Spatial Distribution of Ventilation and Perfusion: Mechanisms and Regulation

Glenny, Robb W.; Robertson, H. Thomas

doi:10.1002/cphy.c100002

Cited by 53 publications

(59 citation statements)

References 119 publications

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“…These asymmetries grow in magnitude with each succeeding generation, resulting in heterogeneous distribution in ventilation and perfusion both in horizontal and vertical dimensions. 43 Of particular importance is that only 1-25% of pulmonary perfusion is mediated by gravitational forces compared with the influence of vascular architecture. 44 This has been referred to as the Slinky effect, 44 in which the lung is analogous to a deformable spring that distorts under its own weight (eg, more spring coils gather at the base of a vertically oriented spring).…”

Section: Ventilation/perfusion Relationshipmentioning

confidence: 99%

A Comprehensive Review of Prone Position in ARDS

2015

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Prone position (PP) has been used since the 1970s to treat severe hypoxemia in patients with ARDS because of its effectiveness at improving gas exchange. Compared with the supine position (SP), placing patients in PP effects a more even tidal volume distribution, in part, by reversing the vertical pleural pressure gradient, which becomes more negative in the dorsal regions. PP also improves resting lung volume in the dorsocaudal regions by reducing the superimposed pressure of both the heart and the abdomen. In contrast, pulmonary perfusion remains preferentially distributed to the dorsal lung regions, thus improving overall alveolar ventilation/perfusion relationships. 1660RESPIRATORY CARE • NOVEMBER 2015 VOL 60 NO 11Moreover, the larger tissue mass suspended from a wider dorsal chest wall effects a more homogeneous distribution of pleural pressures throughout the lung that reduces abnormal strain and stress development. This is believed to ameliorate the severity or development of ventilator-induced lung injury and may partly explain why PP reduces mortality in severe ARDS. Over 40 years of clinical trials have consistently reported improved oxygenation in approximately 70% of subjects with ARDS. Early initiation of PP is more likely to improve oxygenation than initiation during the subacute phase. Maximal oxygenation improvement occurs over a wide time frame ranging from several hours to several days. Meta-analyses of randomized controlled trials suggest that PP provides a survival advantage only in patients with relatively severe ARDS (P aO 2 /F IO 2 <150 mm Hg). Moreover, survival is enhanced when patients are managed with a smaller tidal volume (<8 mL/kg), higher PEEP (10 -13 cm H 2 O), and longer duration of PP sessions (>10 -12 h/session). Combining adjunctive therapies (high PEEP, recruitment maneuvers, and inhaled vasodilators) with PP has an additive effect in improving oxygenation and may be particularly helpful in stabilizing gas exchange in very severe ARDS.

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Section: Ventilation/perfusion Relationshipmentioning

confidence: 99%

A Comprehensive Review of Prone Position in ARDS

2015

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“…In the healthy resting lung, V A/Q matching is thought to occur largely as a result of passive mechanisms, such as vascular branching structure and the effect of gravity (18). The effects of gravity are manifested as gradients in hydrostatic pressure (affecting blood flow), intrapleural pressure (affecting alveolar size and therefore ventilation) interacting to affect alveolar expansion, airway or blood vessel diameter, and local resistances, facilitating V A/Q matching.…”

Section: Effects Of Posturementioning

confidence: 99%

“…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).…”

mentioning

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

118

130

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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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“…The latter mechanism was considered as true as intuitively it was thought that the change in gravity direction will affect the lung perfusion in the same way, i.e., less perfusion towards dorsal lung regions, now non-dependent, in the prone position. Several experiments found that the dorsal lung regions when in the prone position still had the highest amount of blood flowing through them (2)(3)(4)(5)(6)(7)(8). Therefore, this unexpected finding argued against the second mechanism to explain the reduction in intra-pulmonary shunt.…”

Section: Rationalementioning

confidence: 99%

Prone positioning acute respiratory distress syndrome patients

Guérin¹

2017

Ann. Transl. Med.

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Prone position has been used in acute respiratory distress syndrome (ARDS) patients for more than 40 years in ICU. After having demonstrated its capability to significantly improve oxygenation in a large number of patients, sometimes dramatically, this procedure has been found to prevent ventilatorinduced lung injury, the primary concern for the intensivists managing ARDS patients. Over the time, This strategy has eventually been found efficient to improve patient outcome in selected ARDS patients.In this review, we will go over the rationale and then the evidence of using prone position in ARDS patients. RationaleThe early reason that prompted clinicians to turn ARDS patient to prone was oxygenation improvement. This effect, sometimes dramatic (1), was observed in the large majority of patients. Therefore from the early onset the clinicians used proning to improve oxygenation. This effect resulted from a reduction in intra-pulmonary shunt. For the intra-pulmonary shunt to go down two possibilities do exist, either more ventilation in well perfused areas or less perfusion in poorly ventilated lung regions. The latter mechanism was considered as true as intuitively it was thought that the change in gravity direction will affect the lung perfusion in the same way, i.e., less perfusion towards dorsal lung regions, now non-dependent, in the prone position. Several experiments found that the dorsal lung regions when in the prone position still had the highest amount of blood flowing through them (2-8). Therefore, this unexpected finding argued against the second mechanism to explain the reduction in intra-pulmonary shunt. Therefore, better ventilation towards well perfused areas accounts for the common scenario to explain better oxygenation in prone (9).With the recognition of ventilator-induced lung injury (VILI) it turned out that prone position was also able to modulate it. Animal studies, like that of Broccard et al. (10), demonstrated that prone position, as compared to supine

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Spatial Distribution of Ventilation and Perfusion: Mechanisms and Regulation

Cited by 53 publications

References 119 publications

A Comprehensive Review of Prone Position in ARDS

A Comprehensive Review of Prone Position in ARDS

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

Prone positioning acute respiratory distress syndrome patients

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