Oxygenation
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
Oxygenation refers to the amount of oxygen in a medium. In blood it may be taken to be synonymous with saturation, which describes the degree to which the oxygen-carrying capacity of haemoglobin is utilised, normally 98-100%.
Oxygenation also refers to the process of adding oxygen to a medium such as water or body tissue. Claims have been made that oxygenation of human tissue prevent diseases, including cancer, however some regard these claims as unverifiable. Oxygenation of various fluorocarbon liquids has been used successfully in liquid breathing systems, allowing air-breathing animals, including humans, to breathe via liquids for short periods of time.
Monitoring
Respiration and oxygenation
- PaO2/FiO2 ratio (PF ratio)
- <math>{P/F\ ratio} = \left (\frac{PaO_2}{Fi0_2}\right) \times 100</math>
An example in a healthy person:
- <math>{476} = \left (\frac{100\ mm\ Hg}{21%}\right) \times 100</math>
A higher ratio indicates better gas exchange:
- Normal is 500
- ARDS is < 200
Comparative studies suggest this measure correlates better with pulmonary shunts than does the A-a gradient.[1][2][3]
Oxygenation index
- <math>Oxygenation\ index = \left(\frac{mean\ airway\ pressure}{P/F\ ratio}\right) \times 100</math>
or
- <math>Oxygenation\ index = \left(mean\ airway\ pressure \times \left(\frac{Fi0_2}{PaO_2}\right)\right) \times 100</math>
A lower oxygenation index indicates better gas exchange. The oxygenation index, which includes the mean airway pressure[4][5], may better correlate with intrapulmonary shunting than the PF ratio[2]
Alveolar-arterial oxygen (A-a) gradient (alveolar-arterial oxygen difference - AVO2D)
- <math>\mbox{A-a gradient} = {PAO_2}\ -\ {PaO_2}</math>
- <math>{PAO_2} = {Fi0_2} *\left ({760 - 47}\right) \ -\ \frac{PaCO_2}{0.8}</math>
- Normal is < 10 mm Hg
The A-a gradient is harder to calculate, but accounts for changes in respiration as measured by the partial pressure of carbon dioxide. However, this calculation relies on the respiratory quotient being constant in the prediction of alveolar CO2 When compared to the PF ratio, the A-a gradient is found to correlate less well with pulmonary shunting.[1][2][3]
Among outpatients with possible pulmonary embolism, the A-a gradient may be a better test.[6]
An online calculator for the A-a gradient is at http://www.mdcalc.com/aagrad.
ROX
The respiratory rate‑oxygenation (ROX) index by dividing the oximetry-derived oxygen saturation by the respiratory rate.
- The ROX predicts respiratory failure from COVID-19 when ≤1.4 ("with a sensitivity of 85%, a specificity of 86%, and an AUC of 0.86")[7].
Tissue perfusion
- Central venous oxygen saturation (ScvO2)
In patients with septic shock, maintaining the central venous oxygen saturation (ScvO2) > 70% is a health care quality assurance measure for the Institute for Healthcare Improvement.[8] This is measured from the superior vena cava. This is hard to predict by physical examination.[9]
- Mixed venous oxygen saturation (SvO2)
In patients with septic shock, maintaining the mixed venous oxygen saturation (ScvO2) > 65% is a health care quality assurance measure for the Institute for Healthcare Improvement that is an alternative to the central venous oxygen saturation.[10] This is measured from a pulmonary artery catheter. This is hard to predict by physical examination.[9]
The mixed venous pressure may be lower than the central venous pressure due to mixing with blood from the splanchnic circulation or carotid sinuses that has lower oxygen content.[11]
- Tissue oxygen saturation (StO2)
Tissue oxygen saturation (StO2) at the thenar eminence may be an alternative, non-invasive measurement.[12][13]
- Lactate clearance
Maintaining lactate clearance about 10% may be an easier alternative than invasive measurements of oxygenation according to a randomized controlled trial by EMShockNet.[14]
- <math>\text{Lactate clearance} = \left(\frac{lactate_{initial} - lactate_{two\ hours}}{lactate_{initial}}\right) \times 100</math>
- Capnography
Capnography, which is "continuous recording of the carbon dioxide content of expired air,"[15] may detect respiratory depression before hypoxemia occurs.[16] Proposed criteria for respiratory depression are:[16]
- End tidal CO2 (ETCO2) level 50 mm Hg
- ETCO2 change from baseline of 10%
- Loss of waveform for 15 seconds
See Also
- ↑ 1.0 1.1 Covelli HD, Nessan VJ, Tuttle WK (1983). "Oxygen derived variables in acute respiratory failure". Crit. Care Med. 11 (8): 646–9. PMID 6409506.
