Hypoxaemia

Introduction[edit | edit source]

Respiratory failure.jpg

Hypoxemia is defined as an abnormally low partial pressure of oxygen in the blood (normal arterial partial pressure of oxygen: 75 - 100mmHg). It is different from hypoxia, where the tissue oxygen delivery is inadequate to support normal the aerobic metabolism of the tissues.[1] It 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).[2]

  • 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.
  • Normal values for PaO2 are 10.5–13.5 kPa, and for SaO2 are 94–98%. Obtained via arterial blood gas (providing PaO2and SaO2) and pulse oximetry (providing peripheral capillary oxygen saturation; SpO2).
  • Normal values decline with age and are influenced by the presence of comorbidities such as chronic lung disease[3].

Respiratory Failure: a clinical condition that happens when the respiratory system fails to maintain its main function, which is gas exchange, in which PaO2 is lower i.e., hypoxemia than 60 mmHg and/or PaCO2 higher than 50 mmHg. Respiratory failure is divided into type I and type II.

  1. Type I respiratory failure involves low oxygen, and normal or low carbon dioxide levels.
  2. Type II respiratory failure involves low oxygen, with high carbon dioxide.

Hypoxia and Hypoxemia[edit | edit source]

The term hypoxia and hypoxemia are not synonymous.

  1. Hypoxemia is defined as a decrease in the partial pressure of oxygen in the blood
  2. Hypoxia is defined by reduced level of tissue oxygenation. Severe hypoxia can affect the production of ATP by mitochondrialoxidative phosphorylation, threatening cellular integrity. Non-oxygen dependent bioenergetic pathways are referred to as anaerobic metabolism; they are short-term inefficient systems that are unable to sustain life for prolonged periods of time in humans[3].

Mechanisms of Hypoxemia[edit | edit source]

There are various mechanisms of hypoxemia. These are V/Q mismatch, right-to-left shunt, diffusion impairment, hypoventilation, and low inspired PO2[4].

Pulmonary-embolism-29.png

Ventilation/perfusion (V/Q) mismatch Image 2: Positive study pulmonary embolism situated within the right middle lobe/anterior segment of the right upper lobe and the posterior basal segment of the right lower lobe.

  • This is the most common type of hypoxemia. Ventilation refers to the oxygen supply in the lungs, while perfusion refers to the blood supply to the lungs.
  • Ventilation and perfusion are measured in a ratio, called V/Q ratio. Normally, there’s a small degree of mismatch in this ratio, however if the mismatch becomes too great, problems can occur.
  • There are two causes of ventilation perfusion mismatch:
    1. The lungs are getting enough oxygen, but there’s not enough blood flow (increased V/Q ratio).
    2. There is blood flow to the lungs, but not enough oxygen (decreased V/Q ratio).

Shunt

  • Normally, deoxygenated blood enters the right side of the heart, travels to the lungs to receive oxygen, and then travels to the left side of the heart to be distributed to the rest of the body.
  • In this type of hypoxemia, blood enters the left side of the heart without becoming oxygenated in the lungs.

Diffusion impairment

  • When oxygen enters the lungs, it fills the alveoli. Capillaries surround the alveoli. Oxygen diffuses from the alveoli into the blood running through the capillaries.
  • In this type of hypoxemia, the diffusion of oxygen into the bloodstream is impaired.

Hypoventilation

  • Hypoventilation is when oxygen intake occurs at a slow rate. This can result in higher levels of carbon dioxide in the blood and lower levels of oxygen.

Low Inspired PO2

  • This type of hypoxemia typically occurs at higher altitudes. Available oxygen in the air decreases with increasing altitude.
  • Therefore, at higher altitudes each breath provides you with lower levels of oxygen than when you’re at sea level[5].

Clinical Signs[edit | edit source]

A patient with acute hypoxaemia will display some or all of the following symptoms[6][7];

[8]

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, Restlessness

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[9]. 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. [10]

Causes of Hypoxaemia[edit | edit source]

Some of the respiratory causes of hypoxaemia are:

  • Anaemia: a condition in which there aren’t enough red blood cells to effectively carry oxygen. Because of this, a person with anemia may have low levels of oxygen in their blood.
  • Asthma
  • 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).[11] Symptoms of a PE may include shortness of breath and chest pain, particularly on inspiration.
  • Collapsed lung (atelectasis)
  • Congenital heart defects or disease
  • High altitudes
  • Interstitial lung disease eg 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.[12] 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.
  • Medications that lower breathing rate eg some opiods and anesthetics
  • 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.[13] Due to the lobe consolidation, the lungs are not adequately ventilated.
  • Sleep apnea
  • 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.[14]
  • Chronic respiratory conditions: such as COPD ( flow of air in the lungs is obstructed. Destruction of the walls of alveoli and surrounding capillaries in COPD can lead to problems with oxygen exchange, which can lead to hypoxemia) or CF
  • Distended abdomen, e.g. pancreatitis, ascites. This prevents the diaphragm from descending which reduces the surface area for gas exchange.
  • Hypoxemia can sometimes occur in newborns with congenital heart defects or disease. In fact, measuring the levels of oxygen in the blood is used to screen infants for congenital heart defects.
  • Preterm infants are also vulnerable to hypoxemia, particularly if they’ve been placed on a mechanical ventilator[5]

Medical Treatment of Hypoxaemia[edit | edit source]

