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 a common presentation in critically ill patients, with the potential for severe harm if not addressed appropriately. Hypoxaemia refers to a lower than normal arterial blood oxygen level, measured either as oxygen saturation (SaO2) or partial pressure of oxygen (PaO2).

• It is a common feature of acutely unwell hospitalised patients and can result in substantial morbidity and mortality if not treated rapidly and appropriately. Hypoxaemic patients may require admission to an intensive care unit, with more than 60% of those that do eventually requiring invasive ventilation.
• The mortality of hypoxaemic critically ill patients is 27%, rising to as high as 50% in patients with severe hypoxaemia No specific threshold of SaO2 or PaO2 defines hypoxaemia.
• Suggested normal values for PaO2 are 10.5–13.5 kPa, and for SaO2 are 94–98%. This information can be obtained via arterial blood gas (providing PaO2and SaO2) and pulse oximetry (providing peripheral capillary oxygen saturation; SpO2).
• It should be noted that normal values decline with age and are influenced by the presence of comorbidities such as chronic lung disease.

The term hypoxia and hypoxemia are not synonymous. Hypoxemia is defined as a decrease in the partial pressure of oxygen in the blood whereas hypoxia is defined by reduced level of tissue oxygenation. It can be due to either defective delivery or defective utilization of oxygen by the tissues. Hypoxemia and hypoxia do not always coexist. Patients can develop hypoxemia without hypoxia if there is a compensatory increase in hemoglobin level and cardiac output (CO). Similarly, there can be hypoxia without hypoxemia

Physiology

Hypoxaemia may result from a multitude of pathologies; however, the basic physiology underlying these can be split into:

• Hypoventilation
• Ventilation–perfusion (VQ) mismatch (either an increased or decreased VQ ratio)
• Right to left circulatory shunting of blood
• Impaired diffusion of oxygen across the alveolar membrane
• Reduced FiO2 (Sarkar et al, 2017).

Respiratory failure is a broad term that describes inadequate gas exchange that either consists of hypoxaemia alone (type 1) or in combination with hypercapnia (type 2).

Type 1 Respiratory Failure

Type 1 Respiratory Failure (hypoxemic): is associated with damage to lung tissue which prevents adequate oxygenation of the blood.  However, the remaining normal lung is still sufficient to excrete carbon dioxide. This results in low oxygen, and normal or low carbon dioxide levels.

• Arterial oxygen pressure (PaO2) is <8 kPa (60 mm Hg) with normal or low arterial carbon dioxide pressure (PaCO2).
• Causes of type 1 respiratory failure include: pulmonary oedema, pneumonia, COPD, asthma, acute respiratory distress syndrome, chronic pulmonary fibrosis, pneumothorax, pulmonary embolism, pulmonary hypertension

Type 1 respiratory failure is related to respiratory distress, with increased work of breathing and deranged gas exchange.

• May occur with or without the presence of excessive pulmonary secretions and/or sputum retention
• 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.

Clinical Signs

A patient with acute hypoxaemia 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 hypoxaemia 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 hypoxaemia.

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 Hypoxaemia

Some of the respiratory causes of hypoxaemia are:

1. Pneumonia: is caused by an infection which can start off as a lower respiratory tract infection, which when untreated can cause consolidation and significant sputum retention. Due to the lobe consolidation, the lungs are not adequately ventilated.
2. Pulmonary embolus (PE): is a blockage of an artery in the lungs by a clot that has moved from elsewhere in the body through the bloodstream (embolism). Symptoms of a PE may include shortness of breath and chest pain, particularly on inspiration.
3. 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.
4. 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.
5. Chronic respiratory conditions: such as COPD or CF
6. Distended abdomen, e.g. pancreatitis, ascites. This prevents the diaphragm from descending which reduces the surface area for gas exchange.

Medical Treatment of Hypoxaemia

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

1. 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.
2. 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.
3. Treat the Cause
   Whilst the delivery of oxygen therapy is the primary treatment for hypoxaemia it is essential to treat the cause ,e.g. bronchospasm, sputum retention, volume loss. 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
• Home oxygen therapy

Complications with Hypoxaemia

If left untreated hypoxaemia 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

Resources

Respiratory failure

Respiratory system

Arterial blood gases