Comparing peak and sustained values of maximal respiratory pressures in healthy subjects and chronic pulmonary disease patients

ORIGINAL ARTICLE

Comparing peak and sustained values of maximal respiratory pressures in healthy subjects and chronic pulmonary disease patients^*

Antonio Fernando Brunetto^I; Luiz Antonio Alves^II

^IAssociate Professor the Physiotherapy Department of UEL

^IIResident Physician in Pulmonary Physiotherapy at UEL

^{Correspondence}

ABSTRACT

BACKGROUND: The measurement of maximal inspiratory and expiratory pressures is useful for the evaluation of pulmonary function. However, the methods to obtain them are not always properly described.

OBJECTIVE: To identify the difference between the peak pressure values (Ppeak, the highest pressure reached) and the sustained pressure (Pmaxs, the highest pressure sustained for one second) in MIP and MEP evaluation.

METHOD: 55 healthy individuals and 50 patients who were taking part in a pulmonary rehabilitation program, all of them with chronic pulmonary disease, were studied by recording their best maximal inspiratory pressure and maximal expiratory pressure tests. The peak and sustained pressure values were compared and analyzed to determine whether there was a difference between them.

RESULTS: The maximum inspiratory pressure records of healthy individuals showed that the maximal peak inspiratory pressure and the maximal inspiratory pressure found were, respectively, 102 ± 33 cmH₂O and 92 ± 29 cmH₂O (p < 0.001), and those of the patients participating in the pulmonary rehabilitation program were 75 ± 23 cmH₂O and 67 ± 22 cmH₂O (p < 0.001). The recorded values of maximal expiratory pressure, peak and maximal were 119 ± 42 cmH₂O and 110 ± 39 cmH₂O (p < 0.001), respectively, for healthy subjects, and 112 ± 40 cmH₂O and 103 ± 36 cmH₂O (p < 0.001) for the patients.

CONCLUSION: There is a significant difference between Ppeak and Pmaxs, that can lead to different interpretations in the evaluation of the respiratory muscle strength. To minimize interpretation errors, the authors suggest the use of devices which record both values (peak and sustained).

Key words: Pulmonary function tests. Respiratory muscles strength. Maximal respiratory pressures.

Acronyms and abbreviations used in this study

MIP – Maximum inspiratory pressure

MEP – Maximum expiratory pressure

Pmax – Maximum expiratory pressure sustained for one second

Ppeak – Peak maximum inspiratory or expiratory pressure

EPpeak – Peak maximum inspiratory pressure

IPpeak – Peak maximum expiratory pressure

Introduction

Measurement of maximum respiratory pressures (MIP and MEP, respectively inspiratory and expiratory) has gained importance due to its simplicity and great usefulness in the laboratory and in clinical and hospital settings to assess the strength of the respiratory muscles

In the first study that described the method, which continues to be the most widely used up to date, Black and Hyatt⁽¹⁾ utilized a set of aneroid manometers and recommended the assessment of maximum pressures after sustaining expiration or inspiration for at least one second, ⁽¹⁾ in an attempt to eliminate the interference of inertia from the respiratory system, also from the measurement equipment.^(2-4)

With few exceptions, studies on MIP and MEP records about the highest pressure sustained for at least one second do not generally specify how these values were established. Some authors stated that Pmax would be the pressure value achieved exactly after a one second effort.^(3,5,6) Enright et al⁽⁴⁾ reported IPpeak and EPpeak as being the MIP and MEP, respectively, assuring that their system of digital measurement system had no significant inertia.⁽⁴⁾ Hamnegård et al,⁽⁷⁾ using a portable digital system, reported the mean maximum pressure during one second as MIP and MEP, reaching with their method, slightly higher values than those with sustained Pmax, and stated that their methods better represent the maximum muscle strength; however, without finding a significant difference with regard to Ppeak.⁽⁷⁾

In clinical practice or even in reports in literature some discrepancies can be found regarding values of respiratory pressure. To carry out reading of the sustained pressure achieved with the aneroid manometers,⁽⁸⁾ is apparently difficult. These manometers are the most often used in daily practice, and some obvious errors are detected when manometers with drag indicators, or digital systems that only express the peak pressure are used ⁽⁹⁾ , or when values of peak pressures are registered, instead of the sustained ones.⁽⁴⁾

As such, this study aims to identify the differences between peak pressure measurements (Ppeak, the highest, but not sustained, measurement obtained) and sustained pressure (Pmax, the highest measurement sustained for one second), reached using a digital measurement system for the assessment of MIP and MEP in healthy subjects and patients with chronic pulmonary disease.

