1. Acute respiratory failure

2. Topics
 Definition of acute respiratory failure
 Basic respiratory physiology
 Pathophysiology
 Respiratory monitoring
 Treatment

3. Definitions
 acute respiratory failure occurs when:
– pulmonary system is no longer able to meet
the metabolic demands of the body
 hypoxaemic respiratory failure:
– PaO2 60 mmHg when breathing room air
 hypercapnic respiratory failure:
– PaCO2 50 mmHg

4. Basic respiratory physiology

5. CO2 O2

6. Pulmonary Ventilation and pressures

7. Oxygen in
• Depends on
– PAO2
– Diffusing capacity
– Perfusion
– Ventilation-perfusion matching

8. Oxygen
Carbon
dioxide
Water
vapour
Nitrogen
Alveolar pressure PAO2 PACO2 PAH2O PAN2

9. Alveolar Oxygen tension
PAO2= (PB-PH2O)FIO2 – CO2/0.8
PAO2= (760-47)0.21 -40/0.8=100

10. How much oxygen is in the
blood
 PaO2
 SaO2
 Oxygen content (CaO2)

11. How much oxygen is in the blood
PaO2
• The amount of dissolved oxygen in the plasma phase --
and hence the PaO2 -- is determined by alveolar PO2
and lung architecture only
SaO2
• The percentage of hemoglobin molecule bounded with
oxygen.
Oxygen Content CaO2
• CaO2 = Hb (gm/dl) x 1.34 ml O2/gm Hb x SaO2 + PaO2 x
(.003 ml O2/mm Hg/dl)

12. Oxygen dessociation Curve

13. Oxygen in
• Depends on
– PAO2
• FIO2
• PACO2
• Ventilation
• Alveolar pressure
– Ventilation-perfusion matching
– Perfusion
– Diffusing capacity

14. Carbon dioxide out
 Largely dependent on alveolar ventilation
Alveolar ventilatio n RR x (V - V )T D
 Anatomical dead space constant but
physiological dead space depends on
ventilation-perfusion matching

15. Carbon dioxide out
• Patient Vt f Ve Description
–A (400) (20) = 8.0 L/min slow and deep
–B (200) (40) = 8.o L/min fast/shallow
• Patient Va-Vd f Va Description
–A (400-150)(20) = 5.0 L/min slow and deep
–B (200-150)(40) = 2.0 L/min fast/shallow

16. Acute Lung Compromise
Increase work of breathing
Muscle fatigue
Shallow breathing followed
by increase in RR
Increase PaCO2

17. Carbon dioxide out
 Respiratory rate
 Tidal volume
 Ventilation-perfusion matching

18. Ventilation-perfusion matching

19. Dead space

20. Shunt

21. Pathophysiology

22. Pathophysiology
• Low inspired Po2
• Hypoventilation
• Ventilation-perfusion mismatch
– Shunting
– Dead space ventilation
• Diffusion abnormality

23. PAO2=105 mmHg
PACO2=37 mmHg
75% 100%

24. Pathophysiology
• Low inspired oxygen concentration
• Hypoventilation
• Shunting
• Dead space ventilation
• Diffusion abnormality

25. F I O2
Ventilation
without
perfusion Hypoventilation
(deadspace
ventilation)
Diffusion
abnormality
Normal
Perfusion
without
ventilation
(shunting)

26. Brainstem
Spinal cord
Airway Nerve root
Lung Nerve
Pleura
Neuromuscular
Chest wall junction
Respiratory
muscle
Sites at which disease may cause ventilatory disturbance

27. Causes of respiratory failure
 Respiratory Center in Brain
Brain

28. Causes of respiratory failure
 Respiratory Center in Brain
Brain
 Neuromuscular Connections
(peripheral nervous system)
Nerves

29. Causes of respiratory failure
 Respiratory Center in Brain
Brain
 Neuromuscular Connections
 Thoracic Bellows
(intact rib cage and chest wall musculature)
Nerves
Bellows

30. Causes of respiratory failure
 Respiratory Center in Brain
Brain
 Neuromuscular Connections
 Thoracic Bellows
 Airways (upper & lower)
Nerves
Bellows
Airways

31. Causes of respiratory failure
 Respiratory Center in Brain Brain
 Neuromuscular Connections
 Thoracic Bellows
 Airways (upper & lower) Nerves
 Alveoli
All the links are disrupted ! Bellows
Airways
Alveoli

32. Shunting
• Intra-pulmonary
– Pneumonia
– Pulmonary oedema
– Atelectasis
– Collapse
– Pulmonary haemorrhage or contusion
• Intra-cardiac
– Any cause of right to left shunt
• eg Fallot’s, Eisenmenger,
• Pulmonary hypertension with patent foramen ovale

33. Respiratory monitoring

34. Clinical
• Respiratory compensation
• Sympathetic stimulation
• Tissue hypoxia
• Haemoglobin desaturation

35. Clinical
• Respiratory compensation
– Tachypnoea
– Accessory muscles
– Recesssion
– Nasal flaring
• Sympathetic stimulation
• Tissue hypoxia
• Haemoglobin desaturation

36. Clinical
• Respiratory compensation
• Sympathetic stimulation
– HR
– BP (early)
– sweating
• Tissue hypoxia
• Haemoglobin desaturation

37. Clinical
• Respiratory compensation
• Sympathetic stimulation
• Tissue hypoxia
– Altered mental state
– HR and BP (late)
• Haemoglobin desaturation

