2010A09 Explain the physiological factors that may lead to a decrease in
mixed venous blood oxygen saturation.



·         Intro

·         ↓CaO2

·         ↓CO

·         ↑VO2

·         ↓SvO2:PvO2




·   Mixed venous oxygen saturation

·   Blood sampled from pulmonary artery via Swann-Ganz catheter

Normal values

·   SvO2 75%

·   PvO2 40mmHg

·   CvO2 15mL/100mL


·   Marker of adequacy of O2 delivery hence cardiac output

·   Often monitored during and after cardiac surgery

·   Preferred over ScvO2 (central venous) since includes coronary sinus blood

Factors reducing CvO2

·   Fick equation: VO2 = CO x (CaO2 – CvO2)

·   Rearranged: CvO2 = CaO2 – VO2/CO


·   Hence:

·   ↓O2 delivery (↓CaO2, ↓CO)

·   ↑O2 consumption

·   (Minor factor: ↓SvO2:PvO2 ratio)




·   CaO2 = [Hb] x 1.34 x SaO2 + PaO2 x 0.03 (per L)


·   ↑Loss: bleeding, haemolysis

·   ↓Production: haematinic deficiency, marrow failure


·   ↓PiO2 (e.g. altitude, diffusion hypoxia)

·   ↓Alveolar ventilation (e.g. opioid)

·   ↑Low V/Q area and shunt (e.g. atelectasis)

·   ↑Diffusion impairment (e.g. pulmonary oedema)


·   Right shift oxyhaemoglobin dissociation curve

o Allosteric interactions between haem groups

o Relaxed -> tense state

o Occurs if ↑tissue metabolic rate

·   Causes:

o Bohr effect: ↑pCO2, ↓pH

o ↑Temp

o ↑2,3-DPG

↓Hufner’s constant

·   Methaemoglobin (congenital, nitrodilator toxicity)

·   Sulfhaemoglobin (sulfonamide toxicity)

·   Carboxyhaemoglobin (poisoning)


↓Cardiac output:


·   ↓Blood volume (blood loss, dehydration

·   ↓Venous tone (general or neuraxial anaesthesia)

·   Obstruction to venous return: IPPV/PEEP

·   Reverse Trendelenburg posture


·   ↑Arterial tone: hypothermia, vasopressors

·   Extrinsic vascular compression: pneumoperitoneum


·   e.g. B-blocker

·   (also depressed by ↑↑rate, ventricular arrhythmia)


·   e.g. heart failure


↑VO2 consumption (↑ATP consumption, ↑rate of aerobic metabolism)


·   ↑ATP consumption -> ↑rate of oxidative phosphorylation -> ↓tissue pO2 -> ↑concentration gradient for O2 uptake

Physiological causes

·   Exercise

·   High lean mass

·   Pregnant

·   Neonate

Pathological causes

·   Post-op SIRS, sepsis

·   Malignant hyperthermia


↓SvO2:PvO2 ratio:

Right shift oxyhaemoglobin dissociation curve

·   Same factors as for arterial blood above



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