2019A01 Explain the effects of intermittent positive pressure ventilation on cardiac output.

 

List:

·         Intro

·         Right heart

·         Left heart

·         Indirect effects

·         Hormonal effects

 

Intro:

Determinants of cardiac output

Factors increasing it:

·   ↑Preload

·   ↓Afterload

·   ↑Heart rate

·   ↑Contractility

Bivalent effect of IPPV

·   ↓LV output if healthy LV

o ↓Preload predominates

·   ↑LV output if failing/overloaded LV

o ↓Transmural pressure predominates

·   Inspiration: ↑LV output, ↓RV output

o ↑Intrathoracic pressure

o Opposite to spont vent

·   Expiration: ↓LV output, ↑RV output

o ↓Intrathoracic pressure

o Opposite to spont vent

Exacerbating factors

↓Preload exaggerated by:

·   PEEP (compression of great veins)

·   Hypovolaemia (↓MSFP, ↓VR)

·   Reverse Trendelenburg (venous pooling)

 

Right ventricle:

 

Inspiration: ↑ITP (intrathoracic pressure)

Expiration: ↓ITP

PL

Early:

-Compress great veins -> emptying -> ↑preload

-Compress RV -> ↓compliance -> ↓preload

Late:

-Compress great veins -> collapse -> ↓preload

-Compress RV -> same

-↑LV vol -> ↓RV compliance -> ↓preload (interdependence)

Other:

-Loss of diaphragm pump -> ↓preload

Early:

-Relax great veins -> pooling -> ↓preload

-Relax RV -> ↑compliance -> ↑preload

Late:

-Relax great veins -> ↑venous return -> ↑preload

-Relax RV -> same

-↓LV vol -> ↑preload (interdep.)

Other:

-Loss of diaphragm pump -> ↓preload

AL

-Compress RV -> ↓transmural pressure -> ↓afterload

-Compress lung vessels -> ↑afterload

-Relax RV -> ↑afterload

-Relax lung vessels -> ↓afterload

 

Left ventricle:

 

Inspiration: ↑ITP

Expiration: ↓ITP

PL

Early:

-Compress lung vessels -> emptying -> ↑preload

-Compress LV -> ↓compliance -> ↓preload

Late:

-Compress LV -> ↓compliance -> ↓preload

-Compress lung vessels -> collapse -> ↓preload

-↓RV vol -> ↑LV compliance -> ↑preload (interdep.)

-↓RV output -> ↓LVEDP -> ↓preload

Early:

-Relax lung vessels -> pooling -> ↓preload

-Relax LV -> ↑preload

Late:

-Relax LV -> ↑preload

-Relax lung vessels -> ↑venous return -> ↑prelaad

-↑RV output -> ↑preload (interdep).

-↑RV vol -> ↓preload

AL

-Compress LV -> ↓transmural pressure -> ↓afterload (more important)

-Compress great arteries -> ↑afterload (less important)

-Relax LV -> ↑afterload

-Relax great arteries -> ↓afterload

 

Indirect effects:

Baroreceptor response

·   ↓CO -> ↓mAP -> ↓baroreceptor stretch -> ↑SNS/↓PSNS output

Bainbridge effect

·   ↓PL -> ↓atrial stretch -> ↓HR

Bowditch effect

·   ↑HR -> ↓time for Na+ efflux by Na+K+ATPase -> ↑Ca2+ influx by Na+/Ca2+ exchanger -> ↑contractility

Anrep effect

·   ↓AL -> ↓activation of tension-dependent Na+/H+ channel -> ↓Ca2+ extrusion -> ↓contractility

 

Hormonal effects:

Renin-angiotensin system

·   Activation:

o ↓mAP -> ↓renal baroreceptor stretch -> ↑renin release

·   Angiotensin effects:

o Peripheral vasoconstriction -> ↑AL

o ↑Na+, H2O reabsorption from prox tubule -> ↑blood volume -> ↑preload

·   Aldosterone effects:

o ↑Na+ reabsorption

o Indirect H2O reabsorption via ↑ECF osmolality -> ↑ADH

·   Net effect

o ↑Blood volume -> ↑PL, ↑CO, ↑mAP

Atrial natriuretic peptide

·   PPV -> ↑CVP -> ↑stretch right atrium -> ↑ANP release

o ↑HR -> ↑CO

o ↑natriuresis

 

 

 

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