel-by-voxel basis with smoothed perfusion (ml·min Ϫ1 ·ml Ϫ1 ) images to obtain regional V A/Q ratio. Data were averaged for voxels within 1-cm gravitational planes, starting from the most gravitationally dependent lung. The slope of the relationship between alveolar ventilation and vertical height was less prone than supine (Ϫ0.17 Ϯ 0.10 ml·min, P ϭ 0.02) as was the slope of the perfusion-height relationship (Ϫ0.14 Ϯ 0.05 ml·min, P ϭ 0.02). There was a significant gravitational gradient in V A/Q ratio in both postures (P Ͻ 0.05) that was less in prone (0.09 Ϯ 0.08 cm Ϫ1 supine, 0.04 Ϯ 0.03 cm Ϫ1 prone, P ϭ 0.04). The gravitational gradients in ventilation, perfusion, and regional V A/Q ratio were greater supine than prone, suggesting an interplay between thoracic cavity configuration, airway and vascular tree anatomy, and the effects of gravity on V A/Q matching. magnetic resonance imaging; arterial spin labeling; specific ventilation imaging; ventilation-perfusion ratio; gravity WHILE THE LUNG HAS A NUMBER of functions, it is primarily a gas exchange organ. 1 Ventilation-perfusion (V A/Q ) matching, such that regions of the lung that receive fresh gas also receive deoxygenated capillary blood, is the most important mechanism determining gas exchange efficiency (53). Although several mechanisms are thought to accomplish V A/Q matching in the healthy lung (see Ref. 18 for review), it is thought that passive mechanisms dominate under normal conditions. Such passive mechanisms include vascular branching structure and the effect of gravity on ventilation and perfusion (53).Modeling studies suggest that, because of the shape of the lungs within the thorax, the gradient of intrapleural pressures is more uniform in prone posture compared with supine (48). This predicts that the g...

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“…In this study gases with FIO2 = 0.21 (normoxia/room air), FIO2 = 0.125 (hypoxia), and FIO2 = 1.00 (hyperoxia) are presented in balanced order between subjects, although these can be varied as desired, consistent with the research goals 2. After a subject reaches steady state for a specified condition (˜20 minutes for a particular gas) 7 , MR measurements of perfusion and proton density are acquired. In this case, the 20 minute period of exposure to the gas before imaging is chosen because although the initiation of hypoxic pulmonary vasoconstriction response occurs within seconds, the response to alveolar hypoxia is not maximal until 20 minutes 8 , consistent with the goal of this particular study.…”

Section: Lung Proton Densitymentioning

confidence: 99%

“…Three different indexes of perfusion heterogeneity are calculated. These are 1) relative dispersion 4,10,11 , also known as the coefficient of variation, a global scale of heterogeneity defined as the ratio of the standard deviation to the mean perfusion in which the larger the relative dispersion, the more heterogeneous the perfusion distribution; 2) fractal dimension (Ds) 7 , an index of the spatial heterogeneity that is scale independent, where the value varies between 1.0 (homogeneous) and 1.5 (spatially random); and 3) a geometric standard deviation, also global scales of heterogeneity but based on log normal model distribution 2 .…”

Section: Coil Inhomogeneity Correctionmentioning

confidence: 99%

Magnetic Resonance Imaging Quantification of Pulmonary Perfusion using Calibrated Arterial Spin Labeling

Arai

Prisk

Holverda

et al. 2011

JoVE

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This demonstrates a MR imaging method to measure the spatial distribution of pulmonary blood flow in healthy subjects during normoxia (inspired O2, fraction (FIO2) = 0.21) hypoxia (FIO2 = 0.125), and hyperoxia (FIO2 = 1.00). In addition, the physiological responses of the subject are monitored in the MR scan environment. MR images were obtained on a 1.5 T GE MRI scanner during a breath hold from a sagittal slice in the right lung at functional residual capacity. An arterial spin labeling sequence (ASL-FAIRER) was used to measure the spatial distribution of pulmonary blood flow 1,2 and a multi-echo fast gradient echo (mGRE) sequence 3 was used to quantify the regional proton (i.e. H2O) density, allowing the quantification of density-normalized perfusion for each voxel (milliliters blood per minute per gram lung tissue).With a pneumatic switching valve and facemask equipped with a 2-way non-rebreathing valve, different oxygen concentrations were introduced to the subject in the MR scanner through the inspired gas tubing. A metabolic cart collected expiratory gas via expiratory tubing. Mixed expiratory O2 and CO2 concentrations, oxygen consumption, carbon dioxide production, respiratory exchange ratio, respiratory frequency and tidal volume were measured. Heart rate and oxygen saturation were monitored using pulse-oximetry. Data obtained from a normal subject showed that, as expected, heart rate was higher in hypoxia (60 bpm) than during normoxia (51) or hyperoxia (50) and the arterial oxygen saturation (SpO2) was reduced during hypoxia to 86%. Mean ventilation was 8.31 L/min BTPS during hypoxia, 7.04 L/min during normoxia, and 6.64 L/min during hyperoxia. Tidal volume was 0.76 L during hypoxia, 0.69 L during normoxia, and 0.67 L during hyperoxia.Representative quantified ASL data showed that the mean density normalized perfusion was 8.86 ml/min/g during hypoxia, 8.26 ml/min/g during normoxia and 8.46 ml/min/g during hyperoxia, respectively. In this subject, the relative dispersion 4 , an index of global heterogeneity, was increased in hypoxia (1.07 during hypoxia, 0.85 during normoxia, and 0.87 during hyperoxia) while the fractal dimension (Ds), another index of heterogeneity reflecting vascular branching structure, was unchanged (1.24 during hypoxia, 1.26 during normoxia, and 1.26 during hyperoxia).Overview. This protocol will demonstrate the acquisition of data to measure the distribution of pulmonary perfusion noninvasively under conditions of normoxia, hypoxia, and hyperoxia using a magnetic resonance imaging technique known as arterial spin labeling (ASL).Rationale: Measurement of pulmonary blood flow and lung proton density using MR technique offers high spatial resolution images which can be quantified and the ability to perform repeated measurements under several different physiological conditions. In human studies, PET, SPECT, and CT are commonly used as the alternative techniques. However, these techniques involve exposure to ionizing radiation, and thus are not suitable for repeated measurements...

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Hypoxic pulmonary vasoconstriction does not contribute to pulmonary blood flow heterogeneity in normoxia in normal supine humans

Cited by 53 publications

References 48 publications

Gas exchange and ventilation–perfusion relationships in the lung

Gas exchange and ventilation–perfusion relationships in the lung

The gravitational distribution of ventilation-perfusion ratio is more uniform in prone than supine posture in the normal human lung

Magnetic Resonance Imaging Quantification of Pulmonary Perfusion using Calibrated Arterial Spin Labeling

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