Constancy of Effort and Variability of Maximal Expiratory Flow Rates

Constancy of Effort and Variability of Maximal Expiratory Flow Rates* Dan Stanescu, M.D.;·· Glaude Venter, M.B.; Rene Van Leemputten, M.A.; and Lucien Brasseur, M.D. t

In 14 normal subJecfs and In 13 patients with obstructive pulmonary diseases, we studied the variability within an individual of values for the maximal expiratory 80w rate (Vmu) recorded simultaneously VI expired pulmonary volume
Vmax at dUferent levels of Inftation, with respect to either expired or thoracic volume. There was no statistical dUference in Vmax between the first and the last day. A larger variability of Vmu measured vs expired volume impHes a change in the expiratory etfort from one forced expiration to another and a different degree of compression of intrathoncic air. Since this was not the case, we conclude that muscular etfort during repeated forced expirations is simDar. The good reproducibDity of effort explains in great measure the good reproducibility of "maL

variability of measurements of the maximal T heexpiratory How rate ("max) has been the object

coefficient of variation of Vmax at 75 percent of the expired VC after bronchodilatation in patients with obstructive pulmonary disease was twice as large as the coefficient of variation of the PEFR. Similarly, McCarthy et al6 found that the mean coefficient of variation in normal subjects was the lowest for the PEFR and the largest for Vmax at 75 percent of the expired VC. In all of these studies, the flow rate was plotted against expired volume at the mouth. Using a body plethysmograph to record changes in pulmonary volume, Green et al 4 found that the variability of "max within a subject increased at low pulmonary volumes. Thus, these data seem to imply that, rather paradoxically, the reproducibility of Vmax is poorer on the effort-independent than on the effortdependent part of the flow-volume curve. In this report, we studied the variability of "max measured simultaneously vs expired volume at the mouth and vs changes in thoracic pulmonary volume. From these comparative data, we tried to assess whether muscular effort during forced expiration is reproducible or not Our approach is as follows. During forced expiration, intrathoracic gas is compressed and changes in thoracic volume may exceed changes in expired volume at the mouth. This results in a changing localization of "max on the pulmonary volume axis. In normal subjects and in patients with obstructive pulmonary diseases who performed maneuvers for VC that were of varying effort, Ingram and Schilder" showed that at a given pulmonary volume, there was a much larger difference in flow rate when expired volume at the mouth, instead of thoracic gas

of numerous studies.l" Over the last 70 or 80 percent of the vital capacity (VC), values for Vmax are determined by the mechanical properties of the lung and are said to be independent of effort. 8 This means that once a threshold of effort is reached, any further increase in effort does not result in a further increase in flow (of course, effort below this threshold produces lower flow rates). Effort in excess of that necessary to achieve Vmax is usually developed during forced expiratory maneuvers. One would expect that flow rates on the initial effort-dependent segment, like the peak expiratory flow rate (PEFR), would be less reproducible. Implicit in this assumption is that effort during successive forced expiratory maneuvers is poorly reproducible. In fact, studies by Sobol and Emirgil! and by Afschrift et al2 have shown that the_ PEFR has about the same coefficient of variability as Vmax located on the effort-dependent part of the Howvolume curve. To explain their results, Afschrift et al 2 have suggested that subjects standardize their way of performing forced expiratory maneuvers; however, in the same report by Afschrift et al,2 the °From the Cardiopulmonary Laboratory, Department of Internal Medicine, Cliniques Universitaires Saint Luc, Universite Catholique de Louvain, Brussels, Belgium. Supported b~ grant 7246-22-2-002 from the Communaute Europeenne de Charbon et de l'Acier, °°Associate Professor of Medicine. tProfessor of Medicine. Manuscript received October 11; revision accepted November28.

