Lung 155, 23-32 (1978) LI; ® Spdnger-V~

Measurement of Maximum Inspiratory Pressure During Routine Spiromet *

R o b e r t G i l b e r t , J. H o w l a n d A u c h i n c l o s s , J r . , and S h a r o n B l e b

Department of Medicine, State University of New York, Upstate Medical Center, Syracuse, New York, U.S.A.

A b s t r a c t. Maximum inspiratory airway pressure (PI max) was mea- sured as part of routine spirometry in a series of 236 subjects. With men and women considered separately, PI max was tested for correlation with age and height, FEV I, FIV 1 and vital capacity. The best linear correlation was with FIV I. Prediction formulas for the lower limit are as follows. For men: PI max (cm H20) = - I0 + .021 FIV 1 (ml). For women: PI max =

- 6 + . 019 FIVI. Appropriate formulas are also presented using FEV 1 and vital capacity. Values for PI max below these predicted values can be con- sidered reduced out of proportion to the ventilatory defect with approximate- ly 95% confidence. Measurement of PI max may add useful information to the spirogram, but low values are not specific and must be interpreted in the clinical context.

Key words: Pulmonary function testing -Airway pressure - Mechanics of breathing - Muscles of breathing - Inspiratory muscles -Ventilatory muscles

Introduction

Although Rahn and associates described the measurement of maximum static airway pressures more than 35 years ago [13], these measurements have never become part of routine pulmonary function testing. Only a few reports of normal values are available [2, 7, 14], and predicted values for subjects with pulmonary disease have been based on total lung capacity or

residual volume [ 3, 5], measurements not ordinarily obtained during clini- cal spirometry.

For the past several years we have measured maximum inspiratory (PI max) and maximum expiratory (PE max) airway pressures as part of rou-

Supported in part by U.S. Public Health Service Grant HL05954

0 3 4 1 - 2 0 4 0 / 7 8 / 0 1 5 5 / 0 0 2 3 / $ 02. O0

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tine spirometry. The results for PI max in a series of 236 subjects are reported here with simple prediction formulas describing lower limits for

subjects with both normal and abnormal spirograms.

M~h~s

Spirometry. Simultaneous volume-time and flow-volume spirograms were obtained with a Med Science Wedge spirometer recording on a Tektronix storage oscilloscope [12]. Forced inspiratory and expiratory maneuvers were displayed on the storage oscilloscope and repeated until maximum re- producible curves were obtained. Maximum voluntary ventilation (IVfVV) was obtained by a standard technique using a Collins 13. 5 liter water-filled

spirometer. Predicted vital capacity (VC) was obtained by the formulas of Kory and associates [8, 9]; the predicted value minus I. 64 times the stan- dard error of estimate (SEE) was taken as the lower limit of normal [15]. Predicted values for maximum mid-expiratory flow (MMF) and flow rates at 50 and 25% of vital capacity (1750, 1725) were obtained from the formulas of Cherniack and Raber [6]; again the predicted value minus i. 64 times SEE was taken as the lower limit of normal. The lower limit of normal for MVV was taken as 80% of the predicted values of Baldwin and associates [I].

Maximum airway pressures. Pressure was measured following spirometry.

PI max and PE max were measured by the method of Black andHyatt [2]. Using a nose clip and a short, hard rubber or plastic mouthpiece, the sub- ject inhaled or exhaled forcibly into a rigid plastic cylinder 16 cm long and 1.5 cm in diameter. The distal end of the cylinder was closed except for a small orifice of i. 5 mm diameter which dissipates pressure generated by the cheek muscles. Pressure within the cylinder was measured by an aneroid gauge (Dwlfer Instruments, Michigan City, Indiana) connected by a short length of plastic tubing to the inside of the cylinder and calibrated in centimeters of water. The gauge was capable of measuring both positive (expiration) and negative (inspiration) pressure.

The breathing maneuver for PE max was started from the maximum in- spiratory point and that for PI max, from the maximum expiratory position. The maneuvers were repeated until the peak pressures stabilized or began

to fall off. The highest recorded values for each subject were used in the analysis.

