Hypoxaemia

Original Editor - Adam Vallely Farrell

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

Introduction

Hypoxaemia is an abnormally low amount of oxygen in the blood, specifically arterial blood. Hypoxaemia is defined as the inability to maintain the PaO2 above 8kPa (see ABGs for more details).  

It can cause hypoxia (low oxygen) in body tissues if unresolved, which can in turn lead to cell death of the peripheral then central tissues. Hypoxia is also known as type 1 respiratory failure.

Type 1 Respiratory Failure

Respiratory failure is characterised by a reduction in the function of the lungs due to lung disease, skeletal or neuromuscular disorder.

It occurs when gas exchange in the lungs is significantly impaired causing a drop in the levels of oxygen in the blood: hypoxaemia or type 1 respiratory failure.

When there is a drop in oxygen and a rise in carbon dioxide levels this is known as: hypercapnia or type 2 respiratory failure.

Respiratory failure is demonstrated in arterial blood gas (ABG). Type I respiratory failure is defined as PaO2 <8.0kPa with a normal or lowered PaCO2.

Type 1 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.

Classification of Hypoxaemia

There are many causes of hypoxaemia, often due to respiratory disorders:

	Definition	Cause
Hypoxic hypoxemia	Occurs where blood flows through parts of the lung which are un-ventilated Inability to transfer oxygen across the pulmonary membrane (gas diffusion limitation) Acute bronchoconstriction: insufficient gas flow in and out of the lung Insufficient inspired oxygen therapy (including faulty oxygen delivery equipment)	Primary respiratory disease: COPD, pulmonary fibrosis, asthma, 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 hypoxemia	Usually due to inadequate blood flow through the lung	Pulmonary embolus Destruction of the pulmonary vasculature (COPD, pulmonary trauma)
Anaemic hypoxemia	Reduction in the oxygen carrying capacity of the blood	Shock (significant blood loss with a reduced Hb) Primary haematological diseases, e.g. sickle cell crisis, anaemia
Toxic hypoxemia	Difficulty in the utilisation of oxygen It is common in patients admitted with inhalation burns/ smoke inhalation injuries	E.g. carbon monoxide poisoning, cyanide poisoning

Causes of Hypoxemia

Bronchopneumonia

Bronchopneumonia is a form of pneumonia, sometimes referred to as lobar pneumonia, that also causes inflammation in the bronchi. A patient presenting with bronchopneumonia will present with troubled breathing due to airway constriction. Due to inflammation, their lungs are not adequately ventilated. Symptoms can range from mild to severe. Physiotherapy treatment can include;

• Ensure medication is optimised (oxygen, analgesia, bronchodilators, antibiotics, etc.)
• Positioning to decrease work of breathing
• Airway clearance techniques
• Humidification

Acute lobar pneumonia

Lobar pneumonia is an acute exudative inflammation of an entire pulmonary lobe, produced in 95% of cases by Streptococcus pneumoniae (pneumococci). If not treated lobar pneumonia progresses in 4 stages; namely consolidation, red hepatization, grey hepatization and resolution. Treatment is dependent on the severity of symptoms and may include some of the following techniques;

• During the unproductive phase advice on positioning may reduce WOB
• CPAP is useful for hypoxaemia
• Sputum clearance s only indicated if the patient becomes productive

Pulmonary embolus

Pulmonary embolism (PE) is a blockage of an artery in the lungs by a substance that has moved from elsewhere in the body through the bloodstream (embolism). Symptoms of a PE may include shortness of breath, chest pain particularly upon breathing in, and coughing up blood. Physiotherapy is not indicated. CPAP may help with severe hypoxaemia.

Pulmonary fibrosis

Pulmonary fibrosis describes a collection of diseases which lead to interstitial lung damage and ultimately fibrosis and loss of the elasticity of the lungs. It is a chronic condition characterised by shortness of breath. The lung tissue becomes thickened, stiff and scarred over a period of time. The development of scar tissue is called fibrosis. As the lung tissue becomes scarred and thicker, the lungs start to lose their ability to transfer oxygen into the bloodstream. Treatment of pulmonary fibrosis may involve;

• Present with profound hypoxaemia. Humidified CPAP is effective
• Ensure sufficient oxygen is available when CPAP removed

Pulmonary oedema

Pulmonary oedema occurs when fluid accumulates in the alveoli of the lungs causing an increased work of breathing. This fluid accumulation interferes with gas exchange and can cause respiratory failure. Treating a patient with pulmonary oedema may include;

• CPAP is effective in the treatment of pulmonary oedema
• If hypotensive, check that BP does not drop with increased intrathoracic pressure.
• NIV (pressure support with EPAP) may be useful in the patient tiring on CPAP

CO2 retention

Uncontrolled oxygen therapy, or receiving too much oxygen, can make people who usually have higher CO2 levels retain more until it reaches dangerous levels.

