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Spontaneous breathing reduces pulmonary shunt independent of hypoxic vasoconstriction in the lung collapse model
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Surgical Sciences, Anaesthesiology and Intensive Care.
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Surgical Sciences, Anaesthesiology and Intensive Care.
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Medical Sciences, Clinical Physiology.
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Surgical Sciences, Anaesthesiology and Intensive Care.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Background: Compared with mechanical ventilation (MV), spontaneous breathing (SB) improves oxygenation even without recruiting atelectatic lung areas, probably by redistributing perfusion to well ventilated areas whose alveolar vessels are not compressed by the higher alveolar pressures of MV. We assumed that regional vasoconstriction, presumably hypoxic vasoconstriction (HPV), causes a redistribution of perfusion, and when the vasoconstriction is blocked by sodium nitroprusside (SNP) infusion, pulmonary shunt in the porcine lung collapse model is similar during SB and MV.

Methods: Lung collapse was induced in 8 piglets by negative pressure application. The animals were allowed both to breathe spontaneously or set to MV at settings matched to SB, in random order. Calculated venous admixture (Qva/Qt), a proxy for pulmonary shunt, was measured, and any vasoconstriction was blocked by SNP infusion. For determining vascular tone, venous return was gradually reduced and cardiac output (CO) over pulmonary perfusion pressure (Pppulm) plots were recorded.

Results: With SNP, Qva/Qt increased both during MV (from 7.7 to 20.2%, P=0.009) and SB (from 3.5 to 9%, P=0.013) with similar slopes of the Pppulm over CO plot (~1.5 mmHg*min/L), indicating a similar hypoxic response during both conditions.

Conclusion: Despite attenuating any vasoconstriction, unexpectedly, pulmonary shunt was lower during spontaneous breathing than during MV. This suggests a mechanism active during SB, favorably redistributing pulmonary blood flow and not silenced by SNP.

Keyword [en]
pulmonary shunt, pulmonary hypoxic vasoconstriction, sodium nitroprusside, mechanical ventilation, spontaneous breathing
National Category
Anesthesiology and Intensive Care
Research subject
Physiology
Identifiers
URN: urn:nbn:se:uu:diva-182303OAI: oai:DiVA.org:uu-182303DiVA: diva2:559443
Available from: 2012-10-09 Created: 2012-10-09 Last updated: 2013-01-23Bibliographically approved
In thesis
1. Benefits of Spontaneous Breathing: Compared with Mechanical Ventilation
Open this publication in new window or tab >>Benefits of Spontaneous Breathing: Compared with Mechanical Ventilation
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

When spontaneous breathing (SB) is allowed during mechanical ventilation (MV), atelectatic lung areas are recruited and oxygenation improves thereby. Whether unsupported SB at its natural pattern (without PEEP and at low pressure/small tidal volume) equally recruits and improves oxygenation, and if so by which mechanism, has not been studied.

A porcine lung collapse model was designed to study this question. The cardiac output dependency of the pulmonary shunt was investigated with healthy lungs and with major shunt (during one-lung ventilation) and with SB, MV and continuous positive airway pressure (CPAP). The hypoxic pulmonary vasoconstriction (HPV) was blocked with sodium nitroprusside (SNP) to see whether HPV is the only mechanism available for ventilation/perfusion (VA/Q) matching during MV and SB. In all experiments, respiratory rate and tidal volume during MV were matched to SB. Oxygenation was assessed by serial blood gas measurements, recruitment by thoracic CTs; pulmonary shunt was assessed by multiple inert gas elimination or venous admixture.

SB attained better oxygenation and lower pulmonary shunt compared with MV, although it did not recruit collapsed lung. Pulmonary shunt did not correlate with cardiac output during SB, whereas a correlation was found during MV and CPAP. With blocked HPV, pulmonary shunt was considerably lower during SB than MV.

In conclusion, SB improves VA/Q matching as compared with MV, even when no recruitment occurs. In contrast to MV and CPAP, cardiac output has no major effect on pulmonary shunt during SB. The improved VA/Q matching during SB despite a blocked HPV might indicate the presence of a SB-specific mechanism that improves pulmonary blood flow redistribution towards ventilated lung regions independent of or supplementary to HPV.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2012. 47 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Medicine, ISSN 1651-6206 ; 824
Keyword
spontaneous breathing, mechanical ventilation, pulmonary shunt, oxygenation, hypoxic pulmonary vasoconstriction
National Category
Anesthesiology and Intensive Care
Research subject
Anaesthesiology and Intensive Care
Identifiers
urn:nbn:se:uu:diva-182564 (URN)978-91-554-8498-9 (ISBN)
Public defence
2012-11-23, Enghoffsalen, Entrance 50, Akademiska sjkhuset, Uppsala, 13:00 (English)
Opponent
Supervisors
Available from: 2012-11-01 Created: 2012-10-11 Last updated: 2013-01-23Bibliographically approved

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Vimlati, LaszloLarsson, AndersHedenstierna, GöranLichtwarck-Aschoff, Michael

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