assessing the need for mechanical ventilation
TRANSCRIPT
Basic respiratory physiology
O2CO2
Oxygen in
• Depends on– PAO2– Diffusing capacity– Perfusion– Ventilation-perfusion matching
Carbon dioxide
Water vapour
Oxygen
Nitrogen
2A2A2A2A NPOHPCOPOPpressure Alveolar
Oxygen in
• Depends on– PAO2• FIO2• PACO2• Alveolar pressure• Ventilation
– Diffusing capacity– Perfusion– Ventilation-perfusion matching
Carbon dioxide out
• Largely dependent on alveolar ventilation
• Anatomical deadspace constant but physiological deadspace depends on ventilation-perfusion matching
)V-(V xRR nventilatio Alveolar DT
Carbon dioxide out
• Respiratory rate• Tidal volume• Ventilation-perfusion matching
Pathophysiology
FIO2
Ventilation without perfusion
(deadspace ventilation)
Diffusion abnormality
Perfusion without
ventilation (shunting)
Hypoventilation
Normal
PAO2=14.74 kPa PACO2=5 kPa
75% 100%
FIO2
Ventilation without perfusion
(deadspace ventilation)
Diffusion abnormality
Perfusion without
ventilation (shunting)
Hypoventilation
Normal
PAO2=6 kPa PACO2=5.5 kPa
75% 75%
75% 75%
100% 75%
87.5%
75% 75%
100% 75%
90%
Shunting
• Intra-cardiac– Any cause of right to left shunt• eg Fallot’s, Eisenmenger
• Intra-pulmonary– Pneumonia– Pulmonary oedema– Atelectasis– Collapse– Pulmonary haemorrhage or contusion
FIO2
Ventilation without perfusion
(deadspace ventilation)
Diffusion abnormality
Perfusion without
ventilation (shunting)
Hypoventilation
Normal
PAO2=14.74 kPa PACO2=5 kPa
FIO2
Ventilation without perfusion
(deadspace ventilation)
Diffusion abnormality
Perfusion without
ventilation (shunting)
Hypoventilation
Normal
PAO2=14.74 kPa PACO2=5 kPa
75%92%
FIO2
Ventilation without perfusion
(deadspace ventilation)
Diffusion abnormality
Perfusion without
ventilation (shunting)
Hypoventilation
Normal
PAO2=9.74 kPa PACO2=10 kPa
75%92%
Brainstem
Spinal cord
Nerve rootAirway
Nerve
Neuromuscular junction
Respiratory muscle
Lung
Pleura
Chest wall
Sites at which disease may cause ventilatory disturbance
Distribution of Normal Ventilation-Perfusion Ratios
1 100.10
Oxygen-carbon dioxide diagram
The Effect of Increasing Ventilation-Perfusion Inequality on Arterial Po2
and Pco2
Ventilation-Perfusion InequalityAcute Exacerbation of COPD
0 .01 0.1 1 10 100
Ventilation-Perfusion InequalityAsthma
0 .01 0.1 1 10 100
Ventilation-Perfusion InequalityPulmonary Embolism
0 .01 0.1 1 10 100
Shunting ProcessARDS
0 .01 1 10 100
The effect of changing the inspired oxygen concentration on arterial Po2 for lung’s
shunts of 10 to 50%
Demands
Capacity
Alveolar ventilation
Mechanical Ventilation: Indications
• Ventilation abnormalities– Respiratory muscle dysfunction• Respiratory muscle fatigue• Chest wall abnormalities• Neuromuscular disease
– Decreased ventilatory drive– Increased airway resistance and/or
obstruction
• Oxygenation abnormalities– Refractory hypoxemia– Need for positive end-expiratory pressure
(PEEP)– Excessive work of breathing
Mechanical Ventilation: Indications
Demands
Alveolar ventilation