Canonical Question

Resp – Oximetry

V5 D3.viii Historical V4 F12.i 3 appearances

Master answer

Mixed Venous Blood

Mixed Venous Blood = blood taken from the pulmonary artery
Normal mixed venous partial pressure of oxygen = 40mmHg -> 75% saturation
Majority of oxygen in blood bound to haemoglobin
- Hb has 4 haem molecules, each can bind one oxygen molecule
- Affinity of Hb for oxygen changes with oxygen saturation, due to conformational change in the haem group on oxygen binding
- Affinity for oxygen decreased by: low pH, high CO₂, high temperature, 1-3-DPG

Factors

Complex interplay of factors which affect Venous Oxygen Partial pressure of oxygen and hence content
Modified Fick principle: Mixed venous oxygen content = oxygen delivery – oxygen extraction

Oxygen Content

\[ O_2 \; content \; = \; (O_2 \; carried \; by \; Hb) \; + \; (Dissolved \; O_2) \] \[ \; \; \; = \; (Hb \times sats \times 1.34) \; + \; (PO_2 \times 0.003) \]

= 20ml/100ml in arterial blood, 15ml/100ml in venous blood
Vast majority of oxygen carried by haemoglobin, tiny dissolved portion
Non-linear relationship between dissolved O2 and O2 content
- PO2 is the fraction of dissolved O2
- PO2 determines Sats based on Oxygen-Haemoglobin Dissociation Curve

Oxygen Delivery

= Cardiac output x Arterial oxygen content

\[ DO_2 \; = \; CO \; \times C_aO_2\]

Oxygen Demand

= Cardiac output x Arteriovenous O2 content difference

\[ VO_2 \; = \; CO \; \times (C_aO_2 \; – \; C_vO_2)\]

Oxygen flux

\[ = DO_2 \; – \; VO_2 \] \[ = \; CO . C_aO_2 \; – \; [CO . (C_aO_2-C_vO_2)] \] \[ = \; CO \; . \; (C_vO_2) \] \[= CO \; . \; [(Hb*1.34* \underline{S_vO_2}) + (\underline{p_vO_2}*0.003)] \]

Hence SvO2 and pvO2 are surrogate markers for Oxygen flux

O₂ Delivery
- 1000ml/min in health
- Decrease in cardiac output: ↓HR, ↓SV (↓preload, ↓contractility, ↑afterload)
- Decrease in functional Hb: anaemia, carbon monoxide, congenital abnormalities of haemoglobin
- Decrease in oxygen saturation: V/Q mismatch, anatomical shunt, hypoventilation, low F_iO₂(e.g. at altitude), right-shift of oxygen dissociation curve (low pH etc. – see above)
Oxygen extraction:
- 250ml/min
- Increased by:
  - increased tissue metabolism (e.g. sepsis, pregnancy, malignancy, post-operative catabolic state etc.)
  - right shift of oxygen dissociation curve (see above)

Mooney / JC 2020

Answer by JC, Mooney

Exam appearances

Exam	Exact wording	Relationship	Success
2008A Q10	Briefly describe the factors that influence the partial pressure of Oxygen in mixed venous blood.	historical_member	—
2017A Q19	Define mixed venous PO2 (20% of marks). Outline the factors that affect this value (80% of marks).	historical_member	—
2021B Q13	Describe the factors that affect mixed venous oxygen saturation.	historical_member	—