Quick Review: Oxygen Transport
T Fujii, B Phillips
Citation
T Fujii, B Phillips. Quick Review: Oxygen Transport. The Internet Journal of Academic Physician Assistants. 2002 Volume 3 Number 1.
Abstract
This is a brief review on Oxygen Transport.
“The first concern in any life-threatening illness is to maintain an adequate supply of oxygen to sustain oxidative metabolism”
...Marino
Oxygen Transport
The Oxygen Transport Variables:
Oxygen Content [CaO2] Oxygen Delivery [DO2] Oxygen Consumption [VO2] Extraction Ratio [ER]
Oxygen Content
The oxygen in the blood is either bound to hemoglobin or dissolved in plasma
The sum of these two fractions is called the
CaO2 = the Content of Oxygen in Arterial Blood Hb = Hemoglobin (14 g/dl) SaO2 = Arterial Saturation (98%) PaO2 = Arterial PO2 (100 mmHg)
CaO2 = (1.3 x 14 x 0.98) + (0.003 x 100) CaO2 = 18.1 ml/dl (ml/dl = vol %; 18.1 vol %)
* at 100% Saturation, 1 g of Hb binds 1.3 ml of Oxygen !
* at 100% Saturation, 0.003 ml/mmHg of Oxygen is Dissolved in Plasma !
Pulse 126 - BP 164 / 72 - RR 26 Hb = 12 Hct = 36 ABG's: pH 7.38 / PaO2 100 / PaCO2 32 / 96 % Sat
Oxygen Delivery
DO2: the Rate of Oxygen Tranport in the Arterial Blood * it is the product of Cardiac Output & Arterial Oxygen Content
DO2 = Q x CaO2
Cardiac Ouput (Q) can be “indexed” to body surface area Normal C.I. : 2.5 - 3.5 L/min-m2 By using a factor of 10, we can convert vol % to ml/s
Pulse 126 - BP 164 / 72 - RR 26
Hb = 12 / Hct = 36 ABG's: pH 7.38 / PaO2 100 / PaCO2 32 / 96 % Sat C.I. = 2.86
Oxygen uptake is the final step in the oxygen transport pathway and it represents the oxygen supply for tissue metabolism
VO2 = Q x (CaO2 - CvO2) VO2 = Q x [ (1.3 x Hb) x (SaO2 - SvO2) x 10 ] VO2 = 3 x [ (1.3 x 14) x (.98 - .73) x 10 ] VO2 = 3 x [ 46 ] VO2 = 138 ml/min-m2
Pulse 126 - BP 164 / 72 - RR 26 Hb = 12 / Hct = 36 ABG's: pH 7.38 / PaO2 100 / PaCO2 32 / 96 % Sat C.I. = 2.86 SvO2 71 %
Extraction Ratio
ER = the fractional uptake of oxygen from the capillary bed O2ER: derived as the Ratio of Oxygen Uptake to Oxygen Delivery
ER = 18 %, what does this imply ?
ER = 40 %, what does this imply ?
{image:6}
Pulse 126 - BP 164 / 72 - RR 26 Hb = 12 / Hct = 36 ABG's: pH 7.38 / PaO2 100 / PaCO2 32 / 96 % Sat C.I. = 2.86 SvO2 71 %
Question # 4: What is this Patient's Extraction Ratio ?
The uptake of oxygen from the microcirculation is a set point that is maintained by adjusting the
The Normal Response to a
VO2 = Q x Hb x 13 x (SaO2 - SvO2)
Q = 3; VO2 = 3 x 14 x 13 x
The DO-VO Curve
{image:7}
Mixed Venous Oxygen
By rearranging the Fick Equation, the determinants of Venous Oxygen are:
VO2 = Q x Hb x 13 x (SaO2 - SvO2)
* the most prominent factor in determining SvO2 is
Hypoxemia
Increased Metabolic Rate
Low Cardiac Output
Anemia
Another Point: Oximetry
Arterial Oxygen Saturation can be estimated but Venous Oxygen Saturation MUST be Measured !
In Critically-ill patients, augmenting the extraction ratio (in response to a change in oxygen delivery)
{image:8}
** DO2 & VO2 are indexed to body surface area