Respiratory Physiology 1 Flashcards
Which volumes of the lung cannot be measured by spirometry?
- residual volume
- FRC
- total lung capacity
What is the functional residual capacity?
The volume of gas remaining in the lungs at the end of tidal expiration (~2500 ml)
It’s the lung volume at which the PVR is at it’s lowest level.
Determined by the balance between the inward elastic recoil of lung, and outward force of rib cage/diaphragm (due to muscle tone).
Why is FRC important?
- O2 reservoir, prolongs time to desaturation during apnoea
- as it falls, distribution of ventilation within lungs changes, leading to a mismatch with perfusion
- if it falls below closing capacity, airway closure occurs leading to shunt
How can FRC be measured?
- body plethysmography (most accurate -esp in patients with lung disease assoc/w gas trapping)
- helium dilution (only measures ventilated lung volumes)
What will reduce the FRC?
- head down < supine
- obesity/intra-abdominal mass/pregnancy
- GA (esp with muscle relaxants)
- restrictive lung disease (eg pulmonary fibrosis)
- female gender (10% less than men)
- age (less in young children)
What factors will increase FRC?
- PEEP and CPAP
- increased airway resistance (eg asthma)
- age
What is dead space?
The volume of gas involved in ventilation that doesn’t take part in gas exchange. Usually 2-3 ml/kg (30% of VT ~150mls).
What is anatomical dead space?
- mouth
- nose
- pharynx
- airways not lined with respiratory epithelium
How do you measure anatomical dead space?
Fowler’s method
What is alveolar dead space?
The volume of alveoli that are ventilated but not perfused and therefore unable to take part in gas exchange.
There is always some degree of alveolar dead space even in healthy patients.
What is physiological dead space?
Anatomic dead space + alveolar dead space.
Calculated using the Bohr equation.
How does equipment add to dead space?
Face masks, breathing circuits and related equipment all increase the volume of the conducting airways.
What effect will increasing dead space have?
- increase in PACO2 - EtCO2 difference. Alveolar dead space contains no CO2, so exhaled CO2 is further diluted
- Less efficient breathing (as the proportion of tidal volume involved in gas exchange is decreased)
What factors will increase dead space?
- increases in size of transporting airway
- bronchodilation (eg pregnancy)
- neck extension
- jaw protrusion
- standing
- old age
- appartus (face masks, catheter mounts)
- reduced % of minute volume reaching alveoli
- high RR with low tidal volume
- reduced number of perfused alveoli
- PE/air embolism
- hypotension
- haemorrhage
- other causes
- GA & IPPV
- pulmonary disease
- drugs (atropine, hyoscine)
What factors will decrease dead space?
- decreased size of transporting airways
- ETT, tracheostomy
- bronchoconstriction
- supine position
- increased number of perfused alveoli
- increased cardiac output
What is Fowler’s method?
- technique for measuring anatomical dead space
- at the end of tidal expiration, a vital capacity breath of 100% O2 is taken
- exhaled N2 is measured during a slow maximal exhalation
- N2 is then plotted against volume expired
What is the Bohr equation?
Calculates the volume of physiological dead space.
