SPIROMETRY

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

Note:

• Normal spirometric values are based on age, gender, race, and height.
• "Some regression equations that include "weight" as a determinant yield absurd values in very obese subjects" (Murray & Nadel)
• Display of spirometry data can be as volume-over-time or flow-over-volume

For flow-volume curves:

1. "For each point on the volume axis, a maximal flow exists that cannot be exceeded regardless of the pressure generated by the respiratory muscles. Although this portion of the curve is very reproducible in a given subject, it is altered in a characteristic manner by the effects of diseases on the mechanical properties of the lungs" (Murray & Nadel)
2. "Flow-volume curves during forced inhalation are entirely effort dependent. The inspiratory limb of the flow-volume curve has great diagnostic usefulness when central airway obstruction is suspected, a situation in which ordinary spirometry reveals a nonspecific pattern" (ibid.)
3. "When the tidal volume loop is superimposed on the flow-volume curve, comparison of the two may be useful in clinical evaluation.  'Negative effort dependence' is said to be present when expiratory air flow rates during quiet breathing exceed those during maximal effort. This phenomenon suggests that the airways are less rigid than normal, as may be seen in emphysema and in some forms of chronic bronchitis." (ibid.)

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:

1. Pressure, generated by
   1. Elastic recoil of the lungs (reduced in emphysema)
   2. Muscular effort
2. Resistance, dependent on
   1. Small airways function (reduced in asthma)
   2. Large airways function
   3. Interdependence of airways and alveoli (i.e., disturbances in the normal ability of the alveoli, through their attachments to the terminal bronchioles, to prop them open during exhalation.
3. "FEV measurements taken at 0.5, 0.75, 2.0, and 3.0 seconds add little additional clinically valuable information to the FEV₁ measurement" (Murray & Nadel)

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.)

Determining effort: 

1. Time-Volume and Flow-volume curves should be smooth, without evidence of cough or premature cessation of airflow
2. Peak flow should occur within 0.1sec of initiation of forced expiration
3. Poor reproducibility with repeat testing

Reversibility of low FEV1/FVC with inhaled bronchodilator suggests asthma; nonreversibility suggests "COPD" (chronic bronchitis, cystic fibrosis)

1. Per Am. Coll. Chest Physicians, sig. reversibility = 15% or greater increase in two of either FEV1, FVC, or FEF 25-75% (Murray & Nadel)
2. Per Am. Thoracic Soc., 12% increase in FVC or FEV1 is sig.
3. "Ratios such as FEV1/FVC should not be used to evaluate reversibility" (Murray & Nadel)
4. "Failure to demonstrate significant responses to acute bronchodilator therapy does not rule out reversible airway obstruction" (Murray & Nadel)

Restrictive pulmonary diseases:

1. Idiopathic pulmonary fibrosis
2. Idiopathic fibrosing alveolitis
3. Interstitial pneumonitis (e.g. from meds, asbestosis, rheumatoid arthritis)
4. Scarring e.g. from TB
5. Sarcoidosis
6. Thoracic deformities
7. Congestive heart failure

Recommendations from the National Lung Health Education Program (includes the American College of Chest Physicians & National Heart, Lung, and Blood Institute):

1. Office spirometers should not be utilized for diagnostic testing (i.e. refer for standard spirometry if results abnormal)
2. Many lung function indexes may be derived from spirometry; however, the most valuable indexes are the total volume of exhaled air and the FEV₁
3. The measurement of FVC should be replaced by that of FEV₆ so that each maneuver need last for only 6 seconds
4. Airway Obstruction will Be interpreted when the FEV₁/FEV₆ ratio and the FEV₁ percent predicted Are both below their lower limit of normal
5. Report FEV₁ as a percent of predicted to patients. This is "the number" the patient should remember.
   1. FEV₁ LLN to 60% predicted FEV₁ = mild obstruction
   2. 40 to 59% predicted FEV₁ = moderate obstruction
   3. < 40% predicted FEV₁ = severe obstruction
6. If FEV₁/FEV₆ ratio is above the LLN but the FEV₆ is below the LLN, the patient has a low vital capacity, perhaps due to restriction of lung volumes
7. The measurement of FVC should be replaced by that of FEV₆ so that each maneuver need last for only 6 s

References

Sources include:

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.

SEE ALSO:

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