2012B12 Discuss the causes of early post-operative hypoxaemia.

 

List:

·      Intro

·      Relevant equations

·      Pathophysiology and causes for each factor

 

Intro:

Definition

PaO2 <60mmHg

Importance

·  Corresponds to SaO2 90%, CaO2 18mL/100mL

·  Approaching steep part of oxyhaemoglobin dissociation curve

Most common causes

·  Atelectasis causing shunt

·  Iatrogenic hypoventilation

Solution

·  All causes effectively treated by ↑FiO2 except shunt

 

Equations:

Alveolar gas equation

PAO2 = FiO2(Patm – SVPH2O) – PaCO2/RQ

Alveolar ventilation equation

PaCO2 ≈ PACO2 VCO2/VA

Bohr equation

VA = RR x (VT-VD)

Fick’s law

Q = (P1-P2) x area/thickness x (membrane solubility)/(√molecular weight)

Shunt equation

Qs/Qt = (CcO2 – CaO2) / (CcO2 – CvO2)

 

↓PiO2:

Pathophysiology

↓FiO2 -> ↓PiO2 -> ↓PAO2 -> ↓PaO2

Causes

(1) Diffusion hypoxia:

o Converse of the concentration effect

o Seen at the end of a case

o N2O ceased, N2 resumed

o N2 is slowly take up from alveoli (blood-gas partition coefficient [BGPC] 0.014)

o N2O rapidly returns to alveoli (BGPC 0.47)

o Alveolar O2 diluted -> ↓ PaO2

o Prevent by FiO2 100% at end of case

(2) High altitude

o ↓Patm -> ↓PiO2 -> ↓PaO2

 

Hypoventilation:

Pathophysiology

↓RR or ↓VT or ↑VD –> ↓VA –> ↑PACO2/PaCO2 -> ↓PAO2/PaO2

Causes

(1) Residual anaesthetic or opioid

o ↓Response to ↓PaO2 and ↑PaCO2

o Note ↓response to ↓PaO2 even at 0.1MAC

(2) Residual neuromuscular blockade

o Inadequate reversal

o Re-curarisation

(3) Painful breathing:

o e.g. thoracotomy, laparotomy

(4) Existing pathology

o e.g. COPD

 

Diffusion impairment:

Pathophysiology

·   Time-limited O2 uptake in health

·   Diffusion-limited uptake in disease

·   ↑Area, ↓thickness -> ↓rate of uptake -> ↓PaO2

Causes

·   Negative pressure APO

·   Excess IV fluid -> APO

·   Existing disease: pulmonary fibrosis, emphysema

 

V/Q mismatch (V<Q):

Pathophysiology

·   Low V/Q: Area of lung which is well perfused but poorly ventilated

·   Well matched blood + poorly matched blood = ↓pO2 in LV blood

Cause

·   Normal: VQ scatter with gravity [V, Q and V/Q graph]

·   Post-op: residual volatile -> impaired HPV and local matching -> ↓V/Q overall

·   Existing disease: COPD, asthma

 

Shunt:

Pathophysiology

·   Where some systemic blood returns to the LV without any oxygenation

·   V/Q = 0

·   Minimal correction with supplemental O2 (cf. all other causes)

Cause

·   Physiological: bronchial, pleural, Thebesian veins

·   Post-op: atelectasis, aspiration pneumonitis

·   Existing disease: lung (e.g. AVM), heart (e.g. VSD)

 

 

 

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