Materials and methods

Population

Fifty-five healthy subjects and 50 patients with chronic pulmonary diseases (chronic bronchitis, emphysema, bronchiectasis, and asthma) participating in a pulmonary rehabilitation program. were included in this study. At the time all patients were clinically stable, with no episodes of worsening or respiratory infection during the last two months, and all of them gave their informed consent for participation in the study.

Pulmonary function test

The pulmonary function test was performed using a standardized technique, according to the I Brazilian Consensus on Spirometry⁽¹⁰⁾, and normal values according to Knudson et al⁽¹¹⁾, with the Pony graphic (Cosmed, Rome/Italy) spirometer.

Assessment of maximum respiratory pressures

Maximum respiratory pressures were assessed according to the Black and Hyatt’s technique.⁽¹⁾

For MIP assessment, subjects should perform expiration almost to residual volume then they should place the mouthpiece between the lips and make a maximum inspiratory effort for about three seconds, having been previously oriented to make this maximum effort from the beginning and sustain it during the entire maneuver. For MEP assessment the process was similar, but in this case subjects had to perform maximum inspiration, almost to peak pulmonary capacity, and then perform a maximum expiratory effort, with the cheeks compressed by the examiner to avoid air leaks.⁽¹²⁾ For each measurement, ten acceptable and reproducible maneuvers were performed ^(13,14), with a one minute rest between each. The highest value was chosen for analysis as long as it was not the ⁽¹⁵⁾ the last one.

Prior to each maneuver, subjects were oriented about the technique, and had to remain seated for at least 10 minutes. During all maneuvers, subjects remained seated, with a nasal clip, and received constant standardized stimulation from experienced professionals, in addition to a visual feed­back on a computer screen.

Equipment used

The measurement and recording system had a cylindrical disposable mouthpiece with a 20 mm internal diameter, with a lateral 1.0 mm diameter opening for prevention of glottis closure and/or use of peri-buccal muscles; non-extendable 1m length trachea; air-liquid interface; digital pressure transducer ± 300 mmHg (Lynx Tecnologia Eletrônica, São Paulo/Brasil); amplifier and processor for pressure signs (Lynx); analogical-digital conversion plaque(Lynx); microcomputer PC 486DX2 with a data acquisition software Aqda­dos 4 (Lynx), adjusted to obtain pressure and time data, with a 100Hz frequency. Before each test, a mercury column was used for calibration.

Analysis of collected data

After collection, data was converted from mmHg to cmH₂O (1 mmHg = 1.31 cmH₂O) and transferred for analysis to an electronic spreadsheet (Microsoft Excel^TM) to calculate Ppeak (0.01s of registered pressure) and Pmax (1.0 s sustaining, as proposed by Black and Hyatt).⁽¹⁾ Absolute values were expressed as mean and standard deviations (X ± DP). Ppeak and Pmaxs examples are shown in Figure 1, a typical graph of an MEP test.

Comparisons between Ppeak and Pmax values were made by initial use of the Shapiro-Wilk’s test to determine normality of the distribution of values;, followed by Student’s t test for paired data (normal distribution) or Wilcoxon’s test (for abnormally distributed data), both with significance level of p < 0.05.⁽¹⁶⁾

This study was previously approved by the Ethics Committee of the Hospital Universitário Regional do Norte do Paraná.

