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?

List:

· Intro

· Status awake

· Status with sevoflurane anaesthesia

· Other responses

Intro:

Quantification

· 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:

Graph	VR = (MSFP – RAP) / RVR 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
Restlessness	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

Detrimental:

· Direct effects exacerbated

· All compensatory responses attenuated

· Effects are dose-dependent

Protective:

· Relative preservation of baroreceptor reflex

· ↑SNS output in Guedel’s stage 2

Other responses:

Decompensation

· ↑↑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)