RESPIRATION PHYSIOLOGY: VENTILATION

VENTILATION PARAMETERS

A. Lung Volumes (==>)

1. Basic volumes: elements

a. Tidal Volume (V_T, TV): volume of gas exchanged each breath -- can change as ventilation pattern changes

b. Inspiratory Reserve Volume (IRV): maximum volume that can be inspired, starting from the end inspiratory position -- potential volume at the end of inspiration

c. Expiratory Reserve Volume (ERV): maximum volume that can be expired, starting from the end expiratory position -- potential volume at the end of expiration

d. Residual Volume (RV): volume remaining in the lungs and airways following a maximum expiratory effort -- lungs cannot empty completely because of (1) stiffness when compressed and (2) airway collapse and gas trapping at low lung volumes

2. Capacities: combined volumes

a. Vital Capacity (VC): maximum volume of gas that can be exchanged in a single breath
VC = TV + IRV + ERV

b. Total Lung Capacity (TLC): maximum volume of gas that the lungs (and airways) can contain
TLC = VC + RV
= TV + IRV + ERV + RV

c. Functional Residual Capacity (FRC): volume of gas remaining in the lungs (and airways) at the end expiratory position
FRC = RV + ERV

d. Inspiratory capacity (IC): maximum volume of gas that can be inspired from the end expiratory position
IC = TV + IRV

3. Parameter Measurement

a. spirometer: can determine all basic volumes except residual volume

( See figure ==> )

Note: Now volumes usually measured with electronic flowmeters

b. residual volume

1) whole body plesthysmograph

2) gas dilution: inhale inert gas (e.g. helium) from a closed container of known volume (V) and note the concentration initially (C_initial) and after equilibration (C_equil) ; then solve the equation below for FRC and use this to calculate RV:

C_initial x V = C_final x ( V + FRC)

4. Typical Values (young adult male of average size)

TV = 0.5-0.6 L (liters)
IRV = 3.0 L
ERV = 1.3 L
RV = 1.2 L

VC = 4.8 L ( approx. 5 Liters)
TLC = 6.0 L
FRC = 2.5 L
IC = 3.5 L

Note: "normal" values depend upon age, sex, and size

B. Ventilation (compare with cardiac output)

1. frequency or respiration rate (f or RR): breaths per unit time

typical value at rest: 12/min (but much individual variation)

2. ventilation rate: total volume inspired or expired per unit time; sometimes called Minute Volume (MV) when measured per minute; to avoid ambiguity, usually measured as volume expired, V'_E

MV or V'E = f x TV

typical value at rest: MV or V'E = 12 breaths/min x 0.5L = 6 L/min

3. maximum ventilation (Maximum Voluntary Ventilation Capacity, Maximum Breathing Capacity, MVVC, MBC): maximum volume of gas that can be inspired and expired per unit time

typical value: 120-180 L/min (usually measured for 15 seconds and scaled per minute, because of work of maximum ventilation)

4. maximum air flow rate, single breath

a. peak velocity (e.g. peak expired flow rate) normal value 400-600 liters/minute ( ==> ) (Must compare with predicted value based on age, size, and sex)

Note Limitation: does not distinguish between obstructive conditions and small or stiff lungs (restrictive conditions)

b. timed vital capacity: volume of gas that can be expired from the lungs with maximum effort in a given time (usually 1 second; "one-second forced expired volume", FEV₁)

1) usually expressed as a fraction of the total volume expired in a maximum effort, the Forced Vital Capacity (FVC)

2) normal value of FEV₁ / FVC ³ 80%

3) because FEV₁ / FVC incorporates both volume and flow, it can distinguish between obstruction and small or stiff lungs