Abstract

This is a brief review on Oxygen Transport.

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 Oxygen Content

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 !

• Note that the PaO2 contributes little to the Oxygen Content !

• Despite it's popularity, the PaO2 is NOT an important measure of arterial oxygenation !

• The SaO2 is the more important blood gas variable for assessing the oxygenation of arterial blood!

The PaO₂ should be reserved for evaluating the efficiency of pulmonary gas exchange

Example # 1: 35 yr old male s/p GSW to Chest

Pulse 126 - BP 164 / 72 - RR 26 Hb = 12 Hct = 36 ABG's: pH 7.38 / PaO₂ 100 / PaCO₂ 32 / 96 % Sat

Question # 1: What is this Patient's Oxygen Content ?

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

DO₂ = Q x CaO₂ DO₂ = 3 x (1.3 x Hb x SaO₂) x 10 DO₂ = 3 x (1.3 x 14 x .98) x 10 DO₂ = 540 ml/min-m²

Normal Range: 520 - 720 ml/min-m²

Example # 2: 35 yr old male s/p GSW to Chest

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

Question # 2: What is this Patient's Oxygen Delivery ?

Oxygen Consumption

Oxygen uptake is the final step in the oxygen transport pathway and it represents the oxygen supply for tissue metabolism

The Fick Equation: Oxygen Uptake is the Product of Cardiac Ouput and the Arteriovenous Difference in Oxygen Content

VO₂ = Q x [(CaO₂ - CvO₂)]

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

Normal VO2: 110 - 160 ml/min-m²

Example # 3: 35 yr old male s/p GSW to Chest

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 # 3: What is this Patient's Oxygen Consumption ?

Extraction Ratio

ER = the fractional uptake of oxygen from the capillary bed O₂ER: derived as the Ratio of Oxygen Uptake to Oxygen Delivery

Questions:

ER = 18 %, what does this imply ? ER = 40 %, what does this imply ?

Example # 4: 35 yr old male s/p GSW to Chest

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 Extraction Ratio to match changes in oxygen delivery

The ability to adjust O₂ Extraction can be impaired in serious illness

The Normal Response to a Decrease in Blood Flow is an Increase in O₂ Extraction sufficient enough to keep VO₂ in the normal range

VO₂ = Q x Hb x 13 x (SaO2 - SvO2) Q = 3; VO₂ = 3 x 14 x 13 x (.97 - .73) = 110 ml/min-m² Q = 1; VO₂ = 1 x 14 x 13 x (.97 - .37) = 109 ml/min-m²

• The Drop in Cardiac Index is BALANCED by an Increased (SaO2 - SvO2) Difference and VO2 remains Unchanged

• Note the drop in SvO2 from 97 % to 37 % !!

• The Association between SvO2 & O2ER is the Basis for SvO2 Monitoring

• The Ability to Adjust Extraction is a feature of all vascular beds except the Coronary Circulation & the Diaphragm !

The DO₂-VO₂ Curve

• Flat Portion of the Curve

  • VO2 Flow – Independent

  • O2 Extraction varies in response to Blood Flow, keeping VO2 Constant

• Linear Portion of the Curve

• • VO2 Flow - Dependent

• Indicates a defect in oxygen extraction from the microcirculation

• Extraction is fixed and the VO2 becomes directly dependent on Delivery

• • Critical Level of Oxygen Delivery

  • • The Threshold DO2 needed for Adequate Tissue Oxygenation If DO2 falls below this level, oxygen supply will be sub-normal

Another Point: Oximetry

Arterial Oxygen Saturation can be estimated but Venous Oxygen Saturation MUST be Measured !

* Remember the shape of the Oxyhemoglobin Curve * The SaO₂ falls on the flat portion & can be safely estimated, while the Venous % Sat (68 - 77 % falls on the Steep Portion and can vary significantly even with small errors in estimation !

In Critically-ill patients, augmenting the extraction ratio (in response to a change in oxygen delivery) may not be possible! In these patients, the Venous Oxygen Levels may change little in response to changes in Cardiac Output ! Thus, the Relationship between CO(Q) and Mixed Venous Oxygen must be determined before using SvO₂ or PvO₂ to monitor changes in DO₂ or VO₂

The Transport Variables:

** DO₂ & VO₂ are indexed to body surface area