1. Analysis and Monitoring of Gas Exchange
RESD 60
Cardiopulmonary Assessment
Egan Chapter 11

2. Respiration
Process of moving oxygen to tissues for aerobic metabolism & removal of carbon dioxide.
Involves gas exchange at lungs & tissues
O2 from atmosphere to tissues for aerobic metabolism
Removal of CO2 from tissues to atmosphere

3. Diffusion Gas moves across system by simple diffusion
Oxygen moves from PO2 of 159 mm Hg in atmosphere to intracellular PO2 of ~5 mm Hg
CO2 gradient is reverse from intracellular CO2 ~60 mm Hg to atmosphere where it = 1 mm Hg

4. Alveolar oxygen tensions (PAO2)
PIO2 is primary determinant
In lungs, it is diluted by water vapor & CO2
Alveolar air equation accounts for all these factors
PAO2 = FIO2  (PB – 47) – (PaCO2/0.8)

5. Alveolar Oxygen Tension (PA02)
PAO2 = FIO2  (PB – 47) – (PACO2/0.8)
Dalton’s law of partial pressures accounts for first part of formula; second part relates to rate at which CO2 enters lung compared to oxygen exiting
Ratio is normally 0.8.
If FIO2 > 0.60, (PACO2/0.8) can be dropped from equation

6. Diffuison Diffusion occurs along pressure gradients
Barriers to diffusion
A/C membrane has 3 main barriers:
Alveolar epithelium
Interstitial space & its structures
Capillary endothelium - RBC membrane

7. Fick’s Law
Fick’s law: The greater the surface area, diffusion constant, & pressure gradient, the more diffusion will occur.
What factors can decrease each of these?

8. Time limits to diffusion:
Pulmonary blood is normally exposed to alveolar gas for 0.75 second, during exercise may fall 0.25 second
Normally equilibration occurs in 0.25 second
With diffusion limitation or blood exposure time of less then 0.25 seconds, there may be inadequate time for equilibration

10. Shunts
PaO2 normally 5–10 mm Hg less than PAO2 due to presence of anatomic shunts, Portion of cardiac output that returns to left heart without being oxygenated by exposure to ventilated alveoli
Two right-to-left anatomic shunts exist:
Bronchial venous drainage
Thebesian venous drainage
These drain poorly oxygenated blood into arterial circulation lowering CaO2

11. V/Q ratio & regional differences
Ideal ratio is 1, where V/Q is in perfect balance
In reality lungs don’t function at ideal level
High V/Q ratio at apices >1 V/Q (~3.3)
↑PAO2 (132 mm Hg), ↓PACO2 (32 mm Hg)
Low V/Q ratio at bases <1.0 (~0.66)
Blood flow is ~20 times higher at bases
Ventilation is greater at bases but not 20
↓PAO2 (89 mm Hg), ↑PACO2 (42 mm Hg)
See Table 11-1

12. Oxygen Transport
Oxygen transport is the mechanism by which oxygen is carried from the lungs to the capillary bed.
How oxygen is used by the tissues is oxygen consumption.

13. Factors that Determine 02 Transport
Oxygen Content
Cardiac Output – CO or Qt
Distribution of cardiac output
Oxyhemoglobin dissociation curve

14. Oxygen Content – Ca02 Transported in 2 forms
Physically dissolved in plasma
Gaseous oxygen enters blood & dissolves.
Henry’s law allows calculation of amount dissolved
Dissolved O2 (ml/dl) = PO2  0.003
Chemically bound to hemoglobin (Hb)
Each gram of Hb can bind 1.34 ml of oxygen.
[Hb g]  1.34 ml O2 provides capacity.
70 times more O2 transported bound than dissolved

15. Hemoglobin Saturation
Saturation is % of Hb that is carrying oxygen compared to total Hb
SaO2 = [HbO2/total Hb]  100
Normal SaO2 is 95% to 100%

16. Total oxygen content of blood
Combination of dissolved & bound to Hb
CaO2 = (0.003  PaO2) + (Hb  1.34  SaO2)
Normal is 1620 mL/dL
Normal arteriovenous difference (~5 mL/dL)
C(a  v)O2

17. Abnormalities of Gas Exchange & Transport
Impaired oxygen delivery (DO2)
DO2 = CaO2  Qt
When DO2 is inadequate, tissue hypoxia ensues
What can cause impaired oxygen delivery?

18. Impaired Oxygen Delivery
Decreased Qt – (CO)
What can decrease CO?
How can you assess this? What are the signs and symptoms?

19. Impaired Oxygen Delivery
Decrease in Ca02
What can decrease Ca02?
How do you assess?

20. Hypoxemia Defined as abnormally low PaO2
Most common cause is V/Q mismatch
Other causes: hypoventilation, diffusion defect, shunting, & low PIO2 (altitude)

21. Physiologic shunt Where perfusion exceeds ventilation, includes:
Capillary or absolute anatomic shunts
Relative shunts seen in disease states diminish pulmonary ventilation
Relative shunts can be caused by:
COPD
Restrictive disorders
Any condition resulting in hypoventilation

22. Shunt equation Quantifies portion of blood included in V/Q mismatch
Usually expressed as % of total cardiac output
What is the shunt equation?

23. Deadspace Ventilation
Ventilation that doesn’t participate in gas exchange
Waste of energy to move gas into lungs
Two types: alveolar & anatomic

24. Alveolar Deadspace: Disorders leading to alveolar deadspace :
Ventilation that enters into alveoli without any, or without adequate perfusion
Disorders leading to alveolar deadspace :
Pulmonary emboli
Partial obstruction of the pulmonary vasculature
Destroyed pulmonary vasculature (as can occur in COPD)
Reduced cardiac output

25. Anatomic Deadspace
Ventilation that never reaches alveoli for gas exchange
Normal individuals have fixed anatomic deadspace
Becomes problematic in conditions where tidal volumes drop significantly
Significant % of inspired gas remains in anatomic deadspace

26. Increased Deadspace
In face of increased deadspace, normal ventilation must increase to achieve homeostasis
Additional ventilation comes at cost:
Increase in WOB
Consumes additional oxygen
Further adding to burden of external ventilation

27. Quick Quiz
Oxygen delivery to the tissues (DO2) may be substantially impaired due to all of the following, except:
Low Hb levels (anemia)
V/Q matching
abnormal cardiac output
presence of Carboxyhemoglobin (COHb)