Hypoxemia

From Wikipedia, the free encyclopedia - View original article

 
Jump to: navigation, search

Hypoxemia (or hypoxaemia in British English) is an abnormally low level of oxygen in the blood.[1][2] More specifically, it is oxygen deficiency in arterial blood.[3] Hypoxemia has many causes, often respiratory disorders, and can cause tissue hypoxia as the blood is not supplying enough oxygen to the body.

Definition[edit]

Hypoxemia refers to low oxygen in the blood, and the more general term hypoxia is an abnormally low oxygen content in any tissue or organ, or the body as a whole.[2] Hypoxemia can cause hypoxia (hypoxemic hypoxia), but hypoxia can also occur via other mechanisms, such as anemia.[4] Informally, hypoxemic hypoxia is sometimes called hypoxic hypoxia.[citation needed]

Hypoxemia is usually defined in terms of reduced partial pressure of oxygen (mm Hg) in arterial blood, but also in terms of reduced content of oxygen (ml oxygen per dl blood) or percentage saturation of hemoglobin (the oxygen binding protein within red blood cells) with oxygen, which is either found singly or in combination.[2][5]

While there is general agreement that an arterial blood gas measurement which shows that the partial pressure of oxygen is lower than normal constitutes hypoxemia,[5][6][7] there is less agreement concerning whether the oxygen content of blood is relevant in determining hypoxemia. This definition would include oxygen carried by hemoglobin. The oxygen content of blood is thus sometimes viewed as a measure of tissue delivery rather than hypoxemia.[7]

Signs and symptoms[edit]

In an acute context, hypoxemia can cause symptoms such as those in respiratory distress. These include breathlessness, an increased rate of breathing, use of the chest and abdominal muscles to breathe, and lip pursing.[8]:642

Chronic hypoxemia may be compensated or uncompensated. The compensation may cause symptoms to be overlooked initially, however, further disease or a stress such as any increase in oxygen demand may finally unmask the existing hypoxemia. In a compensated state, blood vessels supplying less-ventilated areas of the lung may selectively contract, to redirect the blood to areas of the lungs which are better ventilated. However, in a chronic context, and if the lungs are not well ventilated generally, this mechanism can result in pulmonary hypertension, overloading the right ventricle of the heart and causing cor pulmonale and right sided heart failure. Polycythemia can also occur.[8] In children, chronic hypoxemia may manifest as delayed growth, neurological development and motor development and decreased sleep quality with frequent sleep arousals.[9]

Other symptoms of hypoxemia may include cyanosis, digital clubbing, and symptoms that may relate to the cause of the hypoxemia, including cough and hemoptysis.[8]:642

Serious hypoxemia occurs (1) when the partial pressure of oxygen in blood is less than 60 mm Hg, (the beginning of the steep portion of the oxygen–haemoglobin dissociation curve, where a small decrease in the partial pressure of oxygen results in a large decrease in the oxygen content of the blood);[6] or (2) when hemoglobin oxygen saturation is less than 90%.[medical citation needed] Severe hypoxia can lead to respiratory failure [8]

Causes[edit]

Hypoxemia refers to insufficient oxygen in the blood. Thus any cause that influences the rate or volume of air entering the lungs (ventilation) or any cause that influences the transfer of air from the lungs to the blood (perfusion) may cause hypoxemia. As well as these respiratory causes, cardiovascular causes such as shunts may also result in hypoxaemia.

The most common causes of hypoxemia are ventilation-perfusion mismatch, hypoventilation, and shunts.[10] :229

Ventilation[edit]

If the alveolar ventilation is insufficient, there will not be enough oxygen delivered to the alveoli for the body's use. This can cause hypoxemia even if the lungs are normal, as the cause is in the brainstem's control of ventilation or in the body's inability to breathe effectively.

Respiratory drive[edit]

Respiration is controlled by centres in the medulla, which influence the rate of breathing and the depth of each breath. This is influenced by the blood level of carbon dioxide, as determined by chemoreceptors in the aorta. Hypoxia occurs when the breathing center doesn't function correctly or when the signal is not appropriate:

Physical states[edit]

A variety of conditions that physically limit airflow can lead to hypoxemia.