Revrint requests: Dr. Stat1e8cu, CUniques UnioerritDires St. Luc, 1200 Brussels, Belgium

CHEST, 76: 1, JULY, 1979

CONSTANCY OF EFFORT AND VARIABILITY OF MEFR 59

Vmouth

Vthor

FiGURE 1. Flow rate (V) was recorded simultaneously on two oscilloscopes vs expired volume at mouth (VmOu th) and vs thoracic volume (V thor). We instructed subject to vary his expiratory effort, as can be seen from difference in PEFR between successive expirations. One may note large difference in flow rate at 50 and 75 percent of VC from graph of How vs expired volume (right), which contrasts with good reproducibility of plot of flow rate vs thoracic volume ( left) (curves drawn from photographs). Amplification of flow and volume signals was not exactly same on two oscilloscopes.

volume, was displayed vs How rate. This is illustrated in Figure 1. We simultaneously recorded How rate vs expired volume at the mouth (Fig 1, right) and vs thoracic volume (Fig 1, left) during three forced expirations of varying effort; at 50 and 75 percent of the expired VC, there was a large difference in How, which contrasts with the good reproducibility seen on the plot of thoracic volume (Fig 1, left). We reasoned that if, during repeated forced expirations, effort is poorly reproducible, compression of thoracic gas would ,be different from one expiration to another; and, therefore, the variability of "max would be larger for a given expired volume at the mouth than for a given thoracic volume. In other words, a supplementary cause of variability, due to the variable compression of intrathoracic gas, would be added to the variability of Vmax. On the contrary, if effort is similar during successive forced expirations, the variability of '9'max at any pulmonary volume would be comparable, whether pulmonary volume is measured at the mouth or as thoracic gas volume.

sis was put on full emptying of the lung at the end of the expiration. Flow rate from the pneumotachygraph was displayed on the vertical axis of a storage oscilloscope. Plethysmographic and expired volume (from the electrical integration of the flow signal) were simultaneously displayed on the horizontal axis of the oscilloscope, using a dual-beam display unit (Fig 2 ). From the flow-volume curves the same observer measured PEFR and Vmax at 25, 50, and 75 percent of the expired VC, taking as a volume reference the largest forced VC. We thus obtained three flow-volume curves using the expired volume and three others using the thoracic volume. In ten healthy subjects values for Vmax vs expired and thoracic pulmonary volumes were measured in duplicate at an interval of 12 days, on the average (range, 8 to 14 days), at the same time in the morning. The largest values for "max are reported.

25

•

MATERIALS AND METHODS

We studied a group of 14 healthy male subjects (aged between 27 and 39 years) who were all nonsmokers and 13 patients with chronic obstructive pulmonary disease, but not bronchial asthma (range of ages, 45 to 57 years). Except for three subjects, the others had no previous experience in respiratory maneuvers. Subjects were instructed in how to perform forced expirations, blowing out three to four times in a water-sealed spirometer. Ten to fifteen minutes later, the subjects sat in a pressurecorrected body plethysmograph10 and breathed ambient air through a pneumotachygraph (Fleisch No.4). They performed three successive maximal forced expirations. Empha-

80 STANESCU ET AL

Vm

Vm Vthor

Vthor

RV

TLC

FiGURE.2. Schematically, flow rate (V) was displayed simultaneously vs expired (Vm) and thoracic (V~or) pulmonary volumes and was measured at 25, SO, and 75 percent of expired VC.

CHEST, 76: 1, JULY, 1979

REsuLTS The average coefficient of variation within subjects was about the same whether Vmax was plotted against thoracic volume or expired volume at the mouth. In patients with obstructive pulmonary disease, the coefficient of variation was 7.5 percent for the PEFR and was 7.8, 7.5, and 11 percent for "max at 25, 50, and 75 percent of the expired VC, respectively (thoracic volume). For the expired volume at the mouth, the corresponding figures were 7.6, 7.9, 7.0, and 12.8 percent. In the group of healthy subjects, the coefficient of variation was 5.6,5.1,5.1, and 7.1 percent for the PEFR and Vmax at 25, 50, and 75 percent of the expired VC, respectively (thoracic volume). For the expired volume at the mouth, the corresponding figures were 5.5, 5.3, 5.4, and 8.9 percent. Table 1 presents the average variance of Vmax within a subject at 25, 50, and 75 percent of the expired VC for the expired volume at the mouth and for the thoracic pulmonary volume in healthy subjects and in patients with obstructive pulmonary disease. At each level of pulmonary inHation, there was no statistical difference between the variance of Vmax measured vs the expired volume at the mouth and vs thoracic volume (paired t-test ). In Table 2 are the individual values of Vmax measured at two sessions. The paired t-test showed no significant differences in Vmax between the first and the last day, whether pulmonary volume was the expired or the 'thoracic pulmonary volume. DISCUSSION

At different levels of inflation, we found that the variability of Vmax within individuals was comparable and did not differ statistically whether flow rates were measured vs expired or thoracic pulmoTable l-Yarianee of Ymcu: .1Id. aralrulit1idual a' 25, 50, tmd 75 Percent of YC, aa Determined from. Espired Yofume a' lIoul1& and .,. Thoraeie Yofume

P'...,.

