G5i: Cardiovascular response to haemorrhage

Classification

• Adult blood volume ~5L
• ∴1L blood loss = 20% circulating volume = MAJOR ACUTE LOSS
• ATLS classifies as Class II haemorrhagic shock

Clinical Features

• Hmmrg → ↓MSFP → ↓VR → ↓CO → ↓BP
• ↓Arterial systolic, diastolic & pulse pressures
• Pulse rapid & thready
• ↓Capillary refill
• ↑RR
• Skin pale, clammy

Aim of Compensatory Mechanisms

1. Minimise ∆ effective blood volume
2. Restore MAP
3. Recover normal tissue perfusion

List of Compensatory Mechanisms

1. Sympathetic reflexes
2. Humoral response
3. Redistribution of body fluids
4. Renal conservation of H₂O
5. Restore plasma proteins & RBCs

Sympathetic Reflexes → 30 secs to activate

• ↑sympathetic output
• ↓vagal tone
• Initiated by:

1. 1. BARORECEPTORS
      • ↓MAP → ↓firing rate of BaroR afferents → ↑symp. outflow
   2. CHEMORECEPTORS
      • ↓tissue perfusion → hypoxia → aerobic metabolism → acidosis → stimulates Central & Peripheral ChemoR
      • ChemoR stimulation = ↑symp. outflow, ↑MV
   3. CNS ISCHAEMIC RESPONSE
      • When MAP falls outside autoregulatory range → CNS ischaemia
      • Activates MASS SYMPATHETIC OUTFLOW

• Sympathetic stimulation results in:

1. 1. ↑HR & CONTRACTILITY → β₁ receptor activation → ↑CO
   2. ARTERIOLAR → α₁ mediated → ↑SVR → maintain BP
      • VC of cutaneous, skeletal m., splanchnic vessels
      • Renal A VC → outer layers of renal cortex most VC
      • Cardiac/Cerebral vessels spared
   3. VENOCONSTRICTION → ↑VR → ↑CO
      • Mobilises reservoirs; lungs, liver, cutaneous

Humoral Response → 1hr for maximal effect

Catecholamines

• Adr & NA released from Adrenal Medulla & symp n. terminals
• ↑ x 50 fold

RAAS

• • ↓Renal perfusion
  • Secretion of Renin
  • ↑AII
  • Constriction of arteries, veins, stimulates Aldosterone & ADH release
  • ↑SVR, ↑Na/H₂O retention → restore MAP
  • AII also stimulates thirst → ↑circulating volume

ADH

• Stimulation of Arterial BaroR & low P receptors
• ↑ADH from PPG
• Constricts arteries, veins, ↑H₂O retention

Redistributon of Body Fluids – 1 – 24hrs

Interstitial Fluid

• Arteriolar VC → ↓Hydrostatic P → ∆Starling’s forces so that there is NET REABSORPTION
• Can draw 1L fluid into vessels

Intracellular → Extracellular Shift

• • Secretion of Cortisol

Intestinal Tract

• Fluid absorption

Renal Conservation of H2O – 12 – 72hrs

↓GFR

• Due to HypoT & symp. VC → oliguria

↑Tubular Reabsorption

• • VC → ↓peritubular hydrostatic P & ↑peritubular oncotic P → ↑reabsorption → oliguria

ADH

• ↑H₂O reabsorption

AII

• • ↑Na⁺ reabsorption
  • ↑ADH & aldosterone release

Aldosterone

• ↑active Na⁺ reabsorption
• ↑Thirst

Restoration of Plasma Proteins & RBC

• Liver: 3 – 4 days
  • ↑synthesis of plasma proteins
• Bone marrow: 4 – 8 weeks
  • ↑EPO → ↑Reticulocytes