Pulmonary Ventilation: Volumes, Flows, Dead Space and Pre-oxygenation Flashcards
Draw a Spirometry graph and label the following: TV, ERV, IRV, RV, FRC, VC, TLC. Give approximate values for these for a 70kg man.
TV=500ml
ERV= 1200ml
IRV= 3100ml
RV=1200ml
FRC=2400ml
VC=4800ml
TLC=6000ml
A capcity is 2 or more values combined.
What are the different methods of measuring lung volumes and capacities?
- Water sealed spirometry
- Dry spirometry
- Body Plethysmography
- Helium Dilution
- Nitrogen Washout (Fowler’s method)
Describe how spirometry works?
Water sealed spirometer: The patient blows into a closed chamber in contact with water, as they inhale/exhale, the water displaces. The chamber is attached via pullies to a pencil. As the water is displaced the pencil moves up and down over moving paper.
Dry spirometer: The patients blows into a mouthpiece which is attached to a bellows which is connected to a pencil. (This is the simplest iteration of a dry spirometer)
A water sealed spirometer is a historic apparatus
Consider the pressure changes in the lungs and the box.
How does body plethysmography work and how can it be used to measure FRC?
Utilsises Boyles Law: Pressure is inversely proportional to volume at a fixed temperature.
A patient is placed in an air tight box with a known volume, which contains a mouth piece which goes outside the box, it also contains a pressure sensor in the mouthpiece, and pressure sensor measuring the pressure in the box.
The patient breathes in and out via the mouthpiece, at the point of FRC (end of a normal tidal volume expiration) a shutter in the mouth piece closes. As the patient tries to inhale the chest volume will increase , as the chest volume increases the volume in the box will be reduced. This reduced box volume will indicate a reciprocal increase in the box pressure (Pbox2).
Vbox2 = Vbox1 - (change in volume lung volume)
Therefore Pbox1.Vbox1= Pbox2 .(Vbox1-change in lung volume) This allows us to calculate the change in lung volume.
If we now consider the pressure sensor in the mouthpiece.
Plung1 at FRC (end of normal expiration)
Vlung1 =FRC
Plung 1 x Vlung 1= inspiratory airway P x inspiratory V of chest
Inspiratory volume of the chest = Vlung 1 + change in lung volume. As previously stated Vlung1 =FRC.
Therefore:
Inspiratory volume of the chest = FRC + change in lung volume.
From looking at the pressure volume relationship in the box we have a value for the change in lung volume which will allow us to calculate FRC.
The P1 values are all measured at the end of a normal expiration i.e at the the point of the FRC.
How does Helium dilution work?
The patient breathes air with a known concentration of helium from FRC in a closed system containing a spirometer.
The CO2 produced during the test is absorbed by soda lime and replaced with oxygen. The helium is then distributed through the lungs.
The initial concentration x the initial volume = the post equilibration concentration x the post equilibration volume**
I.e: C1 x V1 = C2 x (V1+V2)
V2= total lung capacity
** The initial concentration and volume is of that in the closed system. The post equilibration volume will be the volume of the closed system and the lungs.
Helium is used as it is very insoluble therefore stays in the system.
The helium is only distributed to the ventilated areas of lung.
How does Helium dilution work?
The patient breathes air with a known concentration of helium from FRC in a closed system containing a spirometer.
The CO2 produced during the test is absorbed by soda lime and replaced with oxygen. The helium is then distributed through the lungs.
The initial concentration x the initial volume = the post equilibration concentration x the post equilibration volume*
I.e: C1 x V1 = C2 x (V1+V2)
V2= total lung capacity
*The initial concentration and volume is of that in the closed system. The post equilibration volume will be the volume of the closed system and the lungs.
The FRC can be calculated using the other spirometry values.
Helium is used as it is very insoluble therefore stays in the system.
The helium is only distributed to the ventilated areas of lung.
How does Nitrogen washout work?
The subject breathes 100% oxygen from FRC (end expiration) in a closed breathing circuit connected to a spirometer.
After several minutes, the nitrogen concentration and volume of gas within the equipment is measured. This is equal to the amount of nitrogen that was initially present
FRC x 0.79 = measured gas volume and nitrogen concentration
Note similarly this only measures ventilated lung
Calculate the FRC utilising the Nitrogen washout technique with following figures.
Measured gas volume=4000mls
Measured nitrogen concentration = 5%
FRC x 0.79 = 40000mls x 0.05
FRC = (40000mls x 0.05)/0.79
FRC= 2000/0.79= 2531ml
Why is the FRC of importance in anaesthetics?
- Oxygen Reservoir
- Prevention of Airways Collapse
- Optimal Compliance
- Optimal pulmonary vascular resistance
What is Functioncal residual capacity?
The lung volume at barometric pressure at the end of expiration.
It is the balance of the natural tendency of the rib cage to spring out by the tendency of the lung to collapse.
What is a normal apnoea time if you don’t pre-oxygenated and how can apnoea time be increased by utilising pre-oxygenation?
A patients FRC acts as an oxygen resevoir.
The normal FRC is ~2500mls
Normal PAO2 is 15KPa (at sea level) this equates to ~15%
Therefore without pre-oxygenation O2 available in FRC = 2500mls x0.15 = 375mls
Normal oxygen consumption is** ~250ml/min therefore apnoea time without pre-oxygenation = 90secs**
With pre-oxygenation assuming you achieve a ETO2 of 90% would be:
2500mls x 0.9 = 2250mls
2250mls/250mls/min= 9mins
Note if a patient has a reduced FRC they will have significantly reduced apnoea times.
What is closing capacity, and in what situations does it become more relevant?
Closing Capacity (CC): volume in the lungs at which its smallest airways, the bronchioles, collapse. If the closing capacity > FRC there will be small airway collapse.
In fit patients, this is always less than the FRC.
If the FRC is reduced or the CC is increased, there may be airway collapse whilst breathing at tidal volume.
CC may increase in smoking, asthma and age.
PEEP is used to increase FRC and prevent CC being > FRC.
How is FRC related to compliance?
FRC is at a point where the balance between the tendency of lungs to collapse and the rib cage to spring out.
In a healthy individual this is at the steepest part of a lung volume vs transpulmonary pressure curve (compliance curve) where compliance is greatest.
In patients with reduced FRC it falls lower down on the curve where the compliance is poorer.
Which factors increase and decrease FRC?
Increase:
* Height
* Male gender
* Asthma
* Emphysema
* Intermittent Positive Pressure Ventilation (IPPV)
Decrease:
* Obesity
* Anaesthesia (loss of muscle tone therefore redcues the tendency of the rib cage to spring out)
* Supine positioning
* Kyphoscoliosis
* Lung fibrosis
* Pregnancy
NOTE: Age does not have a direct effect on FRC but does increase Closing capacity
What is dead space, what proportion of TV is dead space and define physiolgical dead space?
The volume of inspired air not taking place in gas exchange and typically is 30% of tidal volume.
Physiological dead space = Anatomical dead space - Alveolar dead space
