lung volumes, lung capacities, pulmonary function tests

difference between volume and capacity

volume — 1 value

capacity — sum of multiple volumes

tidal volume

amount of air trapped and expired during normal breath

400-700 mL/kg

inspiratory reserve volume

additional air a person can inhale after a normal inspiration

includes the tidal volume

expiratory reserve volume

additional air a person can exhale after normal exhalation

residual volume

amount of air remaining in lungs after ERV is expired

“dead air”

1-1.2 L

what happens to residual volume as we age

it increases naturally

dead space

the volume of air within the respiratory system that does not participate in gas exchange

inspiratory capacity

amount of air inspired and expired with normal breathing PLUS the amount of air a person can inspire after normal inspiration

IC = tidal volume + IRV

vital capacity

max volume of air that can be forecully expired after max inspiration

decreases with age

VC = tidal volume + IRV + ERV

functional residual capacity

not limited to but INCLUDES amount of air remaining in lungs after normal passive expiration

FRC = ERV + RV

total lung capacity

max volume of air the lungs can hold

TLC = VC + RV

TLC = IC + FRC

what is the average total lung capacity for men

7 L

what is the average total lung capacity for women

5 L

physiological dead space

anatomical + alveolar dead space

anatomical dead space

the volume of gas contained in the non respiratory conducting airways (no gas exchange)

total volume of air from nose / mouth to terminal bronchioles

~150 ml in healthy people

alveolar dead space

volume of gas in alveolar ventilation with little to no perfusion

referred to as “wasted ventilation”

V/Q mismatch

negligible in healthy population (25-50 mL)

higher in obstructive diseases

pulmonary function tests

non invasive

measure lung volumes, lung capacities, rates of flow

determines restrictive and obstructive, severity, progression

what specific things are measured by PFTs

tidal volume

FEV1

vital capacity

how easily CO2 can diffuse from lungs to bloodstream (DLCO)

spirometry PFT

provides basic lung volumes and ESTIMATES other volumes

open circuit, closed circuit

cheaper and quicker

ESTIMATES RESIDUAL VOLUME

body plethysmography PFT

complete test for all lung capacities and volumes

uses nitrogen and helium

DIRECT MEASURE OF TLC, FRC, RV

DLCO

measures diffusion capacity of carbon monoxide to evaluate alveolar capillary membrane diffusion integrity

(carbon monoxide because hemoglobin has high affinity for it)

FEV1 (forced expiratory volume in 1 second)

should be 80-120% normal value

FVC (forced vital capacity)

max volume expired as rapidly as possible after max inspiration

should be 80-120% normal value (4-5 L is norm)

FEV1 / FVC ratio based on predicted values

age

biological sex

height

weight

ethnicity

FEV1/FVC ratio based on absolute value

norm is 0.8

0.85 if children not fully grown

0.7 is lower limit of normal

GOLD 1 stage of COPD (mild)

FEV1 % of predicted >/= 80

GOLD 2 stage of COPD (moderate)

FEV1 % of predicted 50-79

GOLD 3 stage of COPD (severe)

FEV1 % of predicted 30-49

GOLD 4 stage of COPD (very severe)

FEV1 % of predicted <30

what GOLD stage do PTs normally see people in

GOLD stage 2 (FEV1 % predicted of 50-79)

flow loop graph

shows what happens with forced inspiration and forced expiration

what happens to the flow loop graph in restrictive diseases

entire loop is shrunk

“shrunken egg”

what happens to the flow loop graph in obstructive diseases

“scooped out”

if FEV1 increases by 12% or more, the FVC increases by 200 ml or more, or there is a 15% increase in FEF 25-27, what does this tell us

the disease is reversible

FEF 25-27%

forced expiratory flow volume from point 25% to 75% of FVC has been exhaled

used to capture medium to small airways

differential diagnosis for asthma and smokers

maximum voluntary ventilation

max breathing capacity at rest x 1 min (L/min)

patient pants as fast and deep as possible for 15 seconds

indicates inspiratory muscle strength at max exertion

what happens when minute ventilation = max voluntary ventilation

there is no reserve

VERY DANGEROUS

what type of disorder does an FEV1/FVC ratio of >0.7 indicate

restrictive

what type of disorder does an FEV1/FVC ratio of <0.7 indicate

obstructive

what should you do if a patient’s FEV1/FVC ratio is >0.7

they likely have a restrictive disorder:

evaluate FVC and TLC

evaluate DLCO

what should you do if a patient’s FEV1/FVC ratio is <0.7

they likely have an obstructive disorder:

grade severity with GOLD criteria

check reversibility (>12% — asthma, <10% — COPD)