Important concepts for facilitation of ventilation

Definition of FRC

Volume of gas in lung after normal expiration 

→ participate in gas exchange in insp + exp

Factors impacting FRC

Factors

Positioning

Age

Weight

How does positioning impact FRC

Lowest: Supine w/ head down< supine < supine head 30’ up  Highest: Erect standing > Erect sitting > supine 60’ head up Why:  In standing/ erect sitting: abdominal contents are forced down → diaphragm can extend further down In supine: abdominal contents force the diaphragm up at a higher position  Bed height: 60’ ideal

How does age impact FRC

Younger ppl: CC < FRC 

45 years old: supine → CC > FRC 

65 years old: upright → CC > FRC 

→ some dependent areas of lungs collapse

How does weight impact FRC

Obesity → FRC reduced

Definition of closing capacity

Volume of air in lung when small airways in dependent lung starts to collapse in expiration 

Normal: 

1. CC = Residual volume 

2. CC < FRC 

If CC > = FRC in normal expiration 

→ Collapse of airways in dependent lung areas 

→ ↓  ventilation

Definition of hypoxaemic pulmonary vasoconstriction

Protective response

Process: 

Ventilation/ perfusion mismatch → ↓ PaO2 → constriction of pulmonary vessels ( hypoxaemic pulmonary vasoconstriction ) → blood diverted to areas w/ greater ventilation ( ↑ perfusion in areas w/ ↑ ventilation ) → ↑ PaO2

Concepts for management of ventilation

1. Critical opening pressure 

2. Slow laminar flow 

3. Inspiratory hold