2009B09 Classify the causes of hypotension in the early post-operative period, giving relevant examples.

List:

· Intro

· Preload

· Rate

· Contractility

· SVR

Intro:

Definition	<70mmHg or >20% below baseline
Equation	mAP = (CO x SVR) + CVP
Cause	· ↓Preload · ↓Heart rate (or severe tachyarrhythmia impairing LV filling) · ↓Contractility · ↓ SVR (note may ↑CO hence partial offset)
Problem	· ↓mAP -> ↓perfusion pressure -> ↓blood flow -> ↓O2 supply Especially if: · Organ tolerates ischaemia poorly (e.g. brain) · Organ unable to autoregulate (e.g. placenta “pressure-passive”) · mAP below autoregulatory ranges (e.g. mAP 70-170mmHg for kidney)

↓ Preload:

Residual general anaesthetic e.g. propofol, sevoflurane	· Venodilatation (Ca^2+ channel block, ↓SNS outflow) · Immobility -> ↓skeletal muscle pump · ↓minute ventilation or apnoea -> ↓diaphragm pump
Residual neuraxial anaesthetic e.g. bupivacaine	· Venodilation (↓thoracolumbar SNS output) · Immobility -> ↓skeletal muscle pump
Hypovolaemia	· Dehydration: fasting, evaporation, diuretics · Blood loss · Third spacing: sepsis, anaphylaxis
Obstruction to LV filling	· Big PE · Tension pneumothorax · Tamponade

↓ Heart rate:

Residual GA	· ↓medullary SNS output -> ↓β1
Residual NA	· If affecting cardiac SNS T1-4 -> ↓ β1
Drug	· β1 blocker · Ca^2+ channel blocker · AChE reversal i.e. neostigmine
Pathology	· Arrhythmia · Ischaemia/infarction of SA node or AV node

↓ Contractility:

Residual GA	· ↓medullary SNS output -> ↓β1 · ↓L-Ca^2+ activity
Residual NA	· If affecting levels T1-4
Drugs	· β-blocker
Pathology	· Ischaemia, infarction · Septic cardiomyopathy

↓ SVR:

Residual GA	· ↓Medullary SNS output · ↓L-Ca^2+ activity · ↑Nitric oxide release
Residual NA	· ↓Thoracolumbar SNS output
Drugs	· ACE inhibitor or ARB commonly implicated · Others: α1 antagonist, nitrates etc
Pathology	· Sepsis · Anaphylaxis · Post-cardiopulmonary bypass vasoplegia · SIRS, ARDS, pancreatitis · Neurogenic: i.e. spinal cord injury