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
Alveolar dead space definition
Volume of alveoli ventilated but not perfused
Always a degree of alveolar dead space in healthy patients
Physiological dead space definition
Anatomical dead space + Alveolar dead space
How is physiological dead space calculated
Bohr equation
Equipment dead space definition
Face masks, breathing circuits, etc all increase volume of conducting airways
Effects of increased dead space
Increase PACO2-EtCO2 difference (alveolar dead space has no CO2 so exhaled CO2 is diluted)
Less efficient breathing (decreased proportion of tidal volume involved in gas exchange)
Factors which increase dead space
Increased size of transporting airway
Reduced percentage of minute volume reaching alveoli
Reduced number of perfused alveoli
Other causes
Increases size of transporting airway causes of increased dead space
Bronchodilation (e.g. pregnancy)
Neck extension
Standing
Old age
Equipment
Reduced percentage of minute volume reaching alveoli causes of increased dead space
High RR with low tidal volume - e.g. pregnancy
Reduced number of perfused alveoli causes of increased dead space
PE / air embolism
Hypotension
Haemorrhage
Other causes of increased dead space
GA and IPPV
Pulmonary disease
Drugs (e.g. atropine, hyoscine)
Factors which decrease dead space
Decreased size of transporting airways
Increased number of perfused alveoli
Decreased size of transporting airway causes for decreased dead space
ETT / Tracheostomy
Bronchoconstriction
Supine position
Increased number of perfused alveoli causes for decreased dead space
Increased cardiac output
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 use
Calculates volume of physiological dead space
Bohr equation
Changes to lung volumes with age
Total lung capacity remains constant with age
Vital capacity decreases 20 ml/kg from age 20
Residual volume increases 10% per year
FRC increases 3% per year
Effect of closing capacity with age
Closing capacity increases with age at greater rate than FRC increases
Therefore with increased age, closing capacity impeaches on FRC