Variance of

Group and Plot Healthy subjects Thoracic volume Expired volume at mouth Patients with obstructive pulmonary disease Thoracic volume Expired volume at mouth

Vmax, A

LIsee

25 Percent ofVC

50 Percent ofVC

75 Percent ofVC

0.28 0.29

0.07 0.12

0.016 0.021

0.16

0.014

0.002

0.06

0.009

0.003

Values for differences between plots are not significant by paired t-test.

CHEST, 76: 1, JULY, 1979

nary volume. This was true for both healthy subjects and patients with chronic obstructive pulmonary disease. These results suggest that muscular effort, (ie, pleural pressure) was similar during successive forced expirations. Otherwise, change in the expiratory effort from one expiratory maneuver to another would lead to varying degrees of compression of intrathoracic air and, therefore, in a larger variability of How rates measured vs expired volume. This was not the case even in patients with obstructive pulmonary disease, in whom increased airway resistance (Raw) and a large residual volume (RV) promote a larger compression of intrathoracic air. Another argument for a consistent effort during repeated forced expirations is given by the results of the repeatability of Vmax studied at intervals of several days; Vmax, including the PEFR (which is located on the effort-dependent part of the Howvolume curve), did not differ statistically between the first and the last day. This was true for Vmax measured vs both expired and thoracic pulmonary volume. To substantiate our hypothesis, we used a comparison of ~e variance within individuals computed from the Vmax plotted against changes in expired and thoracic volumes. In this way, we avoided the use of the coefficient of variation, which depends on both variance and the average value of the How rate. The coefficient of variation within an individual for the maximum expiratory How rate is larger at low than at high pulmonary volumes. This does not mean that the reproducibility of \Tmax is poorer on the effort-independent part of the flow-volume curve, but simply that variance does not decrease in proportion to the decrease of the average value of How from high to low pulmonary volumes. Only three maximal forced expirations were done by each subject. Recently, Knudson et alII have shown that "there seemed to be little, if anything, to be gained in having a subject perform more than 3 tests."11(p589) Our approach to demonstrate constancy of effort during repeated forced expirations is indirect; however, Zamel et al 12 measured alveolar pressure during ten forced expiratory maneuvers in one healthy subject and found that alveolar pressure was highly reproducible; the coefficient of variation was about 10 percent at 80 and 60 percent of the ve. The reason for the consistency of effort during forced expiration is not clear. Schilder et al 1S were probably the flrst to note that "subjects achieved approximately the same peak How when instructed to give repeated maximal expiratory efforts." These investigatorsIS have suggested that the "respiratory neuro-muscular system responds to a command from

CONSTANCY OF EFFORT AND VARIABILITY OF MEFR 61

Table 2-MeanPEFR and Ymas (±SD) alDi6ererdPulmoJ1GJ7 Yoluma (ltI. . .r.... Thoracic Y olume and .,. Espired Yolume GI MOIdh em T." Oeeaioru) ira Ten HealthY'S."ieela Vmax, Lisee A

