Respiratory

Respiratory Failure

Gas exchange fails in one of two ways: the lung cannot load oxygen into blood (type 1), or the ventilatory pump cannot clear carbon dioxide (type 2).

In a nutshell

Respiratory failure is type 1 (PaO2 below 8 kPa with a normal or low carbon dioxide) or type 2 (PaCO2 above 6.1 kPa). Give oxygen to a target of 94 to 98%, or 88 to 92% if hypercapnic failure is a risk, and treat the cause at the same time.

Classic presentation

A man with severe COPD, 3 days of worsening breathlessness, now drowsy on a 60% mask at 96% saturation, whose gas shows pH 7.24, PaCO2 9.1 kPa and bicarbonate 32 mmol/L.

Key points

Type 2 failure is defined by the carbon dioxide alone. A saturation of 97% on oxygen does not exclude it, and an oximeter never will.
Bicarbonate above 28 mmol/L with a raised PaCO2 and a normal pH means chronic retention. Keep that patient at 88 to 92%.
In neuromuscular disease consider NIV when forced vital capacity is below 1 litre with a respiratory rate above 20, even with a normal carbon dioxide.
Letting the PaO2 rise above 10 kPa in a retainer increases the risk of respiratory acidosis. Overshooting the target range is a prescribing error.
About 20% of acute hypercapnic failure needs level 2 or 3 care (high dependency or intensive care). Plan the escalation before you start NIV.
In pneumonia get the chest film within 4 hours of arrival, but in severe acidosis start NIV without waiting for it.

First-line investigation

Arterial blood gas with the oxygen device and flow rate recorded on it: it separates type 1 from type 2 and gives the pH that decides whether the patient needs ventilating.

Management

Stabilise and get the gas

ABCDE (airway, breathing, circulation, disability, exposure), sit upright, continuous oximetry and cardiac monitoring, arterial blood gas with the oxygen device and flow rate recorded.^1,2
Nurse in an area with monitoring beyond a general ward. Call a senior early and document the plan for what happens if support fails, before you start it.²

Oxygen to a target range, not a flow rate

Not at risk of hypercapnia: target 94 to 98%. Reservoir mask at 15 L/min if saturation is below 85%, then nasal cannulae 1 to 6 L/min or a simple mask 5 to 10 L/min.^3,6,4
At risk of hypercapnia: target 88 to 92% by 24% Venturi mask at 2 to 3 L/min, 28% Venturi at 4 L/min, or nasal cannulae 1 to 2 L/min.^3,6
Repeat the blood gas 30 to 60 minutes after any change in oxygen, and sooner if the patient deteriorates, even if the first carbon dioxide was normal.³

Treat the precipitant in parallel

COPD exacerbation: nebulised salbutamol 2.5 to 5 mg, nebulised ipratropium 500 micrograms (maximum 2 mg daily), and prednisolone 30 mg orally once daily for 5 days.^7,8,9,10
If hypercapnic or acidotic, drive that nebuliser with compressed air, not oxygen, and run oxygen alongside by nasal cannulae to hold 88 to 92%.^7,3
Suspected sepsis at high risk: broad-spectrum intravenous antibiotics and an intravenous fluid bolus, both within 1 hour of the NEWS2 (National Early Warning Score 2) assessment.⁴
Acute asthma: nebulised salbutamol 2.5 to 5 mg every 15 to 30 minutes and ipratropium 500 micrograms 4 to 6 hourly, both oxygen-driven. Add prednisolone 40 to 50 mg daily for at least 5 days.^8,9,10

Non-invasive support

BTS sources differ: use the local NIV threshold and timing. The 2016 guideline uses pH below 7.35, PaCO2 above 6.5 kPa and respiratory rate above 23 after 60 minutes; escalate sooner if worsening.^2,3
Inspiratory pressure 15 cmH2O, expiratory pressure at least 3 cmH2O, then raise the inspiratory pressure to 20 to 30 cmH2O over 10 to 30 minutes.²
Entrain oxygen at the mask to hold 88 to 92% in every cause of hypercapnic failure. Repeat the gas at 1 hour: a falling respiratory rate and rising pH predict success.²
Type 1 failure: consider high-flow nasal oxygen when standard oxygen misses target. CPAP with oxygen to 94 to 98% for unresponsive cardiogenic pulmonary oedema.^5,3

Intubate rather than persist

Indications: imminent respiratory arrest, severe respiratory distress, NIV failed or contraindicated, pH persistently below 7.15 or falling on NIV, Glasgow Coma Scale below 8.²
A pH below 7.25 is the level at which invasive ventilation is considered. Repeated trials of non-invasive support must not be allowed to delay it.²

Wean and prevent the next episode

Stop NIV once pH and PaCO2 have normalised and the patient is generally better, then taper daytime use over 2 to 3 days.²
Step oxygen down to the lowest flow that holds 88 to 92%. Never stop it abruptly: rebound hypoxaemia can be fatal.³
Issue an oxygen alert card after an episode of hypercapnic failure, and refer for consideration of long-term NIV if assisted ventilation was needed.^3,6,7

Exam traps

Never withhold oxygen from a hypoxaemic patient with COPD. The error is uncontrolled oxygen, not oxygen: target 88 to 92% and recheck the gas.
A normal or rising carbon dioxide in acute severe asthma means the patient is tiring. It calls for urgent critical care, not reassurance.
NIV is not used in acute asthma with hypercapnic failure. That patient needs invasive ventilation, not a mask.
Do not turn the oxygen off to correct a high carbon dioxide. Step it down to hold 88 to 92%: sudden withdrawal causes life-threatening rebound hypoxaemia.
An oxygen-driven nebuliser can tip an acidotic COPD patient further. Drive it with air and give the oxygen separately by nasal cannulae.
pH matters more than the carbon dioxide number. A PaCO2 of 9 kPa at a normal pH is chronic; the same value at pH 7.20 is an emergency.

Illustrations

Severe ARDS causing hypoxaemic respiratory failurePortable chest radiograph showing diffuse bilateral air-space opacification in an intubated patient with severe acute respiratory distress syndrome, a cause of type 1 hypoxaemic respiratory failure.James Heilman, MD, Wikimedia Commons · CC-BY-SA-4.0

Key sources

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.