Physiology 11 Flashcards
What are the important spirometric variables?
What is the difference between a volume and a capacity?
TV: Tidal volume IRV: Inspiratory reserve volume ERV: Expiratory reserve volume RV: Residual volume TLC: Total lung capacity VC: Vital capacity FRC: Functional residual capacity
A capacity is the sum of two or more volumes.
What equipment can be used to measure lung volumes?
- Water sealed spirometer
- Dry spirometer
- Body plethysmograph
- Helium dilution
- Nitrogen washout
Which techniques for measuring lung volumes can calculate residual volume/FRC/TLC?
What are the relevant benefits/drawbacks of these techniques?
Body plethysmography - most accurate
Helium dilution - Does not include collapsed/poorly ventilated areas
Nitrogen washout - Also does not include collapsed/poorly ventilated areas. Requires patient to be able to breathe 100% O2 (ie. not chronic T2RF)
Why is helium used for measuring lung volumes?
Its solubility in blood is very low
What are the important functions of the functional residual capacity (FRC)?
- Oxygen reservoir
- Airway splinting
- Optimal lung compliance
- Optimal pulmonary vascular resistance
What is a typical FRC?
What is the volume of oxygen present in the FRC?
2500ml
2500 x 0.15 = 375ml O2
How long does the O2 reserve in the FRC last during apnoea in normal conditions?
90 seconds
Given adequate proxygenation, how long will the O2 reserve FRC last?
2500 x 0.9 = 2250ml O2
2250 / 250 = 9 mins
What factors affect how long a patients O2 reserves will last?
- Reduced FRC (Obesity)
- Increased consumption (Sepsis, childhood)
What is closing capacity?
Residual volume + closing volume = closing capacity
What may cause increased closing capacity?
- Smoking
- Asthma
- Advanced age
Where does FRC usually sit on the lung compliance curve?
Usually at the steepest part of the curve (ie. most compliant)
What is the impact of restrictive lung disease on lung compliance at FRC?
FRC will be below the optimal point on the curve and compliance will decrease, increasing work of breathing
What factors increase FRC?
Height Male gender Asthma Emphysema IPPV
What factors decrease FRC?
Obesity Anaesthesia Supine position Kyphoscoliosis Lung fibrosis
Does age have an effect on FRC?
No (though CC is increased)
What are the constituents of dead space in the ventilated patient?
- Apparatus
- Anatomical
- Alveolar
Outline Fowler’s Method of calculating anatomical dead space
- Patient breathes normally then takes a VC respiratory breath of 100% O2 (from FRC)
- Patient exhales slowly down to RV, exhaled N2 is measured throughout
Initially following expiration the [N2] will be 0, representing the anatomical dead space. The midpoint of the initial steep rise in [N2] represents anatomical dead space volume.
What is a normal dead space:tidal volume ratio?
0.8 in most mammals
What is the shunt equation?
Qs/Qt = (CcO2 - CaO2) / (CcO2 - CvO2)
What is hysteresis?
The phonomenon of two limbs of a curve following different courses eg. the inspiratory and expiratory limbs of a respiratory P-V curve
How is lung compliance measured?
By calculating the gradient of the pressure-volume curve