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

 

 

Feedback welcome at ketaminenightmares@gmail.com