PI max is subatmospheric and therefore should carry a negative sign. For simplicity this sign has been omitted.

Subjects

The subjects were 21 healthy volunteers plus 215 subjects (both inpatients and outpatients) with a variety of diseases or suspected diseases referred to the Pulmonary Function Laboratory. Although neuromuscular disease was not rigidly excluded by extensive clinical and laboratory evaluation, no pa- tient had overt or suspected neuromuscular disease.

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The 236 spirograms were divided into four groups: Group I. Normal, 53 men, 42 women. These subjects had normal values

for VC, MMF, V50, I725 and IVFVV. The ratio of one-second forced expi- ratory volume (FEV I) to VC (FEVI/VC, %) was greater than 75%, and the one-second forced inspiratory volume (FIV I) was greater than FEV I.

Group 2. Obstruction, 51 men, 33 women. These subjects had an FEVI/ VC, %, less than 70% and an FEV 1 less than the FIV I. Values for MMF were below the lower limit of normal and the expiratory flow-volume loop

was concave upward. Group 3. Restriction, 25 men, 19 women. These subjects had a VC be-

low the lower limit of normal and an FEVI/VC, %, greater than 75%. The expiratory flow volume loop showed a straight downslope or only a minimal

upward concavity of the terminal portion. Group 4. Small airway disease, 4 men, 9 women. These subjects had a

normal VC with an FEV I%/VC, %, greater than 75%. The MMF was below the lower limit of normal and/or the V50 and I725 were low. The separation of this group from Group 2 is admittedly arbitrary and the distinction may

be more of a quantitative one than qualitative. It is important to note that in the remainder of this report the terms nor-

mal, restriction, obstruction and small airway disease refer to the spiro- grams, not necessarily to the subjects. These spirograms performed by the Pulmonary Function Laboratory were consecutive except those elimi- nated because they failed to fall into one of the specific categories listed above, and those eliminated because they were repeated tests on the same

subjects.

Results

Normal spirograms. The linear correlations of PI max with age and height, FEVI, FIV 1 and VC were tested for men and women separately in the normal spirogram category. The only significant correlation was PI max vs. FIV 1 for the women, and even here the correlation was weak (R = 0. 34). When all 95 normal spirograms (men and women) were considered together, there

Table i. Linear correlation for PI max

M e a n (SD)

Men Women

89.7 cm H20 (37.0) 64.8 cmH20 (26.8) R SEE R SEE

age, hr. .489 32.4 .517 23.0

FIV 1 .737 25. 1 .709 19. 0

V C . 6 8 7 27. 0 . 697 19. 3

FEV 1 .697 26. 6 . 663 20.1

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were significant correlations with PI max for age and height, FEV I, FIV 1

and VC. The best correlation was with FIV 1 (R = 0. 530).

All spirograms. With men and women considered separately-, but all four

groups combined for each sex, PI max was tested for linear correlations

with FEVI, FIVI, VC and age and height. These correlations are listed in

Table I. The best correlation was with FIV I. The correlations with FEV 1 and VC were only slightly inferior to those with FIV I, whereas the corre-

lation with age and height was considerably poorer. The correlation which

did exist with age and height was due exclusively to the correlation with

height; age alone showing no correlation.

200

0 150

"1"