Environmental oxygen[edit]

Oxygen-Haemoglobin Dissassociation Curve.

In conditions where the proportion of oxygen in the air is low, or when the partial pressure of oxygen has decreased, less oxygen is present in the alveoli of the lungs. The alveolar oxygen is transferred to hemoglobin, a carrier protein inside red blood cells, with an efficiency that decreases with the partial pressure of oxygen in the air.

Perfusion[edit]

Ventilation-perfusion mismatch[edit]

This refers to a disruption in the ventilation/perfusion equilibrium. Oxygen entering the lungs typically diffuses across the alveolar-capillary membrane into blood. However this equilibration does not occur when the alveolus is insufficiently ventilated, and as a consequence the blood exiting that alveolus is relatively hypoxemic. When such blood is added to blood from well ventilated alveoli, the mix has a lower oxygen partial pressure than the alveolar air, and so the A-a difference develops. Examples of states that can cause a ventilation-perfusion mismatch include:

Shunting[edit]

Shunting refers to blood that bypasses the pulmonary circulation, meaning that the blood does not receive oxygen from the alveoli. In general, a shunt may be within the heart or lungs, and cannot be corrected by administering oxygen alone. Shunting may occur in normal states:

Shunting may also occur in disease states:

Physiology[edit]

Key to understanding whether the lung is involved in a particular case of hypoxemia is the difference between the alveolar and the arterial oxygen levels; this A-a difference is often called the A-a gradient and is normally small. The arterial oxygen partial pressure is obtained directly from an arterial blood gas determination. The oxygen contained in the alveolar air can be calculated because it will be directly proportional to its fractional composition in air. Since the airways humidify (and so dilute) the inhaled air, the barometric pressure of the atmosphere is reduced by the vapor pressure of water.

History[edit]

The term hypoxemia was originally used to describe low blood oxygen occurring at high altitudes and was defined generally as defective oxygenation of the blood.[21]

References[edit]

