� Summary
� Direct effects
� Indirect effects
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 |
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
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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 |
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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