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Benefits of Spontaneous Breathing: Compared with Mechanical Ventilation
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Surgical Sciences, Anaesthesiology and Intensive Care.
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 [en]
spontaneous breathing, mechanical ventilation, pulmonary shunt, oxygenation, hypoxic pulmonary vasoconstriction
National Category
Anesthesiology and Intensive Care
Research subject
Anaesthesiology and Intensive Care
Identifiers
URN: urn:nbn:se:uu:diva-182564ISBN: 978-91-554-8498-9 (print)OAI: oai:DiVA.org:uu-182564DiVA: diva2:560032
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
List of papers
1. Spontaneous Breathing Improves Shunt Fraction and Oxygenation in Comparison with Controlled Ventilation at a Similar Amount of Lung Collapse
Open this publication in new window or tab >>Spontaneous Breathing Improves Shunt Fraction and Oxygenation in Comparison with Controlled Ventilation at a Similar Amount of Lung Collapse
Show others...
2011 (English)In: Anesthesia and Analgesia, ISSN 0003-2999, E-ISSN 1526-7598, Vol. 113, no 5, 1089-1095 p.Article in journal (Refereed) Published
Abstract [en]

BACKGROUND: Spontaneous breathing (SB), when allowed during mechanical ventilation (MV), improves oxygenation in different models of acute lung injury. However, it is not known whether oxygenation is improved during mechanically unsupported SB. Therefore, we compared SB without any support with controlled MV at identical tidal volume (V(T)) and respiratory rate (RR) without positive end-expiratory pressure in a porcine lung collapse model.

METHODS: In 25 anesthetized piglets, stable lung collapse was induced by application of negative pressure, and animals were randomized to either resume SB or to be kept on MV at identical VT (5 mL/kg; 95% confidence interval: 3.8 to 6.4) and RR (65 per minute [57 to 73]) as had been measured during an initial SB period. Oxygenation was assessed by blood gas analysis (n = 15) completed by multiple inert gas elimination technique (n = 8 of the 15) for shunt measurement. In addition, possible lung recruitment was studied with computed tomography of the chest (n = 10).

RESULTS: After induction of lung collapse, PaO(2)/FIO(2) decreased to 90 mm Hg (76 to 103). With SB, PaO(2)/FIO(2) increased to 235 mm Hg (177 to 293) within 15 minutes, whereas MV at identical VT and RR did not cause any improvement in oxygenation. Intrapulmonary shunt by 45 minutes after induction of lung collapse was lower during SB (SB: 27% [24 to 30] versus MV: 41% [28 to 55]; P = 0.017). Neither SB nor MV reduced collapsed lung areas on computed tomography.

CONCLUSIONS: SB without any support improves oxygenation and reduces shunt in comparison with MV at identical settings. This seems to be achieved without any major signs of recruitment of collapsed lung regions.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:uu:diva-161927 (URN)10.1213/ANE.0b013e31822ceef8 (DOI)000296236200025 ()
Available from: 2011-11-23 Created: 2011-11-21 Last updated: 2017-12-08Bibliographically approved
2. Haemodynamic stability and pulmonary shunt during spontaneous breathing and mechanical ventilation in porcine lung collapse
Open this publication in new window or tab >>Haemodynamic stability and pulmonary shunt during spontaneous breathing and mechanical ventilation in porcine lung collapse
2012 (English)In: Acta Anaesthesiologica Scandinavica, ISSN 0001-5172, E-ISSN 1399-6576, Vol. 56, no 6, 748-754 p.Article in journal (Refereed) Published
Abstract [en]

Background

We investigated the haemodynamic stability of a novel porcine model of lung collapse induced by negative pressure application (NPA). A secondary aim was to study whether pulmonary shunt correlates with cardiac output (CO).

Methods

In 12 anaesthetized and relaxed supine piglets, lung collapse was induced by NPA (−50 kPa). Six animals resumed spontaneous breathing (SB) after 15 min; the other six animals were kept on mechanical ventilation (MV) at respiratory rate and tidal volume (VT) that corresponded to SB. All animals were followed for 135 min with blood gas analysis and detailed haemodynamic monitoring.

