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Respiratory Failure

Classification

Type I
- PaO2 (< 8.0 kPa, caused by V/Q mismatch)
- PaCO2 (< / =6.5 kPa)

Type II
- PaO2 (< 8.0 kPa, caused by alveolar hypoventilation - with or without V/Q mismatch)
- PaCO2 (> 6.5 kPa)

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Differential diagnosis of hypoxaemia

CONDITION	CAUSES	A-a GRADIENT	RESPONSE TO FiO2
V/Q mismatch	PE, pneumonia, COPD		good
diffusion impairment	fibrosis		good
shunting (R – L)	pulmonary oedema, ARDS		poor
decreased PAO2	decreased FiO2 (altitude), ­ PaCO2 (hypoventilation)	no change	good

*Alveolar – arterial O2 gradient:
= PAO2 – PaO2
= FiO2 – ( 1.2 ´ PaCO2 ) – PaO2

(N value depends on age and FiO2 , but should be < 4 kPa on R.A. in all ages)

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Differential diagnosis of hypercapnia

Control of V
- central drive (drugs, trauma)
- neuromuscular disease (cervical cord lesion, myasthenia gravis, Guillain-Barré)

Mechanics of V
- chest wall disease (kyphoscoliosis, flail chest)

Lung disease (decreased V_A )
- airway obstruction (OSA, COPD, asthma)
- increased dead space (pneumonia, fibrosis)

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Clinical Features of Respiratory Failure:

Hypoxaemia
History
- dyspnoea
- restlessness
- aggression
- sensory (decreased taste, visual hallucinations)
- conscious level (decreased concentration, confusion, coma)

Examination
- tachypnoea
- cyanosis

Chronic
- ­ Hb
- pulmonary HTN

Hypercapnia
History
- headache
- conscious level (drowsiness, confusion, coma)

Examination
- peripheral vasodilatation

- tachycardia
- bounding pulse
- flapping tremor
- papilloedema (raised ICP )

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Management

Type I
- High-concentration O2
- > 60 % (closed system, e.g. reservoir bag)
- recheck ABGs @ 20 mins (if PaO2 < 8 kPa on 60 % O2, then ?need assisted ventilation)

Type II
- controlled O2 (start @ 24 % with a Venturi fitting)
- recheck ABGs @ 20 mins (if PaCO2 stable, can consider increase to 28 % O2 or 35 % O2)
- if PaCO2 ­ > 1 kPa: consider doxapram & assisted ventilation
- reverse any sedative drugs

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Ventilation

Methods
- invasive (endotracheal intubation)
- non-invasive (positive pressure (NIPPV), negative pressure)

Modes
- pressure support (…of patient's self-triggered breaths)
- SIMV (synchronised intermittent mandatory ventilation)
- BiPAP (bipositive airway pressure)

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