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

 

List:

·       Intro

·       ↓CaO2

·       ↓CO

·       ↑VO2

·       ↓SvO2:PvO2

 

Intro:

Definition

·  Mixed venous oxygen saturation

·  Blood sampled from pulmonary artery via Swann-Ganz catheter

Normal values

·  SvO2 75%

·  PvO2 40mmHg

·  CvO2 15mL/100mL

Use

·  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:

Equation

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

↓Hb

·  ↑Loss: bleeding, haemolysis

·  ↓Production: haematinic deficiency, marrow failure

↓PaO2

·  ↓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)

↓SaO2:PaO2

·  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:

↓Preload

·  ↓Blood volume (blood loss, dehydration

·  ↓Venous tone (general or neuraxial anaesthesia)

·  Obstruction to venous return: IPPV/PEEP

·  Reverse Trendelenburg posture

↑Afterload

·  ↑Arterial tone: hypothermia, vasopressors

·  Extrinsic vascular compression: pneumoperitoneum

↓Rate

·  e.g. B-blocker

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

↓Contractility

·  e.g. heart failure

 

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

Mechanism

·  ↑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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