1. Oxygen Transport: A Clinical Review Bradley J. Phillips, M.D.
TRAUMA-ICU NURSING
EDUCATIONAL SERIES
Oxygen Transport:
A Clinical Review
Critical Care Medicine
Boston Medical Center
Boston University School of Medicine
Bradley J. Phillips, M.D.

2. “The First Concern” “the first concern in any life-threatening illness
is to maintain
an adequate supply of oxygen
to sustain oxidative metabolism”
[Marino 2nd ed.]

3. Context
The human adult has a vascular network that stretches over 60,000 miles
More than twice the circumference of the earth
8,000 liters of blood pumped per day
Principle of Continuity
Conservation of mass in a closed hydraulic system
“the volume flow of blood is and must be the same at all points throughout the circuit”

4. Flow Velocity & Cross-sectional Area

5. Respiratory Gas Transport
Respiratory function of blood
Dual system
Transport & delivery of oxygen TO the tissues
Transport & delivery of carbon dioxide FROM the tissues
Oxygen is the most abundant element on the surface of this planet…yet it is completely unavailable to the cells on the interior of the human system
the body, itself, acts as its own natural barrier…
Why ? (remember…oxygen-metabolites are toxic)

6. Oxygen Radicals The metabolism of oxygen occurs at the very end of the
electron transport pathway
i.e. oxidative phosphorylation within
the mitochondrial body

7. Antioxidant Therapy Selenium Glutathione N-acetylcysteine Vit. E
(glu. Peroxidase)
Glutathione
(acts via reduction)
N-acetylcysteine
(a glutahione analog)
Vit. E
(blocks lipid peroxidation)
Vit. C
(pro-oxidant to maintain iron as Fe(II)
Aminosteroids
(? lipid peroxidation)

8. the transport system for oxygen is separated into 4 components: taken together, these form the “oxygen transport variables”

9. The Oxygen Transport Variables
Oxygen Content [CaO2]
Oxygen Delivery [DO2]
Oxygen Uptake [VO2]
Extraction Ratio [ER]

10. the oxygen in the blood is either bound to hemoglobin
Oxygen Content (1)
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)

11. Oxygen Content (2) CaO 2 = (1.34 x Hb x SaO2) + (0.003 x PaO2)
amount carried by Hb amount dissolved in plasma
CaO2 = (1.34 x 14 x 0.98) + (0.003 x 100)
CaO2 = 18.6 ml/dl (ml/dl = vol %; 18.6 vol %)
* at 100 % Saturation, 1 g of Hb binds 1.34 ml of Oxygen !

12. Oxygen Content (3)
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

13. Hemoglobin vs. PaO2: CaO2 “the trifecta”
Arterial oxygenation is based on 3 (and ONLY 3) things: Hb
SaO2
PaO2
A 50% reduction in Hb leads to a direct 50% reduction in CaO2
A 50% reduction in PaO2 leads to a 20% reduction in CaO2

14. CaO2: why do we so often forget ?
PaO2 influences oxygen content only to the
extent that it influences
the saturation of hemoglobin
Hypoxemia (i.e. a decrease in PaO2) has a
relatively SMALL impact
on arterial oxygenation if the accompanying change
in SaO2 is small !

15. 35 yr old male s/p GSW to Chest
Oxygen Content (4)
35 yr old male s/p GSW to Chest
Pulse BP 164 / RR 26
Hb = 12
Hct = 36
ABG’s: pH / PaO / PaCO2 32 / 96 % Sat
Question: What is this
Patient’s Oxygen Content ?

16. 35 yr old male s/p GSW to Chest
Oxygen Content (5)
35 yr old male s/p GSW to Chest
Pulse BP 164 / RR 26
Hb = 12
Hct = 36
ABG’s: pH / PaO / PaCO2 32 / 96 % Sat
Oxygen Content:
CaO2 = (1.34 x Hb x SaO2)
CaO2 = ……..