E o I00

50

0

0

PI max. • •

Men x O0 O0 • o o a ,

• -,F- • • ~ ~ •

. . x " '1'.11 ~ , %ox + :o ~ e Normal

OX 0 × X'~II XCII •qlNl_u~

o ao~o o x [ x •-~#- ' f o Obstructl0n

~?~o o ° ~ • x Restr,ct,on x xclR cOxO ~ • X~x°C~ _~r ~ + Small Airway

~,2 v / _

Disease

I000 2000 3000 4000 5000 6000

F l V l ml

Fig. I. PI max vs. FIV I for men. Symbols represent the four spirometric

groups, as shown. The line represents the predicted value for the lower

limit at the 95% confidence level

150,

0 I00

"I-

E 50

0

0

PI max. • •

Women o + 0

X

I000 2000 3000 4 0 0 0 5 0 0 0

F I V I ml

• Normal

o Obstruction

x Restr ict ion

+ Small Airway

D i s e a s e

Fig. 2. PI max vs. FIV 1 for women. Same format as Figure 1

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In Figures 1 and 2, PI max is plotted against FIV I, men and women plotted separately, with the symbols representing the four groups of spiro- grams. The linear correlation is apparent, although there is a suggestion of a parabolic relationship with an upward convexity. Regression equations for such a parabolic relationship (y = a +bx + cx 2) were calculated by least squares, yielding correlations very slightly better than the linear correla- tions. The differences between the linear and parabolic correlations were not considered to be of sufficient clinical importance to warrant the in- creased complexity of the parabolic relationship.

A comparison of the separate regression lines for men and women showed the slopes to be almost identical. However, the lines for men were significantly higher than for women; thus for any given value of FIV I, for example, the predicted value for PI max was greater for men than for wo- men.

To set an arbitrary level for the lower limit of predicted PI max, the deviation of each point from the regression line was calculated for each relationship (for example, PI max vs. FIV 1 for men), Deviations above the regression line were considered positive; those below were considered negative. The deviations were then ranked from the highest positive to the lowest negative. The lowest 5%, that is, those with the greatest negative deviations, were identified. The deviation of the point just above these was subtracted from the intercept of the regression equation to give a new intercept. This established a new line, parallel to but below the regression line, which cut off the lowest 5% of the values. These lines are shown in the figures; the values for the slopes and intercepts of these lines are shown

in Table 2 for FIV I, FEV 1 and VC. For any individual, therefore, a pre- dicted value can be obtained for the lower limit based on FIVI, FEV 1 or VC. Observed values below these can be considered reduced out of pro- portion to the ventilatory defect with approximately 95% confidence.

Two further considerations should be kept in mind when applying these formulas. First, no patient in the study had a value of FIV 1 or VC below 600 ml, and therefore these formulas are not valid below these levels. Second, only one man had a value of PI max below 25 cm H20, and only two w.omen had values below 20 cm H20. Values below these should be con- sidered reduced out of proportion to the ventilatory defect, even for those subjects whose predicted values using the regression formulas are below them.

Reproducibility. In I0 patients with obstructive disease, a second spiro- gram was available with values of FEVI, FIVI andVC within 10% of those in the spirogram used for the initial analysis. The average absolute dif- ference between the values for PI max in the paired spirograms was 14 cm H20 (SD, 9.9).

Comparison with other studies. Our technique closely approximates that of Black and Hyatt [2]; Table 3 compares the average values for PI max and PE max in the normal subjects of the two studies. The results are almost identical for PI max; for PE max our values are considerably below those of Black and Hyatt. Our values for PI max in normal subjects are also very similar to those reported by Cook, Mead and Orzalesi ~7]; for PE

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Table 2. Prediction formulas for lower limit for PI max (y = a + bx)

y, cm H20 X, ml a b

Men

PI max FIV 1 -I0

PI max VC -17

PI max FEV 1 4

Women

PI max FIV 1 6

PI max VC 9

PI max FEV 1 3

• 021

• 019

• 022

• 019

• 019

• 018

Table 3. Comparison with study of Black and Hyatt [ 43 ~

Age 20-54 > 54

B , H P B , H P

P I m a x M 124 119 103 104

P I m a x F 87 82 71 76

Pressures in cm H20; values represent mean pressures for each age group. B,H = study of Black and Hyatt, P = present study, F = female,

M = male.

max our values are slightly below those of this latter group. The se r ies of Cook et al. was more heavily weighted toward younger subjects than ours, which might explain some of the discrepancy in PE max data.

PI max and PE max correlated closely for individual subjects (R = 0.87), but PE max showed much greater variability for comparable levels of pul-

monary function and age and height• Furthermore, since our average values

for PE max in normal subjects were not as high as those reported by Black and Hyatt [2] and Cook et al. [7], PE max appears to be more susceptible

to subject selection and minor variations in technique. For these reasons,

the results for PI max are stressed in this report.