- ↑ 2.0 2.1 2.2 El-Khatib MF, Jamaleddine GW (2004). "A new oxygenation index for reflecting intrapulmonary shunting in patients undergoing open-heart surgery". Chest. 125 (2): 592–6. PMID 14769743.
- ↑ 3.0 3.1 Cane RD, Shapiro BA, Templin R, Walther K (1988). "Unreliability of oxygen tension-based indices in reflecting intrapulmonary shunting in critically ill patients". Crit. Care Med. 16 (12): 1243–5. PMID 3191742.
- ↑ Marini JJ, Ravenscraft SA (1992). "Mean airway pressure: physiologic determinants and clinical importance--Part 1: Physiologic determinants and measurements". Crit Care Med. 20 (10): 1461–72. PMID 1395670.
- ↑ Marini JJ, Ravenscraft SA (1992). "Mean airway pressure: physiologic determinants and clinical importance--Part 2: Clinical implications". Crit Care Med. 20 (11): 1604–16. PMID 1424706.
- ↑ McFarlane MJ, Imperiale TF (1994). "Use of the alveolar-arterial oxygen gradient in the diagnosis of pulmonary embolism". Am. J. Med. 96 (1): 57–62. PMID 8304364.
- ↑ Mukhtar A, Rady A, Hasanin A, Lotfy A, El Adawy A, Hussein A; et al. (2021). "Admission SpO2 and ROX index predict outcome in patients with COVID-19". Am J Emerg Med. 50: 106–110. doi:10.1016/j.ajem.2021.07.049. PMC 8313790 Check
|pmc=value (help). PMID 34332217 Check|pmid=value (help). - ↑ Maintain Adequate Central Venous Oxygen Saturation Institute for Healthcare Improvement
- ↑ 9.0 9.1 Grissom CK, Morris AH, Lanken PN, Ancukiewicz M, Orme JF, Schoenfeld DA; et al. (2009). "Association of physical examination with pulmonary artery catheter parameters in acute lung injury". Crit Care Med. 37 (10): 2720–6. PMID 19885995.
- ↑ Maintain Adequate Central Venous Oxygen Saturation Institute for Healthcare Improvement
- ↑ Kopterides P, Mavrou I, Kostadima E (2005). "Central or mixed venous oxygen saturation?". Chest. 128 (2): 1073–4, author reply 1074-5. doi:10.1378/chest.128.2.1073. PMID 16100219.
- ↑ Podbregar M, Mozina H (2007). "Skeletal muscle oxygen saturation does not estimate mixed venous oxygen saturation in patients with severe left heart failure and additional severe sepsis or septic shock". Crit Care. 11 (1): R6. doi:10.1186/cc5153. PMC 2147710. PMID 17227587.
- ↑ Leone M, Blidi S, Antonini F, Meyssignac B, Bordon S, Garcin F; et al. (2009). "Oxygen tissue saturation is lower in nonsurvivors than in survivors after early resuscitation of septic shock". Anesthesiology. 111 (2): 366–71. doi:10.1097/ALN.0b013e3181aae72d. PMID 19602965.
- ↑ Jones AE, Shapiro NI, Trzeciak S, Arnold RC, Claremont HA, Kline JA; et al. (2010). "Lactate clearance vs central venous oxygen saturation as goals of early sepsis therapy: a randomized clinical trial". JAMA. 303 (8): 739–46. doi:10.1001/jama.2010.158. PMID 20179283.
- ↑ Anonymous (2024), Capnography (English). Medical Subject Headings. U.S. National Library of Medicine.
- ↑ 16.0 16.1 Deitch K, Miner J, Chudnofsky CR, Dominici P, Latta D (2010). "Does end tidal CO2 monitoring during emergency department procedural sedation and analgesia with propofol decrease the incidence of hypoxic events? A randomized, controlled trial". Ann Emerg Med. 55 (3): 258–64. doi:10.1016/j.annemergmed.2009.07.030. PMID 19783324.