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:

Pathophysiology	· Low V/Q: area of lung which is well perfused but poorly ventilated · Well matched blood + poorly matched blood = ↓pO2 in systemic arterial 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 · 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 with R->L shunt)