2019A07 Explain how a metabolic acidosis develops in hypovolaemic shock.
Describe the consequences of this metabolic acidosis for the body.

Definition

·  Reduction in blood volume resulting in failure of the circulation to meet tissue metabolic requirement

·  Occurs at ~20% loss

Pathophysiology

·  ↓Blood volume (BV) -> ↓preload -> ↓cardiac output -> ↓mAP -> ↓tissue perfusion -> stagnant hypoxia

·  ↓Oxidative phosphorylation, ↓Krebs cycle activity

·  Pyruvate converted to lactate to regenerate NAD⁺

·  (Anaerobic) glycolysis continues

Exacerbating factors

·  ↑Cardiac work: SNS -> ↑HR, ↑contractility, ↑MvO₂ -> ↑lactic acid production

·  ↑Resp work: ↓pH -> ↑minute ventilation -> ↑resp VO₂ -> ↑lactic production
↓Liver perfusion -> ↓clearance of lactic acid

Cardiovascular

·  ↓BV:

o ↓preload -> ↓CO -> ↓mAP and ↓DBP

o Note coronary autoregulation fails below aortic root DBP 60mmHg

·  ↓pH:

o Inhibition of L-Ca²⁺ and SERCA -> ↓ inotropy (predominates at pH <7.2)

o Risk of arrhythmia, arrest

o Systemic vasodilation, pulmonary vasoconstriction

·  ↑↑SNS:

o ↑ HR, ↑inotropy, ↑MVO2 (predominates at pH >7.2)

o Systemic and pulmonary vasoconstriction

o Autotransfusion from capacitance vessels (skin, gut, liver, spleen, lung)

·  ↑Myocardial O₂ demand, ↓O₂ supply -> ischaemia (Type 2)

Respiratory

·  ↓BV:

o ↑Z1 of lung -> ↑dead space -> ↓etCO₂

·  ↓pH

o ↑Peripheral chemoreceptor activity -> ↑MV (doubles for each ↓0.1 pH)

o Right shift OHDC (↑ [H⁺], ↑ [2,3-DPG]

o (partly offset by left shift due to respiratory alkalosis)

CNS

·  ↓BV:

o CNS ischaemia -> 1° and 2° HIBI (failure of autoregulation at CPP <50mmHg)

o Confusion, coma, apnoea, death

o ↑↑SNS output from medulla (central ischaemic response)

Kidneys

·  ↓BV -> ↓mAP:

o Autoregulation mAP 70-175

o Hence ↓↓mAP -> dec GFR -> oliguria

o If prolonged: ATN, renal failure

·  ↓pH:

o ↑ HCO3 reabsorption especially distal nephron (CNT/CCD/MCD)

o ↑Phosphoric acid excretion

o ↑NH₄⁺ production -> ↑HCO₃^- reabsorption

Liver

·  ↓BV -> ↓mAP -> ischaemia

·  ↓pH: lactic acid -> glucose (Cori cycle)

Metabolic

·  ↑[K⁺]: due to H⁺/K⁺ exchange across cells

o +0.6mmol/L K⁺ for each -0.1 in pH

·  ↑[Ca²⁺]: due to ↑ H⁺/Ca²⁺ exchange with bone