1. Oxygen, carbon dioxide, breathing and hypoxia
J Albrett
House surgeon teaching 2016

2. Aims
To develop an appreciation of problems with oxygenation and ventilation.
A stepwise approach to diagnosis and treatment of respiratory failure.
To recognise severe abnormalities and when to call for help.

3. Firstly the basic physiology
Oxygenation is primarily determined by FiO2 and airway pressure
Failure to oxygenate = Type 1 respiratory failure
PaO2 < 60 mmHg when
FiO2 21% (air) & Barometric pressure 760mmHg (sea level)
No intracardiac shunt
Failure to ventilate = Type 2 respiratory failure
PaCO2 > 50 mmHg unless primary metabolic alkalosis

4. Oxygen

5. Oxygen saturation

6. ATSP 65 year old lady with pneumonia
On 6L/min oxygen via hudson mask.
What is her FiO2?
What is the airway pressure?

7. Delivery of oxygen Class Device Oxygen flow (L/min) Approx FiO2
Comments
Variable delivery
Nasal prongs 2 4 6
28%
35%
45%
Stable patients only
Semi-rigid
(Hudson mask) 5 8 10 12
50%
55%
60%
65%
Low cost
Frequently used
Not accurate at controlling FiO2
Reservoir plastic mask
6-15
FiO2 21% + 4% per L/min
6L/min
15L/min
81%

8. Delivery of oxygen Class Device Oxygen flow (L/min) Approx FiO2
Comments
Fixed delivery
Venturi mask
2-8
24-50%
Reliable fixed delivery but more expensive
CPAP/BiPAP
High flow or turbine flow > peak inspiratory flow
Reasonably accurate control of FiO2

9. You decide to put her on an airvo system?
What is the FiO2 possible?
How accurate?
What is the airway pressure?

10. AIRVO system on wards

11. OPTIFLO system in HDU/ICU

12. Airway pressure in optiflo

13. You are asked to review a 22 year old man post appendix surgery 12 hours ago.
He is hypoxic!
What is hypoxia?

14. Hypoxia

15. Introduction
Definition
Mechanisms
Measurement
Treatment

16. Hypoxia Inadequate oxygen supply for tissues Local General
Sat 95%, fall >5% with exercise. In context

17. Outline a system for assessing and treating hypoxia.

18. Mechanisms I start at the wall...
Inadequate supply, check pressure, flow, connection, tubing
Airway
Patency
Suction
ETT blockage, valves etc
Coma
Aspiration

20. Blocked airway

21. Jaw thrust

22. Pharyngeal airway

23. OPA

24. Mechanisms Air to alveolus Bronchospasm Foreign body Hypoventilation
Nitrous oxide

25. Asthma Can get air in but can’t get it out

26. Mechanisms Alveolar/capillary Pulmonary oedema Fibrosis Consolidation
Atelectasis

27. Pulmonary capillaries
Pulmonary embolus
Shunt (Intra-cardiac, V/Q mismatch)

28. Tissues Compartment syndrome / limb ischaemia Dysoxia Sepsis
Cyanide toxicity

29. Measurement of oxygen
Clinical exam
Pulse oximetry
Blood gas

30. Clinical exam Cyanosis Associated with very low oxygen saturations
Need 5g/dL ie oxygen saturation of 67% !!

31. Pulse oximetry Rapid Easy to use Non-invasive But not always reliable
We need to know and anticipate sources of error

32. Pulse oximetry Looks at the pulsatile component of tissue
Measures change in quantity of absorbed light.
Assumes any change must be due to arterial blood (which pulses)

35. Non-patient error Mal-position / off
Interference from external light sources
Motion artefact

36. Patient sources of error
Hypoperfusion
Hypothermia
Venous pulsation
Nail polish
Methylene blue
Abnormal haemoglobin
Carboxyhaemaglobin (false high)
Methaemoglobin (false low)

38. Arterial blood gas Invasive (made easier with arterial line)
Measures pO2 therefore can indicate too much oxygen
Measures pCO2 so can indicate adequacy of ventilation
Measures Hb, HbO2, MetHb, Hb-CO

39. Oxygenation Vs Ventilation
Ventilation: Process of inspiration and expiration exchanging gas from the lungs with the environment
Oxygenation: Process on increasing oxygen to a tissue

40. Time critical processes
Consider: The pulse oximeter takes an average of the last six pulsations.
Ventilation can fall but increasing inspired oxygen results in no change to measured saturation.
With preoxygenation we can maintain oxygen saturation for many minutes with no ventilation

41. FRC

42. FRC 70kg person FRC = 30mLx70kg FRC = 2.1 L With pre-oxygenation
80% oxygen in lungs = 1.6L oxygen
If oxygen consumption is 200mL/minute MEANS 7 minutes before patient desaturates.

43. Special patients Emphysema Obesity Sepsis
Larger lung volumes therefore more time but slower to achieve preoxygenation
Obesity
Smaller lung volumes therefore less time
Sepsis
Higher oxygen consumption therefore less time

44. Ventilation Driven by CO2 Simple measures Sit the patient up
Check LOC +/- jaw thrust
Apply oxygen

45. NIV CPAP, BiPAP Indications COPD with acute CO2 retention
Heart failure
OSA
?Hypoxia (not really)

47. NIV Higher airway pressures improves oxygenation
It makes sense to apply BPAP in order to increase oxygenation
BUT patient still unstable, may only delay respiratory arrest
Patient effectively sicker at point of arrest
Better to involve expert team. Never initiate on ward.

48. Compression only CPR As good as/maybe better than conventional CPR
Layperson
Sole rescuer
NOT
Children
Arrest due to hypoxia

49. DRSABC D Dangers? R Responsive? S Send for help A Open Airway
B Normal Breathing?
C Start CPR
30 compressions : 2 breaths
if unwilling / unable to perform rescue breaths continue chest compressions C
December 2010