• Acute CO2 retention is not a reason to reduce FiO2 unless patients have evidence of acute-on-chronic CO2 retention secondary to chronic respiratory disease
• This can be diagnosed by interpretation of recent blood gas results, assessing pH, in relation to PaCO2, standard bicarbonate and base excess. Only this group of patients require judicious oxygen administration )24-28%), which should be prescribed accordingly

Fatigue

• Hypoxaemic patients may start to fatigue. This is seen by a rising PaCO2 - type II respiratory failure failure. An important clinical sign requiring immediate attention

Chronic chest patients

• Patients who have longstanding chest diseases may have a regular chest clearing routine they adhere to e.g. Bronchiectatic, CF patient
• It is important to discuss this and mould your treatment plan so that it fits within their existing regimen and their current physiotherapy problems

Renal failure

• Patient in renal failure may present with an increased work of breathing
• ABGs will show metabolic acidosis, generally with some form of respiratory ompensation e.g. Decreased CO2
• Pulmonary oedema and pleural effusion may also be present

Distended abdomen, e.g. pancreatitis, ascites

• Positioning in alternate side-lying or well supported high side-lying is useful
• Standing where possible

Clinical Signs

A patient with acute hypoxemia will display some or all of the following symptoms;

Sign	Clinical feature	Observation
Central cyanosis	Blue-ish palor, blue lips	Hypothermic <36.5 degrees C
Peripheral shut down	Cool to touch, clammy	Hypothermic <36.5 degrees C
Tachypnoea	Increased respiratory rate	>20 breaths per min, appears in distress with breathing
Low O2	Low O2 saturations	<90%
Accessory muscle use	Tracheal tug, flared nostrils, bracing through upper limbs
Reduced mental state	Confused, agitated

Chronic hypoxemia can occur from chronic lung conditions such as COPD or seep apnoea, but it can also be caused by environmental factors such as frequent flying or living at high altitudes. which can be compensated or uncompensated. The compensation may result in the symptoms to be overlooked initially, however, further disease progression or mild illness such as a chest infection may increase oxygen demand and unmask the existing hypoxemia.

Medical Treatment of Hypoxemia

The primary treatment for hypoxemia is controlled oxygen therapy, alongside the identification and treatment of the underlying cause.

Controlled Oxygen Therapy

Patients who are unable to maintain a SaO2 >90% with a face mask may require additional respiratory support. This might include either continuous positive airway pressure (CPAP), non-invasive ventilation or intubation with mechanical ventilation.

Controlled oxygen therapy is prescribed for hypoxaemic patients to increase alveolar oxygen tension and decrease work of breathing. It is important to remember that oxygen is a drug and should always be prescribed with the required percentage and/or flow rate.

Humidification

Oxygen therapy is known to dry out the airways. Humidifying oxygen is often used in an attempt to help prevent the drying of the upper respiratory tract. In patients who are requiring high flow oxygen or oxygen for longer periods consider cold or heated humidification. Heated humidification is believed to be better for tenacious secretions or severe bronchospasm.

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

Whilst the delivery of oxygen therapy is the primary treatment for hypoxaemia it is essential to treat the cause. There may be an acute primary respiratory problem, however, the respiratory failure could be due to compensation for another condition such as cardiac or renal. Multi-disciplinary team (MDT) working is vital in managing this.

Physiotherapy

The overall aim of physiotherapy is to identify and treat the cause of the hypoxaemia, thus aiming to increase PaO2 >8kPa while administering appropriate oxygen therapy. The cause of hypoxaemia may be sputum retention in which case various physiotherapy techniques can be used:

• Positioning - to improve V/Q matching
• Active cycle of breathing exercises
• Autogenic drainage
• Manual techniques: percussion or vibrations
• Bubble PEP or other oscillating instruments: acapella, flutter
• Cough assist or IPPB

Complications with Hypoxemia

If left untreated hypoxemia can lead to type 1 respiratory failure. This may result is further symptoms which can be life threatening:

• Respiratory acidosis
• Cardiac arrhythmia
• Cerebral hypoxaemia
• Altered mental state including coma
• Cardiorespiratory arrest