eLFH - Respiratory Physiology Part 1 Flashcards
Lung volumes graph
Which lung volume cannot be measured by spirometry
Residual volume
And therefore also:
Functional residual capacity
Total lung capacity
Functional residual capacity definition
Volume of gas remaining in lungs at end of tidal expiration
Lung volume at which pulmonary vascular resistance is at its lowest
Balance on which FRC is determined
Balance between:
- Inward elastic recoil of lung
- Outward force produced by muscle tone of diaphragm and rib cage
Relevance of FRC to anaesthesia
Acts as oxygen reservoir during apnoea
As FRC falls, distribution of ventilation in lungs changes leading to mismatch with perfusion
If it falls below closing capacity, airway closure occurs leading to shunt
Methods to measure FRC
Helium dilution
Body plethysmography
Limitation of helium dilution to measure FRC
Underestimates FRC in patients with lung disease and gas trapping
Factors which decrease FRC
Body position
Obesity / pregnancy
GA especially with muscle relaxants
Restrictive lung disease
Females
Age (less in young children)
Body positions that reduce FRC
Head down
How much lower is FRC in females than males
Females 10% less than men
Factors which increase FRC
PEEP / CPAP
Increased airway resistance
Age
Dead space definition
Volume of gas involved in ventilation but not in gas exchange
Volume of dead space
2 - 3 ml/kg
Approximately 30% VT (~ 150 ml)
How to measure dead space
Fowler’s method
Anatomical dead space definition
Volume of conducting airways - incapable of gas exchange
Not lined with respiratory epithelium
Fowler’s method
Single breath nitrogen washout
At end of tidal expiration, a vital capacity breath of 100% O2 is taken
Exhaled [N2] is measured during slow maximal exhalation
[N2] plotted against volume expired
Volume with no N2 = pure dead space
Volume from start of N2 to maximal N2 = mix of dead space and alveolar gas
Anatomical dead space volume is point on x axis where area A = area B
Bohr equation