Subject and Day*

PEFR, Lisee

,

25 Percent of VC A

,

50 Percent of VO A

75 Percent of VO A

t

Thoracic

Expired

Thoracic

Expired

Thoracic

Expired

Subject 1 First day Last day

5.54±0.16 5.01 ±0.28

5.37±0.14 4.87 ±0.32

5.35±0.17 4.80 ±0.39

3.82±0.08 4.11 ±0.12

3.62±0.17 3.88 ±0.15

1.64±0.17 1.77 ±0.41

1.44±0.19 1.84 ±0.08

Subject 2 First day Last day

9.55±0.17 9.90 ±O.49

9.48±0.17 9.74 ±0.34

7.52±0.99 7.11 ±0.61

5.39±0.16 5.38 ±0.37

3.56±0.14 3.58 ±0.22

1.71 ±0.07 1.53 ±0.08

1.37±0.07 0.78 ±0.08

Subject 3 First day Last day

9.25 ±0.49 9.48 ±0.63

8.47 ±0.60 9.22 ± 1.00

8.57 ±0.89 8.18 ±0.57

7.01 ±0.15 7.35 ±0.20

6.30 ± 1.10 6.24 ±0.29

2.31 ±0.15 2.31 ±0.06

1.68 ±0.22 2.06 ±0.15

Subject 4 First day Last day

9.10 ±0.77 10.95 ±0.38

8.86 ±0.69 10.71 ±0.25

8.16 ±0.11 9.27 ±0.30

6.23 ±0.30 7.04 ±0.62

5.38 ±0.22 4.89 ± 1.38

2.70 ±0.07 3.00 ±0.19

2.11 ±0.13 2.44 ±0.19

Subject 5 First day Last day

10.33 ±0.44 11.05 ±0.44

10.01 ±0.55 10.31 ±0.30

8.05 ±0.29 8.39 ±0.29

6.90 ±0.21 7.15 ±0.50

5.85 ±0.16 5.45 ±0.17

2.88 ±O 3.02 ±0.17

2.47 ±O 2.38 ±0.08

Subject 6 First day Last day

7.19 ±0.18 7.35±0.27

7.08 ±0.29 7.25±0.30

5.87 ±0.67 5.38±0.25

3.71 ±0.15 3.17±0.15

3.30 ±0.25 3.24±0.17

1.11 ±0.21 1.35±0.05

0.96 ±0.23 1.29±0.10

Subject 7 First day Last day

7.10 ±0.62 6.92±0.08

6.45 ±0.31 6.45±0.17

5.88 ±0.08 5.87±0.10

4.02 ±0.21 3.71 ±O.08

3.56 ±0.08 3.27±0.21

2.09 ±0.12 1.98±0

2.01 ±0.11 1.81 ±0.08

Subject 8 First day Last day

8.68 ±0.65 8.51 ±0.19

8.22 ±0.57 8.30±0.05

5.85 ±0.28 5.19±0.22

3.71 ±0.28 3.60±0.29

3.25 ±0.08 3.05±0.25

1.02 ±0.08 1.05±0.19

1.10 ±0.15 0.85±0.05

Subject 9 First day Last day

12.02 ±0.57 12.26 ±0.58

11.47 ±0.68 11.98 ±0.67

10.69 ±0.76 9.74 ±0.83

8.50 ±0.60 8.91 ±0.44

5.21 ±0.36 5.06 ±0.61

2.97 ±0.08 2.82 ±0.33

1.51 ±0.14 2.04 ±0.15

Subject 10 First day Last day

7.93 ±0.33 8.54±0.63

7.88 ±0.27 8.45±O.55

7.42 ±0.17 7.8O±0.46

4.66 ±0.37 4.8O±0.17

3.24 ±0.05 3.67±0.19

1.76 ±0.09 1.84±0.17

1.51 ±0.13 1.65±0.33

Mean ± SD First day Last day

8.67 ± 1.83 9.00±2.19

8.33 ± 1.78 8.73 ±2.13

7.34 ± 1.65 7.17 ± 1.78

5.40 ± 1.69 5.52 ±1.97

4.33 ± 1.21 4.22 ± 1.10

2.02 ±0.69 2.07 ±O.70

1.62 ±0.46 1.71 ±0.58

-1.59

-2.05

0.83

-1.04

1.25

-0.94

-0.85

Paired t-test**

*Interval between first and last day of measurement was an average of 12 days. **For comparison of first day to last day.

the central nervous system for maximal effort in a way perhaps analogous to a maximally stimulated" skeletal muscle, which Hill14 had "shown that it shortens with a velocity that varies inversely with the load that the muscle must overcome." In our experience, some subjects, especially women and older people, do not achieve reproducible "max even after repeated trials. The cause of this failure is also poorly understood The relationship between the variability of "max, dependence on effort, and independence of effort was the object of debate and controversy. From our 82 STANESCU ET AL

results, it appears that effort is highly reproducible during repeated forced expirations. H Raw is also reproducible, Vmax should be equally reproducible. Indeed, Zamel et al12 found a good repeatability of Raw during forced expiration. Differences in the reproducibility of alveolar pressure and Raw at different pulmonary volumes may thus explain why variability of "max is not the same at different levels of inflation. In the experiment of Zamel et al,12 alveolar pressure was less reproducible at low pulmonary volumes, whereas resistance had about the same repeatability over the whole ve. The good