17. Oxygen Delivery (1) DO2 = Q x CaO2
DO2: the Rate of Oxygen Transport in the Arterial Blood
* it is the product of Cardiac Output & Arterial Oxygen Content
DO2 = Q x CaO2
Cardiac Output, Q, can be “indexed” to body surface area
Normal C.I. : L/min-m2
Bu using a factor of 10, we can convert vol % to ml/min

18. Oxygen Delivery (2) DO2 = Q x CaO2 DO2 = 3 x (1.34 x Hb x SaO2) x 10
DO2 = x (1.34 x 14 x .98) x 10
DO2 = 551 ml/min
Normal Range (CO): 800 – 1000 ml/min
Normal Range (CI) : ml/min/m2

19. 35 yr old male s/p GSW to Chest
Oxygen Delivery (3)
35 yr old male s/p GSW to Chest
Pulse BP 164 / RR 26
H/H = 12/36
ABG’s: pH / PaO / PaCO2 32 / 96 % Sat
CO = CI = 2.1
Question: What is this
Patient’s Oxygen Delivery ?

20. 35 yr old male s/p GSW to Chest
Oxygen Delivery (4)
35 yr old male s/p GSW to Chest
Pulse BP 164 / RR 26
H/H = 12/36
ABG’s: pH / PaO / PaCO2 32 / 96 % Sat
CO = (CI = 2.1)
Oxygen Delivery:
DO2 = Q x CaO2 x 10
DO2 = ……

21. Oxygen Uptake (1) VO2 = Q x [(CaO2 - CvO2)] The Fick Equation:
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 Output
and
the Arteriovenous Difference in Oxygen Content
VO2 = Q x [(CaO2 - CvO2)]

22. Oxygen Uptake (2)

23. Oxygen Uptake (3) VO2 = Q x (CaO2 - CvO2)
The Fick Equation:
VO2 = Q x (CaO2 - CvO2)
VO2 = Q x [(1.34 x Hb) x (SaO2 - SvO2) x 10]
VO2 = 3 x [ (1.34 x 14) x ( ) x 10 ]
VO2 = 3 x [ 46 ]
VO2 = 140 ml/min/m2
Normal VO2: ml/min/m2

24. 35 yr old male s/p GSW to Chest
Oxygen Uptake (4)
35 yr old male s/p GSW to Chest
Pulse BP 164 / RR 26
Hb/Hct = 12/36
ABG’s: pH / PaO / PaCO2 32 / 96 % Sat
CO 4.8
SvO2 56 %
Question: What is this
Patient’s Oxygen Uptake ?

25. 35 yr old male s/p GSW to Chest
Oxygen Uptake (4)
35 yr old male s/p GSW to Chest
Pulse BP 164 / RR 26
Hb/Hct = 12/36
ABG’s: pH / PaO / PaCO2 32 / 96 % Sat
CO 4.8 (CI 2.1)
SvO2 56 %
Oxygen Uptake:
VO2 = Q x (CaO2 - CvO2)
VO2 = Q x [(1.34 x Hb) x (SaO2 - SvO2) x 10]
VO2 = …….

26. the fractional uptake of oxygen
Extraction Ratio (1)
the fractional uptake of oxygen
from the capillary bed
O2ER: derived as the Ratio of Oxygen Uptake to Oxygen Delivery
O2ER = VO2 / DO2 x 100
O2ER = 130 / 540 x Normal Extraction
O2ER = 24 % %

27. 35 yr old male s/p GSW to Chest
Extraction Ratio (2)
35 yr old male s/p GSW to Chest
Pulse BP 164 / RR 26
H/H= 36
ABG’s: pH / PaO / PaCO2 32 / 96 % Sat
C0 4.8
SvO2 71 %
Question: What is this
Patient’s Extraction Ratio ?

28. 35 yr old male s/p GSW to Chest
Extraction Ratio (3)
35 yr old male s/p GSW to Chest
Pulse BP 164 / RR 26
H/H= 36
ABG’s: pH / PaO / PaCO2 32 / 96 % Sat
C0 4.8
SvO2 71 %
Extraction Ratio:
O2ER = VO2 / DO2 x 100
O2ER = …..