  1. ^ Pollak, Charles P.; Thorpy, Michael J.; Yager, Jan (2010). The encyclopedia of sleep and sleep disorders (3rd ed.). New York, NY. p. 104. ISBN 9780816068333. 
  2. ^ a b c Martin, Lawrence (1999). All you really need to know to interpret arterial blood gases (2nd ed.). Philadelphia: Lippincott Williams & Wilkins. p. xxvi. ISBN 978-0683306040. 
  3. ^ Eckman, Margaret (2010). Professional guide to pathophysiology (3rd ed.). Philadelphia: Wolters Kluwer/Lippincott Williams & Wilkins. p. 208. ISBN 978-1605477664. 
  4. ^ Robert J. Mason, V. Courtney Broaddus, Thomas R. Martin, Talmadge E. King, Dean E. Schraufnagel, John F. Murray and Jay A. Nadel (eds.) (2010) Murray & Nadel's Textbook of Respiratory Medicine, 5th ed. Philadelphia: Saunders Elsevier. ISBN 1-4160-4710-7.
  5. ^ a b Morris, Alan; Kanner, Richard; Crapo, Robert; Gardner, Reed. (1984) Clinical Pulmonary Function Testing. A manual of uniform laboratory procedures, 2nd ed.
  6. ^ a b Lorenzo Del Sorbo, Erica L. Martin, V. Marco Ranieri (2010) "Hypoxemic Respiratory Failure" In: Murray & Nadel's Textbook of Respiratory Medicine, Robert J. Mason, V. Courtney Broaddus, Thomas R. Martin, Talmadge E. King, Dean E. Schraufnagel, John F. Murray and Jay A. Nadel (eds.) 5th ed. Philadelphia: Saunders Elsevier. ISBN 1-4160-4710-7.
  7. ^ a b Critical care. New York [u. a.]: Informa Healthcare. 2007. ISBN 0-8247-2920-X. 
  8. ^ a b c d e Nicki R. Colledge, Brian R. Walker, Stuart H. Ralston, ed. (2010). Davidson's principles and practice of medicine (21st ed.). Edinburgh: Churchill Livingstone/Elsevier. ISBN 978-0-7020-3085-7. 
  9. ^ Adde, FV; Alvarez, AE; Barbisan, BN; Guimarães, BR (Jan–Feb 2013). "Recommendations for long-term home oxygen therapy in children and adolescents". Jornal de pediatria 89 (1): 6–17. doi:10.1016/j.jped.2013.02.003. PMID 23544805. 
  10. ^ Harrison's principles of internal medicine. (17th ed.). New York [etc.]: McGraw-Hill Medical. 2008. ISBN 978-0-07-147692-8. 
  11. ^ Craig, Albert B. (Fall 1976). "Summary of 58 cases of loss of consciousness during underwater swimming and diving". Medicine and science in sports 8 (3): 171–5. doi:10.1249/00005768-197600830-00007. PMID 979564. 
  12. ^ Kenneth Baillie and Alistair Simpson. "Altitude oxygen calculator". Apex (Altitude Physiology Expeditions). Retrieved 2006-08-10.  – Online interactive oxygen delivery calculator.
  13. ^ West, JB; Boyer, SJ; Graber, DJ; Hackett, PH; Maret, KH; Milledge, JS; Peters RM, Jr; Pizzo, CJ; Samaja, M; Sarnquist, FH (September 1983). "Maximal exercise at extreme altitudes on Mount Everest". Journal of applied physiology: respiratory, environmental and exercise physiology 55 (3): 688–98. PMID 6415008. 
  14. ^ Grocott, MP; Martin, DS; Levett, DZ; McMorrow, R; Windsor, J; Montgomery, HE; Caudwell Xtreme Everest Research, Group (Jan 8, 2009). "Arterial blood gases and oxygen content in climbers on Mount Everest". The New England Journal of Medicine 360 (2): 140–9. doi:10.1056/NEJMoa0801581. PMID 19129527. 
  15. ^ West, John B. (2000). "Human limits for hypoxia: the physiological challenge of climbing Mt. Everest". Annals of the New York Academy of Sciences 899: 15–27. PMID 10863526. 
  16. ^ Whipp, BJ; Wasserman, K (September 1969). "Alveolar-arterial gas tension differences during graded exercise". Journal of applied physiology 27 (3): 361–5. PMID 5804133. 
  17. ^ Hopkins, SR (2006). "Exercise induced arterial hypoxemia: the role of ventilation-perfusion inequality and pulmonary diffusion limitation". Advances in experimental medicine and biology 588: 17–30. doi:10.1007/978-0-387-34817-9_3. PMID 17089876. 
  18. ^ a b Agustí, AG; Roca, J; Gea, J; Wagner, PD; Xaubet, A; Rodriguez-Roisin, R (February 1991). "Mechanisms of gas-exchange impairment in idiopathic pulmonary fibrosis". The American review of respiratory disease 143 (2): 219–25. doi:10.1164/ajrccm/143.2.219. PMID 1990931. 
  19. ^ Agusti, AG; Roca, J; Rodriguez-Roisin, R (March 1996). "Mechanisms of gas exchange impairment in patients with liver cirrhosis". Clinics in chest medicine 17 (1): 49–66. doi:10.1016/s0272-5231(05)70298-7. PMID 8665790. 
  20. ^ Huang, YC; Fracica, PJ; Simonson, SG; Crapo, JD; Young, SL; Welty-Wolf, KE; Moon, RE; Piantadosi, CA (August 1996). "VA/Q abnormalities during gram negative sepsis". Respiration physiology 105 (1-2): 109–21. doi:10.1016/0034-5687(96)00039-4. PMID 8897657. 
  21. ^ Henry Power and Leonard W. Sedgwick (1888) New Sydenham Society's Lexicon of Medicine and the Allied Sciences (Based on Maye's Lexicon). Vol III. London: New Sydenham Society.