Lung Volumes & Capacities Flashcards
What is the difference between lung volumes and capacities?
❖ Lung volumes are measured by:
* Spirometer
Or
* Gas dilution technique
❖ Lung capacities:
* It’s the sum of 2 or more lung volumes.
Name the 4 lung volumes.
Values given for a 70 kg man.
❖ Tidal volume:
• Is the volume of air inspired per breath during normal quiet breathing.
• It contains the volume of air that fills the alveoli plus the volume that fills the airways.
• Vt = 500 ml.
❖ Inspiratory reserve volume:
• Additional volume that can be inspired above Vt.
• IRV = 2500 - 3000 ml.
❖ Expiratory reserve volume:
• Additional volume that can be expired below Vt.
• ERV = 1500 ml
❖ Residual volume:
• The volume of gas that remains in the lungs after a maximal forced expiration.
• RV = 1500 ml.
• Cannot be measured by spirometry.
Name the four lung capacities.
❖ Functional residual capacity:
• FRC = RV + ERV
• FRC = 3000 ml
• The volume remaining in the lungs after a normal Vt is expired.
❖ Inspiratory capacity:
• IC = Vt + IRV
• IC = 3000 ml
❖ Vital capacity:
• VC = ERV + IC
• VC = ERV + Vt + IRV
• VC = 4500 ml
• The volume that can be expired after maximal inspiration.
• It increases with body size, male gender, physical condition.
• It decreases with age.
❖ Total lung capacity:
• TLC = RV + VC
• TLC = RV + ERV + Vt + IRV
• TLC = 6000 ml
Explain the mechanism of ventilator associated lung injury as a result of volutrauma.
❖ Volutrauma:
• High volumes (e.g. Vt 12 ml/kg) causes overdistension of the lungs leading to diffuse alveolar damage.
• Low Vt ventilation strategy (6 ml/kg reduces mortality in ARDS.
Explain the mechanism of ventilator associated lung injury as a result of barotrauma.
❖ Barotrauma:
• Damage to the lungs as a result of high airway pressure.
❖ Prevention of barotrauma:
• Peak airway pressure =< 35 cmH20.
• Plateau pressure =< 30 cmH20.
Describe what is lung protective ventilation strategy.
❖ In patients with reduced lung compliance (e.g. ARDS) we use ventilation strategies focused on lung protective parameters:
➜ Ppeak =< 35 cmH2O
➜ Pplat =< 30 cmH2O
❖ These patients might not achieve sufficient alveolar ventilation to maintain normocapnoea using these protective parameters.
❖ We allow permissive hypercapnoea rather than increase Vt or inspiratory pressure (which may cause volutrauma and barotrauma) and cause further lung damage.
What is the physiological importance of FRC?
O₂
✤ If FRC did not exist ➝ ↓air in the lungs ➝ ↓ less O₂ in the in the alveoli.
✤ PAO₂ would ↓ during expiration.
✤ As blood doesn’t stop flowing, pulmonary capillary blood would be intermittently oxygenated, only being fully oxygenated during inspiration.
Prevention of alveolar collapse
✤ If FRC did not exist, alveoli would collapse.
✤ Atelectasis would result in V̇ /Q̇ mismatch and hypoxaemia.
✤ Re-expansion of colapsed alveoli with every tidal breath would significantly increase the work of breathing.
Optimal lung compliance
✤ Lung compliance is at its highest at FRC.
✤ Pulmonary vascular resistance is at its lowest at FRC.
