2013C16 Explain how oxygen supply to organs is maintained during isovolumic haemodilution.

List:

· Isovolumic haemodilution

· Equations and normal values

· Direct effect

· Short term compensation

· Medium term compensation

Isovolumic haemodilution:

Definition

Same blood volume but ↓Hct

Problem

↓O₂ content

↓O₂ delivery

↓tissue pO₂

↑Anaerobic metabolism, lactic acidosis

Relevant equations:

Ohm’s law	Blood flow = driving pressure / resistance
Poiseuille’s law	Resistance to laminar flow = (8 x length x viscosity) / (π x radius⁴)
Fick equation	VO₂ = CO x (CaO₂ – CvO₂)
O2 delivery	DO₂ = CO x CaO₂
O2 content	CaO₂ = [Hb] x Hufner x SaO₂ + 0.03 x PaO₂ (per litre)
O2 extraction	EO₂ = VO₂/DO₂
Normal values	DO₂ 1L/min CO 5L/min CaO₂ 200mL/L EO₂ 0.25 (i.e. significant reserve for most tissues)
Alveolar ventilation equation	PaCO₂ ≈PACO₂ ∝ VCO₂/V_A
Alveolar gas equation	PAO₂ = PiO₂ – PaCO₂/0.8

Direct effects:

↓O₂ supply	↓Hb -> ↓CaO₂ -> ↓DO₂
↑O₂ supply	↓Hb -> ↓HCt -> ↓viscosity -> ↓resistance -> ↑CO -> ↑DO₂

Short term compensation (seconds-minutes):

↑CO

(A) Local metabolic autoregulation:

o Anaerobic -> ↑H⁺/K⁺/lactate -> vasodilatation -> capillary recruitment and distension

o ↑preload, ↓afterload -> ↑CO -> DO₂

o ↑area for gas exchange -> ↑EO₂ also

o Important in heart, skeletal muscle, brain

(B) Sympathetic nervous discharge:

o Severe anaemic hypoxia (Hb <50?) -> CNS acidosis -> ↑SNS activation

o ↑ CO -> ↑DO₂

*Note gains are offset by ↑VO₂*

↑EO₂

(A) Local metabolic autoregulation

o Capillary recruitment and distension as above

o ↑Area for gas exchange -> ↑EO₂

(B) Oxyhaemoglobin dissociation curve (OHDC) shift

o Anaerobic metabolism -> ↑tissue [H⁺], ↑RBC [2,3-DPG] -> right shift OHDC

o Note offset by ↑V_A -> ↓PaCO₂ -> left shift OHDC

↑V_A

· Anaerobic metabolism -> ↑[H⁺] -> ↑chemoreceptor activation -> ↑V_A

· ↑V_A -> ↓PACO₂/PaCO₂ -> ↑PAO₂/PaO₂ -> ↑DO₂

*Note gains are offset by ↑VO₂*

Medium term (days-weeks)

↑Erythropoiesis

· ↓pO₂ in renal interstitium -> ↑EPO production

· Hormonal response within hours

· RBC mass restored in 3 weeks

↑Angiogenesis

· e.g. ↑VEGF expression

↑Cell oxidative capacity

· ↑Mitochondrial density

· ↑Oxidative enzymes (TCA cycle, fatty acid oxidation)

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