38. Summary
• worry if
• RR > 30/min (or < 8/min)
• unable to speak 1/2 sentence without pausing
• agitated, confused or comatose
• cyanosed or SpO2 < 90%
• deteriorating despite therapy
• remember
• normal SpO2 does not mean severe
ventilatory problems are not present

39. Treatment

40. Treatment
• Treat the cause
• Supportive treatment
– Oxygen therapy
– CPAP
– Mechanical ventilation

41. Oxygen therapy
• Fixed performance
devices
• Variable performance
devices

42. Variable performance device
30
100% O2
Flow
6 l/min O2
6
0
Time

43. Variable performance device
30 24 l/min air
37% O2
Flow
6 l/min O2
6
0
Time

44. Fixed performance device
Venturi mask
30
60% O2 60% O2 30 l/min
Flow
15 l/min air
100% O2 15 l/min
0
Time

45. Other devices
Reservoir face mask •
Bag valve resuscitator •

46. CPAP
• reduces shunt by recruiting partially
collapsed alveoli

47. Mechanical ventilation
• Decision to ventilate
– Complex
– Multifactorial
– No simple rules

48. Ventilate?
• Severity of respiratory failure
• Cardiopulmonary reserve
• Adequacy of compensation
– Ventilatory requirement
• Expected speed of response
– Underlying disease
– Treatment already given
• Risks of mechanical ventilation

49. Ventilate?
• 43 year old male
• Community acquired pneumonia
• Day 1 of antibiotics
• PaO2 60 mmHg, PaCO2 30 mmHg, pH
7.15 on 15 l/min via reservoir facemask
• Respiratory rate 35/min
• Agitated
No Yes

50. Yes
• 43 year old male
• Community acquired pneumonia
• Day 1 of antibiotics
• PaO2 60 mmHg, PaCO2 30 mmHg, pH
7.15 on 15 l/min via reservoir facemask
• Respiratory rate 35/min
• Agitated

51. Yes
• 43 year old male
• Community acquired pneumonia
• Day 1 of antibiotics
• PaO2 60 mmHg, PaCO2 30 mmHg, pH
7.15 on 15 l/min via reservoir facemask
• Respiratory rate 35/min
• Agitated

52. Yes
• 43 year old male
• Community acquired pneumonia
• Day 1 of antibiotics
• PaO2 60 mmHg, PaCO2 30 mmHg, pH
7.15 on 15 l/min via reservoir facemask
• Respiratory rate 35/min
• Agitated

53. Yes
• 43 year old male
• Community acquired pneumonia
• Day 1 of antibiotics
• PaO2 60 mmHg, PaCO2 30 mmHg, pH 7.15
on 15 l/min via reservoir facemask
• Respiratory rate 35/min
• Agitated

54. Yes
• 43 year old male
• Community acquired pneumonia
• Day 1 of antibiotics
• PaO2 60 mmHg, PaCO2 30 mmHg, pH
7.15 on 15 l/min via reservoir facemask
• Respiratory rate 35/min
• Agitated

55. Ventilate?
• 24 year old woman
• Presents to ER with acute asthma
– SOB for 2 days
• Salbutamol inhaler, no steroids
• PFR 60 L/min, HR 105/min
• pH 7.25 PaCO2 51 mmHg, PaO2 315 mmHg on
FiO2 0.6
• RR 35/min
• Alert
No Yes

56. No
• 24 year old woman
• Presents to A&E with acute asthma
– SOB for 2 days
• Salbutamol inhaler, no steroids
• PFR 60 L/min, HR 105/min
• pH 7.25 PaCO2 51 mmHg, PaO2 315 mmHg on
FiO2 0.6
• RR 35/min
• Alert

57. No
• 24 year old woman
• Presents to A&E with acute asthma
– SOB for 2 days
• Salbutamol inhaler, no steroids
• PFR 60 L/min, HR 105/min
• pH 7.25 PaCO2 51 mmHg, PaO2 315 mmHg on
FiO2 0.6
• RR 35/min
• Alert

58. No
• 24 year old woman
• Presents to A&E with acute asthma
– SOB for 2 days
• Salbutamol inhaler, no steroids
• PFR 60 L/min, HR 105/min
• pH 7.25 PaCO2 51 mmHg, PaO2 315 mmHg on
FiO2 0.6
• RR 35/min
• Alert

59. No
• 24 year old woman
• Presents to A&E with acute asthma
– SOB for 2 days
• Salbutamol inhaler, no steroids
• PFR 60 L/min, HR 105/min
• pH 7.25 PaCO2 51 mmHg, PaO2 315 mmHg on
FiO2 0.6
• RR 35/min
• Alert

60. No
• 24 year old woman
• Presents to A&E with acute asthma
– SOB for 2 days
• Salbutamol inhaler, no steroids
• PFR 60 L/min, HR 105/min
• pH 7.25 PaCO2 51 mmHg, PaO2 315 mmHg on
FiO2 0.6
• RR 35/min
• Alert

61. No
• 24 year old woman
• Presents to A&E with acute asthma
– SOB for 2 days
• Salbutamol inhaler, no steroids
• PFR 60 L/min, HR 105/min
• pH 7.25 PaCO2 51 mmHg, PaO2 315 mmHg on
FiO2 0.6
• RR 35/min
• Alert

62. Thank you