2021A01 Explain the effects of intermittent positive pressure ventilation on cardiac output.

 

List:

·        Summary

·        Direct effects

·        Indirect effects

 

Summary:

Determinants of cardiac output

·     ↑Preload

·     ↓Afterload

·     ↑Rate

·     ↑Contractility

Overall effect

Right ventricle:

·     ↓Preload (from compression of inflow)

·     ↑Afterload (since arteries are intrathoracic)

-> ↓Output and ↑MVO2

Left ventricle:

·     ↓Preload (mainly from ↓RV output)

·     ↓Afterload (since arteries are extrathoracic)

-> ↓Output but ↓MVO2
↓CO 10% without PEEP, ↓CO 20% with 10cmH2O PEEP

Respiratory oscillation

Inspiration:

·     ↑Intrathoracic pressure (opposite to spont vent)

o  ↑LV output

o  ↓RV output

Expiration:

·     ↓Intrathoracic pressure (normalisation if no PEEP)

o  ↓LV output

o  ↑RV output

Cardiac status

Healthy LV = ↓output

·     ↓Preload is bad; worse if

o  PEEP (venous obstruction)

o  Head up (venous pooling)

o  Hypovolaemia

·     ↓Afterload is good

Failing LV = ↑output

·     Pathophysiology:

o  Venous congestion -> ↑preload

o  Inspiratory effort -> ↑transmural pressure (preload and afterload)

·     Effects:

o  ↓Preload is good

o  ↓Afterload is good

 

      Diagram, radar chart Description automatically generated

 

Direct effects:

Inflow

Valsalva:

1.   Early inspiration: venous emptying -> ↑preload

2.   Late inspiration: venous collapse -> ↓preload (major)

3.   Early expiration: venous pooling -> ↓preload

4.   Late expiration: venous filling -> ↑preload (major)

Outflow

·     Pulmonary: ↑PA and alveolar pressure -> ↑PVR -> ↑RV afterload

·     Systemic: ↑aortic pressure -> ↑thoracic outflow -> ↓LV afterload

Ventricles

·     ↓Compliance -> ↓preload

·     ↓Transmural pressure -> ↓afterload

Other

·     Overall ↓RV output -> ↓LV preload

·     Absent negative pressure -> ↓RV preload (i.e. no diaphragm pump)

·     ↑RV afterload -> RV distension -> ↓LV compliance -> ↓LV preload
(i.e. interdependence)

 

Indirect effects:

Neural

·   Baroreceptor response: ↓CO -> ↓mAP -> ↓stretch -> ↑SNS output

Hormonal

·   RAAS: ↓CO -> ↓mAP -> ↓renal baroreceptor stretch -> ↑RAAS activity

o   Vasoconstriction -> ↑afterload

o   Na+/H2O retention -> ↑preload

·   ANP: ↑CVP -> atrial stretch -> ↑ANP

o   ↑HR

o   Natriuresis -> ↓preload

Intrinsic

·   Bainbridge effect: ↓preload -> ↓atrial stretch -> ↓HR

·   Bowditch effect: ↑HR -> ↑Ca2+ influx-> ↑contractility

·   Anrep effect: ↓afterload -> ↓Ca2+ extrusion -> ↓contractility

 

 

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