Oxygen Monitoring — Mixed Venous O2

Current · V5 (2025) → D3.viii Historical · V4 (2023) → F12.i 3 exam appearances

2021B Q13

Exam question

Describe the factors that affect mixed venous oxygen saturation.

CICMWrecks answer

Master answer

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

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

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

= Cardiac output x Arterial oxygen content

DO_2 \; = \; CO \; \times C_aO_2

= Cardiac output x Arteriovenous O2 content difference

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

= 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

Reusable content

CWF-0096

O_2 \; content \; = \; (O_2 \; carried \; by \; Hb) \; + \; (Dissolved \; O_2)

CWF-0098

DO_2 \; = \; CO \; \times C_aO_2

CWF-0099

VO_2 \; = \; CO \; \times (C_aO_2 \; - \; C_vO_2)

Answer by JC, Mooney

Past papers

3 appearances

Exam	Exact exam wording	Candidate success
2021B Q13	Describe the factors that affect mixed venous oxygen saturation.	49%
2017A Q19	Define mixed venous PO2 (20% of marks). Outline the factors that affect this value (80% of marks).	37%
2008A Q10	Briefly describe the factors that influence the partial pressure of Oxygen in mixed venous blood.	33%