2020B01 a) Describe the immediate cardiovascular responses to the sudden loss of 30% blood volume in a healthy awake person.
b) How are these responses different if the patient is undergoing anaesthesia with sevoflurane?



·         Intro

·         Status awake

·         Status with sevoflurane anaesthesia

·         Other responses




·      30% blood loss (~1500mL) = Class 3 shock

·      Severe, life-threatening

If awake

·      Immediate neural response

·      Later hormonal and rheologic responses

If anaesthetised

·      All responses attenuated

·      High risk of decompensation and death


Effects of 30% blood loss:



      VR: venous return

      MSFP: mean systemic filling pressure

      RAP: right atrial pressure

      RVR: resistance to venous return

Direct effect

·   ↓↓MSFP -> ↓VR

·   ↓Cardiac output

·   ↓mAP

·   ↓Tissue perfusion

Baroreceptor response

à ↑CO, ↑SVR, ↑mAP

Afferent limb:

·   Stretch-activated mechanoreceptors on C fibre visceral afferents

·   ↓Stretch -> ↑output to medulla and hypothalamus

·   Arterial pressure: carotid and aortic sinuses

·   Blood volume: receptors in atria and great veins

Efferent limb:

·   ↑SNS output from rostral ventrolateral medulla

·   ↓PSNS output from vagal centres

·   ↑ADH release

·   ↓ANP release

Blood reservoir mobilisation

à ↑Blood volume ~500mL -> ↑MSFP

·   Source: spleen red pulp, liver sinusoids, skin arterioles / venules / A-V anastomoses

·   Mechanism: ↑SNS output -> α1 -> vasoconstriction

Interstitial fluid reabsorption

à ↑Blood volume -> ↑MSFP

·   Mechanism: ↑SNS output -> constriction of pre-capillary sphincters -> ↓capillary hydrostatic pressure -> net fluid reabsorption


à ↑MSFP, ↑venous return

·   ↑SNS -> ↑skeletal muscle pump activity

Central ischaemic response

à ↑CO, ↑SVR, ↑mAP

·   CNS hypoperfusion -> ↑↑SNS output

·   For redirection of flow to vital organs in extremis

Endocrine response

(minutes to hours)

·   ↑ADH -> ↑H2O reabsorption from collecting duct -> water preservation

·   ↑RAAS -> ↑Na+/H2O reabsorption -> ECF preservation

Rheologic response

(days to weeks)

·   ↑Epo -> ↑erythropoiesis -> restoration of red cell mass


If anaesthetized with sevoflurane:

Cellular effects

·   Inhibition of L-Ca2+ channels (heart and vasculature)

·   ↑Nitric oxide release (vasculature)

·   GABA potentiation -> ↓medullary SNS output (heart and vasculature)

Physiological effects

·   ↓Contractility

·   Relative preservation of baroreceptor reflex -> ↑HR

·   Venodilatation -> ↓venous return, ↓preload

·   Vasodilatation -> ↓SVR, ↓afterload

·   ↓Cardiac output

·   ↓mAP

Effects on shock


·   Direct effects exacerbated

·   All compensatory responses attenuated

·   Effects are dose-dependent


·   Relative preservation of baroreceptor reflex

·   ↑SNS output in Guedel’s stage 2


Other responses:


·   ↑↑SNS

·   ↑Metabolic rate (↑cardiac, respiratory, skeletal muscle work)

·   ↑Anaerobic metabolism

·   ↓pH

·   ↓inotropy, arrhythmia, death

·   Greatly increased risk under sevoflurane anaesthesia

Ischaemic pre-conditioning

·   Brief ischaemic episode improves tolerance of subsequent episode

·   Mechanism: activation of sarcolemmal and mitochondrial K+-ATP channel

·   Mimicked by volatile anaesthesia (anaesthetic pre-conditioning)



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