Results

Haemodynamics and gas exchange were stable in both groups during the experiment with arterial oxygen tension (PaO2)/inspired fraction of oxygen (FiO2) and pulmonary artery occlusion pressure being higher, venous admixture (Qva/Qt) and pulmonary perfusion pressure being lower in the SB group. CO was similar in both groups, showing slight decrease over time in the SB group. During MV, Qva/Qt increased with CO (slope: 4.3 %min/l; P < 0.001), but not so during SB (slope: 0.55 %min/l; P = 0.16).

Conclusions

This porcine lung collapse model is reasonably stable in terms of haemodynamics for at least 2 h irrespective of the mode of ventilation. SB achieves higher PaO2/FiO2 and lower Qva/Qt compared with MV. During SB, Qva/Qt seems to be less, if at all, affected by CO compared with MV.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:uu:diva-175676 (URN)10.1111/j.1399-6576.2012.02700.x (DOI)000305070400012 ()22524589 (PubMedID)
Available from: 2012-06-11 Created: 2012-06-11 Last updated: 2017-12-07Bibliographically approved
3. Pulmonary shunt is independent of decrease in cardiac output during unsupported spontaneous breathing in the pig
Open this publication in new window or tab >>Pulmonary shunt is independent of decrease in cardiac output during unsupported spontaneous breathing in the pig
2013 (English)In: Anesthesiology, ISSN 0003-3022, E-ISSN 1528-1175, Vol. 118, no 4, 914-923 p.Article in journal (Refereed) Published
Abstract [en]

Background: During mechanical ventilation (MV), pulmonary shunt is cardiac output (CO) dependent; however, whether this relationship is valid during unsupported spontaneous breathing (SB) is unknown. The CO dependency of the calculated venous admixture was investigated, with both minor and major shunt, during unsupported SB, MV, and SB with continuous positive airway pressure (CPAP). Methods: In seven anesthetized supine piglets breathing 100% oxygen, unsupported SB, MV (with tidal volume and respiratory rate corresponding to SB), and 8 cm H2O CPAP (airway pressure corresponding to MV) were applied at random. Venous return and CO were reduced by partial balloon occlusion of the inferior vena cava. Measurements were repeated with the left main bronchus blocked, creating a nonrecruitable pulmonary shunt. Results: CO decreased from 4.2 l/min (95% CI, 3.9-4.5) to 2.5 l/min (95% CI, 2.2-2.7) with partially occluded venous return. Irrespective of whether shunt was minor or major, during unsupported SB, venous admixture was independent of CO (slope: minor shunt, 0.5; major shunt, 1.1%.min(-1).l(-1)) and mixed venous oxygen tension. During both MV and CPAP, venous admixture was dependent on CO (slope MV: minor shunt, 1.9; major shunt, 3.5; CPAP: minor shunt, 1.3; major shunt, 2.9% .min(-1).l(-1)) and mixed-venous oxygen tension (coefficient of determination 0.61-0.86 for all regressions). Conclusions: In contrast to MV and CPAP, venous admixture was independent of CO during unsupported SB, and was unaffected by mixed-venous oxygen tension, casting doubt on the role of hypoxic pulmonary vasoconstriction in pulmonary blood flow redistribution during unsupported SB.

Keyword
pulmonary shunt, cardiac output, mechanical ventilation, spontaneous breathing
National Category
Anesthesiology and Intensive Care
Research subject
Physiology
Identifiers
urn:nbn:se:uu:diva-182302 (URN)10.1097/ALN.0b013e318283c81f (DOI)000316355000021 ()
Available from: 2012-10-11 Created: 2012-10-09 Last updated: 2017-12-07Bibliographically approved
4. Spontaneous breathing reduces pulmonary shunt independent of hypoxic vasoconstriction in the lung collapse model
Open this publication in new window or tab >>Spontaneous breathing reduces pulmonary shunt independent of hypoxic vasoconstriction in the lung collapse model
(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
pulmonary shunt, pulmonary hypoxic vasoconstriction, sodium nitroprusside, mechanical ventilation, spontaneous breathing
National Category
Anesthesiology and Intensive Care
Research subject
Physiology
Identifiers
urn:nbn:se:uu:diva-182303 (URN)
Available from: 2012-10-09 Created: 2012-10-09 Last updated: 2013-01-23Bibliographically approved

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