• Hypoxaemia
• Hypercarbia
• Exhaustion
• Increase in work of breathing
• Altered level of consciousness

Hypoxaemia

If it proves impossible to adequately oxygenate a patient with a high concentration of oxygen delivered at high flow and the underlying disease is likely to be responsive to positive airway pressure (usually diseases with diffuse infiltrates), hypoxaemia on its own may be an indication for ventilation.

If the underlying disease is unilateral (e.g., lobar pneumonia), and especially if shunt is the major component of the pathophysiology, proceed with caution. The situation may not improve—and may even worsen—with mechanical ventilation.

If the underlying disease is unlikely to respond to either ventilation or treatment for the underlying condition (e.g., pulmonary fibrosis), ventilation should normally be avoided altogether.

Exhaustion and hypoxaemia can lead to agitation. This can put the patient and staff at risk. In this case (if it is appropriate to invasively ventilate the patient), intubation and ventilation should be offered sooner rather than later. Be aware that this can be a difficult situation to handle and senior help should be sought as soon as possible.

Hypercarbia

There is no set level of CO₂ that should prompt institution of mechanical ventilation. The following all make it more likely that ventilation will be necessary:

• A deteriorating trend
• A failure to respond to treatment
• A marked respiratory acidosis
• An associated metabolic acidosis
• A coma caused by CO₂ narcosis

Exhaustion

Any cause of respiratory failure may cause respiratory muscle fatigue. Signs of exhaustion include:

• A high respiratory rate
• Poor tidal volume
• Use of accessory muscles
• An inability to complete sentences and somnolence

Paradoxical or seesaw breathing is a sign of impending respiratory arrest. It is often difficult to predict from a single assessment how well, or for how long, a patient will tolerate this state. An ongoing deterioration and failure to respond to therapy are features more likely to prompt intervention. Eventually, a reduction in respiratory rate may precede respiratory arrest.

It often becomes apparent on the initial assessment that the patient is too exhausted and there is no time to wait for a response to treatment. In this situation, prompt action (intubation and ventilation) can be lifesaving.

Work of Breathing

Work of breathing in respiratory failure can account for 30% of total body oxygen consumption. Respiratory support of some form (CPAP, NIV, or invasive ventilation) will reduce this burden.

There may be advantages in an early introduction of some form of respiratory support, particularly if the total body oxygen consumption is greater than the total body oxygen delivery. Such an imbalance will produce low central or mixed venous oxygen saturation, a metabolic acidosis, and perhaps an increase in lactate. Early severe sepsis or cardiac failure will produce such a picture.

It is often difficult to predict from a single assessment how well, or for how long, a patient will tolerate this state. An ongoing deterioration and failure to respond to therapy are features more likely to prompt intervention. Eventually, a reduction in respiratory rate may precede respiratory arrest.

It often becomes apparent on the initial assessment that the patient is too exhausted and there is no time to wait for a response to treatment. In this situation, prompt action (intubation and ventilation) can be lifesaving.

Altered Conscious Level

Patients with a low conscious level are often unable to maintain and protect a patent airway. Even if they have an unobstructed airway, they are at risk of sudden obstruction and aspiration of gastric and supraglottic secretions.

Intubation for airway control should be considered for patients with a GCS of < 8 or those with a deteriorating GCS.