Relevant Pulmonary Physiology
Flow = Pressure/Resistance
Pa = pressure applied to the respiratory system; must overcome the elastic, resistive, and inertial properties of the respiratory system
Pg = Pg + Pti + Pcw
Pg = pressure generated by gas
Pti = pressure generated by tissue
Pcw = Pressure generated by the chest wall
C = compliance [ability of lungs to change in volume in response to a change in pressure]
V = volume
R = resistance
I = inertance
dV/dt = change in volume over time
"Pleural pressure created during active exhalation not only creates flow by increasing alveolar pressure but also is transmitted to the walls of intrathoracic airways, tending to cause them to become compressed or "collapse" when maximal flow is achieved in the airways. At some point along the airways, the pressure inside the airways equals the pressure outside (pleural pressure), and narrowing or collapse may occur anywhere distal (mouthward) to this point in the airway. The location of these "equal pressure points" (EPPs) is a function of the mechanical properties of the lungs, namely elastic recoil, air flow resistance, and the stiffness of the airway walls themselves. It is the collapse or narrowing of the airways that determines the characteristic limitation of maximal flow during forced expiration. Emphysema probably increases collapsibility of airways and decreases driving pressure in the segment between alveoli and EPP. Conversely, infiltrative interstitial disease increases lung elastic recoil pressure and decreases airway collapsibility, resulting in increased maximal air flow" (Murray & Nadel)
Standard Measures of Spirometry
For flow-volume curves:
VC = Vital Capacity, difference in volume between maximal inhalation and maximal exhalation
FVC = Forced Vital Capacity, difference in volume between maximal inhalation
and maximal "forced" exhalation (as hard/fast as possible)--if lower
than normal can mean either a decrease in TLC (e.g. restrictive lung disease) or
an increase in RV (e.g. obstructive lung disease)--Flow measurements like FEV1
can help differentiate.
FEV1 = Forced Expiratory Volume in 1 second. Depends on:
FEV1/FVC often used as a measure of degree of airways obstruction; tends to decline with age; normal ratio does not exclude obstructive disease, esp. if FVC is low. May be artificially high if pt does not make maximal effort through the exhalation maneuver.
FEF 50% = Forced expiratory flow [rate of flow] at the moment at which 50% of the FVC has been exhaled
FEF 25-75% = Forced expiratory flow [average flow] between 25% and 75% of the FVC; theorized to reflect the most effort-independent portion of the flow-volume curve and most sensitive to airflow in small airways, but not better at identifying small airways disease or predicting clinical outcomes than FEV1/FVC.
TLC = Total lung capacity; max. volume of air that can be contained within the lungs [not measured by spirometry]--Decreased in restrictive lung disease, diseases which weaken mm., and obesity
FRC = Functional Residual Capacity; volume of gas in the lungs at resting end-exhalation [not measured by spirometry]
RV = Residual Volume; The volume of air remaining in the lungs after forced exhalation
MVV = Maximal Voluntary Ventilation; aka "Maximum Breathing Capacity"; volume of air pt can breathe with maximal effort for a given time.
Other Relevant Information
FVC and VC are 7% to 8% higher in the sitting than in the supine position and 1% to 2% higher in the standing than in the sitting position (Murray & Nadel)
Maximal expiratory air flow rates are highest at noon and usually lowest in the early morning (4:00 to 6:00 a.m.). Similar circadian variations have also been described for airway resistance, TLC, and RV, but the mechanisms are unknown. (ibid.)
Reversibility of low FEV1/FVC with inhaled bronchodilator suggests asthma; nonreversibility suggests "COPD" (chronic bronchitis, cystic fibrosis)
Restrictive pulmonary diseases:
Recommendations from the National Lung Health Education Program (includes the American College of Chest Physicians & National Heart, Lung, and Blood Institute):
Petty TL. Simple office spirometry. Clin Chest Med 22: 845, 2001
Ferguson GT et al. Office Spirometry for Lung Health Assessment in Adults: A Consensus Statement From the National Lung Health Education Program. Chest 117:1146, 2000
Murray & Nadel: Textbook of Respiratoyr Medicine, 3rd. ed., 2000, Ch. 28.
Enright PL, Hyatt RE, eds. Office spirometry: a practical guide to the selection and use of spirometers. Philadelphia: Lea & Febiger, 1987:253.
American Association of Respiratory Care. (1996) Clinical practice guidelines: Spirometry, 1996 update. Respir Care 41,629-636