Outline the determinants of oxygen delivery to the tissues. 

[Click here to toggle visibility of the answers]

College Answer

An opening statement such as oxygen delivery = cardiac output x oxygen content then allowed 
a more detailed description of the determinants of both oxygen content and cardiac output. It 
was expected candidates could detail the formula for Oxygen Content = Sat O2 % x 1.34 x Hb 
x 10 (depending on units) + PaO2 x .003, or x .03 (units) and accurately describe each 
element. Determinant of cardiac output completed the answer, often starting with CO = HR x 
SV while discussing the importance of preload, afterload and contractility.
Many candidates spent little time on cardiac output which cost valuable marks. Some 
candidates provided a lot of detail on how oxygen is managed within the lung, the majority of 
which was not required as part of the answer. Some candidates answered the question 
describing the oxygen cascade only which was not sufficient to score well.

Discussion

Total DO2 = Cardiac output (L/min) × Oxygen content (ml/L)

Cardiac output:

= HR × SV

Where SV is influenced by preload, afterload and contractility.

Oxygen content: 

= (sO2 × ceHb × BO) + (PaO2 × 0.003)

Where:

  • ceHb = the effective haemoglobin concentration
  • PaO2 = the partial pressure of oxygen in arterial gas
  • 0.003 = the content, in ml/L/mmHg, of dissolved oxygen in blood
  • BO the maximum amount of Hb-bound O2 per unit volume of blood (normally 1.39)
  • sO2 = oxygen saturation

References

References

Chapler, C. K., and S. M. Cain. "The physiologic reserve in oxygen carrying capacity: studies in experimental hemodilution." Canadian journal of physiology and pharmacology 64.1 (1986): 7-12.

Lifson, Nathan, et al. "The fate of utilized molecular oxygen and the source of the oxygen of respiratory carbon dioxide, studied with the aid of heavy oxygen."Journal of Biological Chemistry 180.2 (1949): 803-811.

Pittman, Roland N. Chapter 4 - Oxygen Transport; in "The Circulatory System and Oxygen Transport." (2011).

Shaw, Joshua J., et al. "Not Just Full of Hot Air: Hyperbaric Oxygen Therapy Increases Survival in Cases of Necrotizing Soft Tissue Infections." Surgical infections (2012).

Buckley, Nick A., et al. "Hyperbaric oxygen for carbon monoxide poisoning."Cochrane Database Syst Rev 4 (2011).

Stoekenbroek, R. M., et al. "Hyperbaric Oxygen for the Treatment of Diabetic Foot Ulcers: A Systematic Review." European Journal of Vascular and Endovascular Surgery 47.6 (2014): 647-655.

Eskes, Anne M., et al. "Hyperbaric oxygen therapy: solution for difficult to heal acute wounds? Systematic review." World journal of surgery 35.3 (2011): 535-542.

Tibbles, Patrick M., and John S. Edelsberg. "Hyperbaric-oxygen therapy." New England Journal of Medicine 334.25 (1996): 1642-1648.

Thom, Stephen R. "Hyperbaric oxygen–its mechanisms and efficacy." Plastic and reconstructive surgery 127.Suppl 1 (2011): 131S.

Gehring, Hartmut, et al. "Hemoximetry as the "gold standard"? Error assessment based on differences among identical blood gas analyzer devices of five manufacturers." Anesthesia & Analgesia 105.6 (2007): S24-S30.

Toffaletti, John, and Willem G. Zijlstra. "Misconceptions in reporting oxygen saturation." Anesthesia & Analgesia 105.6 (2007): S5-S9.

Braunitzer, G. "Molekulare struktur der hamoglobine." Nova Acta Acad Caesar Leopold Card 26 (1963): 113-125. - unfortunately, this publictaion is not available anywhere in an electronic form.

Gregory, I. C. "The oxygen and carbon monoxide capacities of foetal and adult blood." The Journal of physiology 236.3 (1974): 625-634.