Lecture 26: Pulmonary Function Testing

Question 1

Describe what IRV, TV, ERV, RV, TLC, IC, FRC, and VC describe. Which 3 can’t be measured by spirometer?

Answer 1

IRV = inspiratory reserve volume - amount that can be inspired above tidal volume
TV = tidal volume - normal volume changes
ERV = expiratory reserve volume - amount that can be expired above tidal volume
RV = residual volume - amount of air remaining even after forced expiration
TLC = total lung capacity
IC = inspiratory capacity - amount of air that can be forcefully inspired starting from normal expiration
FRC = functional residual capacity = amount of air remaining after normal expiration
VC = vital capacity - amount of air that can be forcefully inspired starting from forced expiration

Question 2

How do you measure FRC?

Answer 2

Helium dilution. Breathe in helium, which doesn’t diffuse, and measure new concentration. Know original volume and concentration, therefore can calculate new volume.

Question 3

How do you measure the anatomical dead space?

Answer 3

Use nitrogen washout. Breathe in pure oxygen, then measure nitrogen levels over full breath. Area above the curve is the dead space, area under the curve is the amount of alveolar air.

Vdeadspace = (Aabove curve)*(total volume expired)/(area above curve + area below curve)

Alveolar ventilation rate = frequency of breaths * (TV-dead space)

Question 4

How do you measure physiological dead space?

Answer 4

Compare the Pco2 in the arteries vs the expired CO2. arterial CO2 levels approximately the same as in the alveoli, since the pressures equilibrate.
Vd/Vt = 1-[PCO2E/PCO2A]
since expired air is diluted by the air that goes into alveoli that aren’t perfused

Question 5

Describe the equal pressure point

Answer 5

Equal pressure point is the point at which airway pressure = intrapleural pressure (transairway pressure is 0). above this point, increasing pleural pressure doesn’t increase flow rate during expiration, since airways are compressed.

At the beginning, alveolar pressure is positive since intrapleural pressures is so large, there is a pressure gradient through the airways since airway resistance exists and CSA decreases which accelerates the gas. Transpulmonary pressure pressure starts to decrease as the alveoli get smaller (less elastic force) and the Vlung decreases, and since (Ptp= PA-Pip) and (PA=Ptp+Pip), as Ptp gets more negative, PA decreases, so there is less of a driving force. Pip can increase up until it equals the Airway pressure to create a larger gradient for flow.

Question 6

Describe what happens to flow-volume loops with changes in effort, as well as with pulmonary fibrosis (restrictive) and obstructive (asthma, COPD)?

Answer 6

Changes in effort are going to increase peak expiratory flow rate (PEFR) but at low lung volumes, will converge to 0 regardless. With restrictive pathology, there is the same flow rates possible, but lower lung volumes. With obstructive pathology, the volumes are the same, but the peak flow rate decreases