Humidifier.jpeg

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[15].
    Controlled oxygen therapy is prescribed for hypoxaemic patients to increase alveolar oxygen tension and decrease work of breathing.[16] It is important to remember that oxygen is a drug and should always be prescribed with the required percentage and/or flow rate.[9]
  2. Humidification (image 3):
    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.[17] Heated humidification is believed to be better for tenacious secretions or severe bronchospasm. [6]
  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.[6]

Physiotherapy[edit | edit source]

NIV1.jpg

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. [6] The cause of hypoxaemia may be sputum retention and in this case various physiotherapy techniques can be used[18]:

Relevance to Physiotherapy[edit | edit source]

Physiotherapists should be aware of the manifestations of hypoxemia which may occur in patients. The causes of hypoxemia may include hypoventilation, ventilation/perfusion mismatch, shunting, diffusion impairment, and low partial pressure of oxygen.[19] The use of supplemental oxygen may be indicated in patients who show signs of hypoxemia at rest, and or with activity, in combination with breathing exercises to optimize oxygen utilization and minimize breathlessness.[20]

Complications with Hypoxaemia[edit | edit source]

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 [6]

Resources[edit | edit source]

Respiratory failure

Respiratory system

Arterial blood gases

References[edit | edit source]

  1. Scheufler KM. Tissue oxygenation and capacity to deliver O2 do the two go together?. Transfusion and apheresis science. 2004 Aug 1;31(1):45-54.
  2. Aronson LA. Hypoxemia. InComplications in Anesthesia 2007 Jan 1 (pp. 637-640). WB Saunders.
  3. 3.0 3.1 Flower L, Martin D. Management of hypoxaemia in the critically ill patient. British Journal of Hospital Medicine. 2020 Jan 2:1-0.
  4. Sarkar M, Niranjan N, Banyal PK. Mechanisms of hypoxemia. Lung India: official organ of Indian Chest Society. 2017 Jan;34(1):47.
  5. 5.0 5.1 Healthline Hypoxemia Available:https://www.healthline.com/health/hypoxemia#types (accessed 19.4.2021)
  6. 6.0 6.1 6.2 6.3 6.4 6.5 Harden B, Cross J, Broad MA. Respiratory physiotherapy: An on-call survival guide. Elsevier Health Sciences; 2009.
  7. Colledge NR, Walker BR, Ralston SH, eds. (2010). Davidson's principles and practice of medicine (21st ed.). Edinburgh: Churchill Livingstone/Elsevier.
  8. marcophage. Signs and symptoms of hypoxemia. Available from: https://www.youtube.com/watch?v=Qj3xxNfWsH8 [last accessed 15/04/2017]
  9. 9.0 9.1 Bonsignore MR, Baiamonte P, Mazzuca E, Castrogiovanni A, Marrone O. Obstructive sleep apnea and comorbidities: a dangerous liaison. Multidisciplinary respiratory medicine. 2019 Dec;14(1):8.
  10. Adde FV, Alvarez AE, Barbisan BN, Guimarães BR. Recommendations for long-term home oxygen therapy in children and adolescents. Jornal de pediatria. 2013 Feb;89(1):06-17. 
  11. Dalen JE, Alpert JS. Natural history of pulmonary embolism. Progress in cardiovascular diseases. 1975 Jan 1;17(4):259-70.
  12. Gribbin J, Hubbard RB, Le Jeune I, Smith CJ, West J, Tata LJ. Incidence and mortality of idiopathic pulmonary fibrosis and sarcoidosis in the UK. Thorax. 2006 Nov 1;61(11):980-5.
  13. Blaisdell FW. Pathophysiology of the respiratory distress syndrome. Arch Surg. 1974 Jan 1;108(1):44-9.
  14. Masip J, Betbesé AJ, Páez J, Vecilla F, Cañizares R, Padró J, Paz MA, de Otero J, Ballús J. Non-invasive pressure support ventilation versus conventional oxygen therapy in acute cardiogenic pulmonary oedema: a randomised trial. The Lancet. 2000 Dec 23;356(9248):2126-32.
  15. Simon M, Braune S, Frings D, Wiontzek AK, Klose H, Kluge S. High-flow nasal cannula oxygen versus non-invasive ventilation in patients with acute hypoxaemic respiratory failure undergoing flexible bronchoscopy-a prospective randomised trial. Critical Care. 2014 Dec 1;18(6):712.
  16. Hardinge M, Annandale J, Bourne S, Cooper B, Evans A, Freeman D, Green A, Hippolyte S, Knowles V, MacNee W, McDonnell L. British Thoracic Society guidelines for home oxygen use in adults: accredited by NICE. Thorax. 2015 Jun 1;70(Suppl 1):i1-43.
  17. O'driscoll BR, Howard LS, Earis J, Mak V. BTS guideline for oxygen use in adults in healthcare and emergency settings. Thorax. 2017 Jun 1;72(Suppl 1):ii1-90.
  18. Derakhtanjani AS, Jaberi AA, Haydari S, Bonabi TN. Comparison the Effect of Active Cyclic Breathing Technique and Routine Chest Physiotherapy on Pain and Respiratory Parameters After Coronary Artery Graft Surgery: A Randomized Clinical Trial. Anesthesiology and Pain Medicine. 2019 Oct;9(5).
  19. Sarkar M, Niranjan N, Banyal PK. Mechanisms of hypoxemia. Lung India: official organ of Indian Chest Society. 2017 Jan;34(1):47.
  20. Hillegass E, Fick A, Pawlik A, Crouch R, Perme C, Chandrashekar R, McNamara SB, Cahalin LP. Supplemental oxygen utilization during physical therapy interventions. Cardiopulmonary Physical Therapy Journal. 2014 Jun 1;25(2):38-49.