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Discussion

We found that PI max is reduced as ventilatory function is impaired; this reduction is relatively independent of the nature of the disease process. Correlation coefficients between PI max and standard parameters of ven- tilatory function such as VC were generally between 0.6 and 0.7, indicating

that about 35 to 50% of individual variation could be accounted for by the variation in ventilatory function. The remainder of the variability may be due to differences in respiratory muscle strength independent of the dis- ease process, differences in motivation, skill in performing ventilatory maneuvers, general health and probably as yet unidentified factors. Leith and Bradley [I0] have demonstrated that training can increase ventilatory muscle strength, but this would not appear to be a factor in our work, since none of the normal subjects or patients was trained.

The finding of comparable decreases in PI max in obstructive and re- strictive spirograms and the continuity with normal subjects and those with small airway disease is an empirical finding not easily explained by a con- sideration of the mechanics of breathing alone. The highest values for PI max are generated at low lung volumes. In obstruetive disease, therefore, the elevated residual volume would be expected to lower PI max. In re- strictive disease, however, although residual volume is decreased, the increased elastic recoil usually present in this type of defect would result in the need for a greater muscular effort to produce a given value of PI max. This might lead to lower values of PI max, especially if the under- lying disease produced generalized debility of which ventilatory muscle weakness was one part.

Relating Pl max to percent total lung capacity or percent predicted total lung capacity might have modified some of the apparent decrease in venti- latory muscle strength, especially in the obstructed patients. Measure- ment of total lung capacity, however, is not part of routine spirometry; the purpose of this report is to present indices which can be applied to rou- tine spirometry plus measu~'ement of PI max. The measurement of PI max takes no more than one to two minutes and should not add to the cost of the spirometry.

Although FIV 1 is measured routinely in our laboratory, this is not done universally. Table 2 presents prediction formulas utilizingFEV 1 andVC which can be used with only a slight loss of accuracy.

Byrd and Hyatt [5] studied 31 males with chronic obstructive airway disease (COAD) and did not find reductions in maximum pressures as com- pared to normal subjects; such results are at variance with ours. In fact, these authors concluded that inspiratory muscle strength might actually be increased in COAD. In reaching this conclusion these investigators related PI max to percent predicted total lung capacity. For the reasons stated above, we have not done this. Furthermore, none of their COAD patients had a VC below 1600 ml, whereas more than one-third of our COAD pa- tients had lower values. Also all their patients were stated to be ambula- tory, presumably studied as outpatients, whereas many of ours were stud- ied during hospital admissions for a variety of illnesses. The patient se- lection and method of analysis is probably the reason for the discrepancy in the results of the two studies.

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Braun and Rochester [4] have recently reported a decrease in PI max in patients with obstructive airway disease. This decrease was roughly proportional to the severity of the disease, a finding similar to ours. They attribute the reduction to the mechanism noted above, namely, the mechan- ical disadvantage of the inspiratory muscles at an elevated residual volume.

We have not used anthropometric data such as age and height to calculate predicted values; rather we used the results of other pulmonary function tests for two reasons. First, the correlations with age and height were much weaker than those with FEVI, FIV 1 or VC. Second and more impor- tant, we believe that the critical question is whether the value of PI max is reduced beyond what would be expected from the disease process alone.

The FIV 1 is probably the most effort-dependent of the routine spirometric measurements over a wide range of disease processes. It is not surprising that this value showed the best correlation with maximum airway pressures, since these latter are, of course, extremely effort-dependent.

A second nonstandard approach to the prediction formulas is to not base the lower predicted limit on some multiple of the standard error of esti- mate (SEE). In a performance test such as that of maximum airway pres- sures, unusually high values should not be considered abnormal, and yet they will increase the range of the SEE. Even using i. 64 times the SEE, as suggested by Sobel [15], rather than twice the SEE to define the lower 5%, will not eliminate the effect of unusually high values on the SEE. We

have chosen therefore to allow the 5% of subjects furthest below the regres- sion line to establish a 95% level below which values of- PI max are con- sidered reduced out of proportion to the ventilatory defect.