Results

In the group of healthy subjects, the obtained MIP value (92 ± 29 cmH₂O) was statistically different from IPpeak (102 ± 33 cmH₂O), with a p value < 0.001. Patients with CPD had values of 67 ± 22 cmH₂O and 75 ± 23 cmH₂O for MIP and IPpeak, respectively, also statistically different, with a p value < 0.001. These data are shown in Figure 2.

The MEP achieved with healthy subjects was 110 ± 39 cmH₂O, with 119 ± 42 cmH₂O for EPpeak, disclosing a statistically significant difference, with a p value < 0.001. Patients with chronic pulmonary diseases had values of 103 ± 36 cmH₂O for MEP and 112 ± 40 cmH₂O for EPpeak, also statistically significantly different, with a p value < 0.001. These data are shown in Figure 3.

Discussion

Results of this study show that in the assessment of maximum respiratory pressures there is a significant difference between Ppeak and Pmax values. This difference is important for two reasons:

1) When Ppeak values are registered as MIP or MEP, respiratory muscles’ strength will be overrated, a relevant fact for patients with proven respiratory muscle weakness, This is the case of patients with chronic pulmonary disorders, as seen in the mean values disclosed in this study (MIP of 70 ± 22 mmHg for healthy subjects, and 51 ± 17 mmHg for patients with chronic pulmonary disorders). However, for patients weaning from mechanical ventilation, Ppeak is a parameter regularly used due to the low endurance capacity of these patients. ⁽¹⁷⁾

2) The degree of the difference will depend on the inertia from the respiratory system and from the measurement equipment that together may cause a sudden effort peak (overshoot).^(2,3,9) In this study, mean PIpeak was 11% higher than mean MIP in healthy subjects. Smyth et al,⁽²⁾ using a mercury column system and ­MIP sustained for 2-3 seconds, found a mean difference of 22% between IPpeak and MIP.⁽²⁾ Apparently, digital measurement systems have lower inertia when compared to aneroid manometers and to a mercury column system, although a comparison of the systems was not performed.

Because of its widespread utilization, maximum respiratory pressure assessment has been the subject of several studies, in an effort to standardize techniques and achieve normal values for the different populations. However, findings are not consistent because a consensus has not been reached about all factors involving these techniques. How such results are read certainly bears a major influence on their variability. For instance, Pmax values found in the current study are comparable to those of Neder et al (mean differences of 14% and 1% for MIP and MEP, respectively) ⁽¹⁸⁾ however very different from Black and Hyatt’s results (mean differences of 31% and 51% for MIP and MEP, respectively),⁽¹⁾ although both authors used the same technique and similar equipment, but with different populations.

Aneroid manometers are still the most widely used in clinical practice however, even when operated by experienced technicians special care should be taken. For some subjects it may be difficult to sustain high pressures for one second, or this may cause great fluctuations, thereby jeopardizing precise identification of the value sustained during a given period.⁽⁸⁾ The difficulty in calibrating aneroid manometers (users rarely do this), and the presence of drag indicators in some models are also interfering factors which, as they record Ppeak, induce the reading as Pmax.

Methods and equipment used in this study were carefully selected and/or adjusted so that all details or circumstances that might interfere with the results, such as learning effect, lips’ compression, and digital reading/interpreting system, were taken into account. Constraints of this system are based upon that fact that it relies on a computer system (hardware and software), not portable and costly. therefore requiring adequate training. A digital pre-programmed portable equipment would certainly redress such constraints, enabling assessment in all environments, including the bedside.

With this study it was concluded that there is a significant difference between Ppeak and Pmax values, both in healthy subjects and in patients with chronic pulmonary disorders that, if not taken into account, may lead to erroneous interpretations of maximum respiratory pressure values.

Received for publication on 09/12/02

Approved, after review, on 12/05/03

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*

This study was performed at the Physiotherapy Department of Universidade Estadual de Londrina – UEL, Londrina, PR. Financial support: CNPq.

Publication Dates

• Publication in this collection
  02 Dec 2003
• Date of issue
  Aug 2003

History

• Received
  09 Dec 2002
• Accepted
  12 May 2003