CHEST, 76: 1, JULY, 1979

reproducibility of Vmax appears to be due to the good reproducibility of effort and Raw and not because they are located or not on the effort-independent segment; however, other factors may intervene and influence the variability of Vmax, such as oscillations of How, slight differences in VC on successive expirations, and even some dependency on effort.3 The PEFR may be as well or even more reproducible than the Vmax at 75 percent of the VC because the PEFR does not depend upon pulmonary volume. On the other hand, the coefficient of variation used by most authors to express variability of Vmax may be misleading. The Vmax at 75 percent of the VC and especially flow rates near RV had necessarily a higher coefficient of variation than How rates located at higher pulmonary volumes; since the absolute values of the former How rates are very small, even a tiny variation of flow rates results in a large coefficient of variation. In a recent review of the How-volume curve, Hyatt and Black" stated that ideally the flow-volume curve should be "measured by the use of a volume displacement plethysmograph" which "would make a correction for gas compression artifacts." On the other hand, from a comparative study of Vmax recorded vs expired and thoracic pulmonary volumes in smokers and nonsmokers, Zamel et aIl 8 have suggested that values of Vmax measured from the plot of How vs expired volume are more sensitive in revealing an obstructive syndrome. The present results show that at least concerning variability within an individual, values for Vmax measured in relation to the expired volume are as good as (and more simple to measure than) Vmax recorded with a body plethysmograph. ACKNOWLEDGMENTS: We thank Dr. K. P. van de Woestijne for his critical reading of the manuscript and Dr. J. Clement for statistical advice.

REFERENCES 1 Sobol BJ, Emirgil C: Subject effort and expiratory flow rate. Am Rev Respir Dis 89:402-408, 1964

CHEST, 76: 1, JULY, 1979

2 Afschri£t M, Clement J, Peeters R, et al: Maximal expiratory and inspiratory flows in patients with chronic obsbuctive pulmonary disease. Am Rev Respir Dis 100:147152, 1969 3 Clement J, van de Woestijne KP: Variability of maximum expiratory flow-volume curves and effort independency. J Appl Physiol 31 :55-62, 1971 4 Green M, Mead J, Turner JM: Variability of maximum expiratory flow-volume curves. J Appl Physiol 37: 67-74, 1974 5 Black LF, Offord K, Hyatt BE: Variability in the maximal expiratory flow volume curve in asymptomatic smokers and in nonsmokers. Am Rev Respir Dis 110:282-292, 1970 6 McCarthy D, Craig DB, Cherniak RM: Intraindividual variability in maximal expiratory flow-volume and closing volume in asymptomatic subjects. Am Rev Respir Dis 112:407-412, 1975 7 Cochrane GM, Prietro F, Clark TJH: Intrasubject variability of maximal expiratory flow volume curve. Thorax 32:171-175, 1977 8 Fry DL, Hyatt RE: Pulmonary mechanics: A uniBed analysis of the relationship between pressure, volume and gas flow in the lungs of normal and diseased human subjects. Am J Med 29:672-689, 1960 9 Ingram RH ]r, Schilder OP: Effect of gas compression on pulmonary pressure, flow and volume relationship. J Appl PhysioI21:1821-1826, 1966 10 Stanescu DC, DeSutter P, van de Woestijne K: Pressurecorrected flow body plethysmograph. Am Rev Respir Dis 105:304-305, 1972 11 Knudson R], Slatin RC, Lebowitz MD, et al: The maximal expiratory flow-volume curve: Normal standards, variability, and effects of age. Am Rev Respir Dis 113:587600, 1976 12 Zamel N, Jones JG, Bach SM Ir, et al: Analog computation of alveolar pressure and airway resistance during maximum expiratory flow. J Appl Physiol 36:240-245, 1974 13 Schilder OP, Roberts A, Fry OL: Effect of gas density and viscosity on the maximal expiratory flow-volume relationship. J Clin Invest 42: 1705-1713, 1963 14 Hill AV: The heat of shortening and the dynamic constants of muscle. Proc Roy Soc BioI 126: 136, 1938 15 Hyatt RE, Black LF: The flow-volume curve: A current perspective. Am Rev Respir Dis 107:191-199, 1973 16 Zamel N, Kass I, Fleischli GJ : Relative sensitivity of maximal expiratory flow-volume curves using spirometer versus body plethysmograph to detect mild airway obstmction. Am Rev Respir Dis 107:861-863, 1973

CONSTANCY OF EFFORT AND VARIABIUTY OF IEFR 83