Although the detection and assessment of neuromuscular disease involv- ing the ventilatory apparatus is considered a prime use for the measure- ment of maximum airway pressures [3, ii], most abnormally low values will probably be due to other factors. Some of these include lack of motiva- tion, generalized weakness, lack of understanding of what is expected and lack of skill in performing respiratory maneuvers. We have studied several subjects with skeletal muscle weakness owing to Guillain-Barr& disease or amyotrophic lateral sclerosis. These patients had normal values for PI max. However, they also had normal spirograms and no respiratory com- plaints, and the finding of normal maximum airway pressures is not sur-

prising. Two patients with myasthenia gravis and one with amyotrophic lateral sclerosis had severe ventilatory insufficiency. All three had low values for both vital capacity and maximum airway pressures, and these measurements fluctuated together as the disease waxed and waned. We. have not as yet found patients with neuromuscular disease where these two types of measurements were dissociated, although such cases have been reported [ 3].

We believe that a value for PI max below the limit predicted by the ven- tilatory defect gives added but nonspeeific insight into a ventilatory problem. Additional information will be required to pinpoint the reason for the low pressure. A value within the range determined by the prediction formulas given here is more difficult to interpret if other parameters of pulmonary function are abnormally low. It is possible that weakness or poor motiva- tion, for example, could be lowering all measurements, including those

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on which the predicted value of PI max is based, and therefore the actual value will be above the predicted lower limit despite its being unusually low.

This latter problem cannot be solved completely by the results of this study, but in some cases this phenomenon can probably be excluded. Val- ues of PI max above the regression line for pressure vs. the parameter used as a guide, such as FIV 1 (not the line for the lower limit), place the subject in the upper half of the group for maximum pressures. It seems unlikely that low values for other functions would be due to weakness, poor

motivation, etc. , in a subject who could generate maximum pressures above the average for his predicted value. For men, the regression lines for predicted values were about 35-40 cm H20 higher than the lines for the lower limit; for women, this value was approximately 25-30 cm H20. Therefore if a subject with low spirometric values was able to generate values of PI max 30-40 cm H20 above the lower predicted limit, it is un- likely that the low spirometric values were due to weakness, poor motiva- tion, neuromuscular disease, etc. Values in the lower half of the predicted range are more difficult to interpret.

In suggesting the measurement of PI max for routine spirometry, some consideration must be given to the risks. Although changes in cardiovascu- lar dynamics must occur, since the entire maneuver takes only three or four seconds, these are brief. During the performance of hundreds of the measurements in all types of patients, we have encountered no adverse reactions, nor have any been reported from other centers. Nevertheless, in patients with borderline cardiovascular compensation, or frank decom- pensation, potential risks might outweigh the benefits.

References

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3. Black, L.F., Hyatt, R.E. : Maximal static respiratory pressures in generalized neuromuscular disease. Am. Rev. Resp. Dis. 103, 641- 650 (1971)

4. Braun, N. M. T., Rochester, D.F. : Respiratory muscle function in chronic obstructive pulmonary disease. Am. Rev. Resp. Dis. 115, 91 (part 2) (1977) (abstract)

5. Byrd, R.B., Hyatt; R.E. : Maximal respiratory pressures in chronic obstructive lung disease. Am. Rev. Resp. Dis. 98, 848-856 (1968)

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7. Cook, C.D., Mead, J., Orzalesi, IV[.M. :Static volume-pressure char- acteristics of the respiratory system during maximal efforts. J. AppI. Physiol. 19, 1016-1022 (1964)

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8. Kory, R.C., Callahan, R., Boren, H.G., Syner, J.C. :The Veterans Administration - Army cooperative study of pulmonary function, i. Clinical spirometry in normal men. Am. J. Med. 30, 243-258 (1961)

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Accepted for publication: 8 December 1977

Robert Gilbert, M.D. State University Hospital 750 East Adams St. Syracuse, NY 13210, USA