29. Extraction Ratio (3) Questions: 1. ER = 16 %, what does this imply ?

30. Control of Oxygen Uptake
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

31. The Normal Response: O2ER (1)
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 13.4 x (SaO2 - SvO2)
Q = 3; VO2 = 3 x 14 x 13.4 x ( ) = 110 ml/min
Q = 1; VO2 = 1 x 14 x 13.4 x ( ) = 109 ml/min

32. The Normal Response: O2ER (2)
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 % !!
This 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 !

33. The DO2 - VO2 Curve (1)

34. The DO2 - VO2 Curve (2)
As O2 delivery decreases below normal, the ER increases proportionally to keep VO2 constant
When ER reaches its maximum level (50 – 60%), further decreases in DO2 are accompanied by proportional decreases in VO2
Critical DO2
The DO2 at which consumption becomes supply-dependent
The point at which energy production within the cell becomes oxygen-limited

35. The DO2 - VO2 Curve (3) Flat Portion of the Curve
VO2 Flow - Independent
O2 Extraction varies in response to Blood Flow (VO2 Constant)
Linear Portion of the Curve
VO2 Flow - Dependent
Indicates a defect in oxygen extraction from the microcirculation
Extraction is fixed and 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

36. The DO2 - VO2 Curve (2)

37. The critical DO2 in anesthesized patients is around 300 ml/min.
“In the ICU…”
The critical DO2 in anesthesized patients is around 300 ml/min.
However, in critically-ill patients, the Critical DO2 varies widely from 150 – 1000 ml/min…
[Leach et al. Dis Mon. 1994;30: ]

38. Mixed Venous Oxygen VO2 = Q x Hb x 13 x (SaO2 - SvO2)
By rearranging the Fick Equation, the determinants of Venous Oxygen are:
VO2 = Q x Hb x 13 x (SaO2 - SvO2)
SvO2 = SaO2 - (VO2/Q x Hb x 13)
* the most prominent factor in determining SvO2 is VO2/Q
Causes of a Low SvO2: Hypoxemia
Increased Metabolic Rate
Low Cardiac Output
Anemia

39. Remember: Mixed Venous
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

40. Oximetry Arterial Oxygen Saturation can be estimated but
Venous Oxygen Saturation
MUST be Measured !
Due to the shape of the Oxyhemoglobin Curve
The arterial Sat falls on the flat portion & can be safely estimated
The venous Sat ( %) falls on the Steep Portion and can vary significantly even with small errors in estimation !

41. OxyHb Curve (1) “Rule-of-Dennis-Betting” 50 % Sat…PO2 25
Mixed Ven. Sat 75…PO2 40

42. OxyHb Curve (2) “Right-shift: off-loading” Acidosis
Elevated temperature
Elevated CO2
Increased 2,3-DPG

43. Optimizing Oxygen Transport: The Steps
Filling Pressures
Cardiac Output
VO2
Serum Lactate

44. Oxygen Transport Variables
Parameter Normal Range
Delivery (DO2) ml/min
Uptake (VO2) ml/min
Extraction Ratio (ER) %
Mixed Venous PO mmHg
Mixed Venous SO %
** DO2 & VO2 can be indexed to body surface area

45. Oxygen Transport “it would be a most difficult task to explain”
Any Questions Yet ?

46. Oxygen Transport: Case 1 (1)
32 yr old male 6 hrs s/p GSW to the Abdomen
Hypotensive & “nearly coded” on the table…
Liver “shattered” & packed…
Multiple holes in the Small Bowel, Stomach, and Right Chest…
Packed and “whip-stitched closed” the fascia…
Now hypotensive and dropping her sats…
“what do you want to do, doc ?”

47. Oxygen Transport: Case 1 (2)
Remember the Steps:
Filling Pressures…
Cardiac Output…
VO2…
Serum Lactate…

48. Oxygen Transport: A Clinical Review Bradley J. Phillips, M.D.
Critical Care Medicine
Boston Medical Center
Boston University School of Medicine
Bradley J. Phillips, M.D.