Hypoxaemia

Original Editor - Adam Vallely Farrell

Top Contributors - Adam Vallely Farrell, Lucinda hampton, Abbey Wright, Kim Jackson, Rishika Babburu and Chelsea Mclene  

Respiratory Failure

Respiratory failure is demonstrated in arterial blood gas (ABG) tensions. Type I respiratory failure is defined as PaO2 <8.0kPa with a normal or lowered PaCO2. Type II respiratory failure (ventilatory failure) is defined as PaO2 <8.0kPa and a PaCO2 >6.0kPa. Acute respiratory failure is related to respiratory distress, with increased work of breathing and deranged gas exchange. It may occur with or without the presence of excessive pulmonary secretions and/or sputum retention., and is not necessarily related to a primary respiratory problem, e.g. neurological problems may be related to respiratory depression, hypoventilation, reduced level of consciousness and inability to protect the airway. Cough depression and risk of aspiration are a serious concern. Unrecognised respiratory failure leads to;

• respiratory muscle fatigue
• hypoventilation
• sputum retention
• Decreased O2 (hypoxaemia)

Acute assessment to establish the underlying cause is imperative as, if left untreated, it may progress to any or all of the following;

• cardiac arrhytmia
• cerebral hypoxaemia
• respiratory acidosis
• coma
• cardiorespiratory arrest

Thus, timely recognition and treatment of respiratory failure is of the utmost importance and a serious part of a patients care.

Hypoxaemia (Type 1 Respiratory Failure)

Hypoxaemia is defined as the inability to maintain the PaO2 above 8kPa

Classification and Causes of Hypoxaemia

	Classification	Cause
Hypoxic hypoxaemia	Where blood flows through parts of the lung which are unventilated Inability to transfer oxygen across the pulmonary membrane (gas diffusion limitation) Acute bronchoconstriction: asthma (insufficient gas flow in and out of the lung) Insufficient inspired oxygen therapy (including faulty oxygen delivery equipment)	Primary respiratory disease: COPD, pulmonary fibrosis, CF, pneumonia, sputum retention, decreased gas transfer across thickened (fibrotic/ oedematous) membrane Primary cardiac disease: heart failure, congestive cardiac failure, pulmonary oedema (causing a diffusion limitation across the respiratory membrane)
Ischaemic hypoxaemia	Usually due to inadequate blood flow through the lung	Pulmonary embolus Destruction of the pulmonary vasculature (COPD, pulmonary trauma)
Anaemic hypoxaemia	Reduction in oxygen carrying capacity of the blood	Shock (significant blood loss with a reduced Hb) Primary haematological diseases, e.g. sickle cell crisis, anaemia
Toxic hypoxaemia	Difficulty in oxygen utilisation - common in patients admitted with inhalation burns/ smoke inhalation injury	E.g. carbon monoxide poisoning, cyanide poisoning

Hypoxic hypoxaemia

Classification

• Where blood flows through parts of the lung which are unventilated
• Inability to transfer oxygen across the pulmonary membrane (gas diffusion limitation)
• Acute bronchoconstriction: asthma (insufficient gas flow in and out of the lung)
• Insufficient inspired oxygen therapy (including faulty oxygen delivery equipment)

Cause

• Primary respiratory disease: COPD, pulmonary fibrosis, CF, pneumonia, sputum retention, decreased gas transfer across thickened (fibrotic/ oedematous) membrane
• Primary cardiac disease: heart failure, congestive cardiac failure, pulmonary oedema (causing a diffusion limitation across the respiratory membrane)

Ischaemic hypoxaemia

Classification

• Usually due to inadequate blood flow through the lung

Cause

• pulmonary embolus
• destruction of the pulmonary vasculature (COPD, pulmonary trauma)

Anaemic hypoxaemia

Classification

• Reduction in oxygen carrying capacity of the blood

Cause

• Shock (significant blood loss with a reduced Hb)
• Primary haematological diseases, e.g. sickle cell crisis, anaemia

Toxic hypoxaemia

Classification

• Difficulty in oxygen utilisation - common in patients admitted with inhalation burns/ smoke inhalation injury

Cause

• E.g. carbon monoxide poisoning, cyanide poisoning

Clinical Signs

A patient with hypoxaemia will display some/all of the following symptoms;

• central cyanosis (blue lips, tongue)
• peripheral shut-down (cool to touch, 'cold and clammy')
• tachypnoea - increased respiratory rate (>20 beats per minute)
• low oxygen saturation (<90%)
• confusion or agitation if profound hypoxaemia, may be non compliant with treatment

Aim of Physiotherapy

To identify and treat, if appropriate, the cause of the hypoxaemia, thus aiming to increase PaO2 >8kPa while administering appropriate oxygen therapy

Treatment of Hypoxaemia

The primary treatment of hypoxaemia is controlled oxygen therapy, plus identification and treatment of the underlying cause. Patients who are unable to maintain SaO2 >90% on face mask oxygen may require additional respiratory support, either continuous positive airway pressure (CPAP) or intubation and mechanical ventilation. Patients with unilateral lung disease can be positioned in side-lying, with the unaffected lung down, to try to improve V/Q matching.

Controlled Oxygen Therapy

• Oxygen is a drug which should be prescribed for the required percentage and/or flow rate
• Usually 24-60% can be given by an oxygen mask
• 2-4L/min via nasal cannulae; however, a mask may be preferable if hypoxic and/or mouth breather
• Over 60% oxygen with persistently low sats (<90%) use a non-rebreather mask to administer constant flow of high concentration oxygen
• CPAP is useful with profound hypoxaemia once pneumothorax excluded

Humidification

• Consider cold or heated humidification
• Heated is better for tenacious secretions or severe bronchospasm

Treat the cause, e.g. bronchospasm, sputum retention, volume loss

• If primary respiratory problem, treat this
• If primary problem is cardiac or renal, discuss this with the medical team

Increased work of breathing

• Use airway clearance techniques if needed
• Positioning is essential to reduce breathlessness and improve ventilation perfusion matching
• IPPB may be useful (with a high flow rate) to rest the muscles and improve efficacy of other treatments