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(Chest. 1999;115:1096-1101.)
© 1999 American College of Chest Physicians

The Unidirectional Valve Is the Best Method To Determine Maximal Inspiratory Pressure During Weaning*

Pedro Caruso, MD; Celena Friedrich, RT; Silvia DC Denari, RT; Soraia AL Ruiz, RT and Daniel Deheinzelin, MD, PhD FCCP

* From the Unidade de Terapia Intensiva, Centro de Tratamento e Pesquisa Hospital do Câncer, Hospital AC Camargo, São Paulo, Brazil. Supported by Fundação Antonio Prudente.


   Abstract
TOP
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Objectives: Although maximal inspiratory pressure (MIP) is used as an index of inspiratory muscular strength, there is no consensus on how to measure it. We compared, during weaning from mechanical ventilation, two methods of measurement to determine which shows the greater values (MIPbest) and is more reproducible. One method measured MIP when negative pressure was maintained for at least 1 s after a forceful expiration, and the other method measured MIP with a unidirectional expiratory valve (MIPuni).

Design: The study had a crossover design, and patients randomly performed three measurements of each method (t1). The procedure was repeated by the same observer after 20 min (t2). The maximal value in each method was considered.

Setting : ICU, Hospital A.C. Camargo, São Paulo, Brazil.

Patients: Fifty-four consecutive patients undergoing short-term mechanical ventilation who became eligible for the study when their physicians decided to restore spontaneous breathing.

Results: MIPbest values were arrived at using MIPuni 75% of the time either in t1 or t2. MIPuni yielded a higher average of MIPbest values in t1 and t2 (p < 0.0001). The effort-to-effort coefficient of variation of one method compared with the other during t1 and t2 was similar (p > 0.2 for t1; p > 0.8 for t2). Also, when comparing t1 and t2, the coefficients of variation were similar for each method (p > 0.62).

Conclusions: Because MIPuni displayed the maximal values, it is the best method for estimating MIP in patients undergoing short-term mechanical ventilation. The reproducibility of consecutive measurements was similar between the methods, even after a short period of time.

Key Words: maximal inspiratory pressure • mechanical ventilation • respiratory monitoring • respiratory muscle • ventilator weaning


   Introduction
TOP
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Maximal inspiratory pressure (MIP) is an effective method for evaluating inspiratory muscle strength.1 It is used as a diagnostic tool,2 as a predictive mechanical index,3 ,4 ,5 ,6 ,7 and as a part of other indexes (eg, tension time index). However, the lack of guidelines for the measurement of MIP in mechanically ventilated and spontaneously breathing patients contributes to irregular results and infrequent use of MIP. There is no consensus on the best method for measuring it.

The most common method is to measure MIP when a negative pressure was maintained for at least 1 s after a forceful expiration to residual volume (MIPsta).1 ,2 ,3 ,6 ,8 ,9 ,10 ,11 ,12 ,13 ,14 In this method, MIP is measured with the patient near the residual volume (RV) after a maximal expiration. The pressure is maintained for at least 1 s against an occluded airway, and expirations are repeated until they are technically satisfactory. It is not easy to perform the measurement in mechanically ventilated patients because the patients may not assimilate instructions well, may be sluggish or noncompliant, and as a result it may not be clear whether accurate measurements of the MIP have been accomplished. Marini et al15 described a method in which MIP was measured with a unidirectional expiratory valve (MIPuni) where low resistance was used to selectively permit exhalation while inspiration was blocked. This caused patients to initiate successive efforts from respiratory volumes progressively closer to the RV, a known factor that helps to generate a more negative pressure. However, reproducibility was not assessed, and it is not clear whether the patients were studied immediately prior to extubation.

We hypothesized the following: MIPuni would yield larger values, considering that patients would be closer to the RV and would have a greater hunger for air (because the respiratory drive increases as occlusion continues); and MIPuni would yield more reproducible consecutive measurements because it requires less coordination between patient and observer, it involves a physiologic response (increased respiratory drive with blocked inspiration), and its larger values could lessen the coefficient of variation. Given the above hypotheses, we designed a protocol to study which of the two methods would yield the larger value and be more reproducible during the weaning of patients from short-term mechanical ventilation.


   Materials and Methods
TOP
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Patients
In order to achieve an 80% power test and a 5% significance level with a crossover design, we studied 54 consecutive patients who were undergoing short-term mechanical ventilation in an oncologic ICU. Patients were eligible for the study when their physicians decided to restore spontaneous breathing. All patients were hemodynamically stable and were able to trigger the ventilator in pressure support ventilation, assist/control, or synchronized intermittent mechanical ventilation modes. No patients had contraindications for undertaking forceful inspiratory maneuvers (ie, suspected intracranial hypertension or flail chest). Patients were studied at least 2 h after the administration of any sedative.

Materials
We used an aneroid manometer (Record; São Paulo, Brazil) capable of registering pressures up to -150 cm H2O. The manometer was calibrated immediately before the beginning of the protocol and two times during 90 days (the length of the protocol). To measure MIPsta, the manometer was attached to a T tube, which had one end free and the other end attached to the patient's endotracheal tube or tracheostomy. When we performed MIPuni, one side of the manometer was attached to the patient's endotracheal tube or tracheostomy, and the other side was attached to a unidirectional, low-resistance valve that only allowed expiration.

Protocol
The protocol was randomized and used a crossover design. Prior to the measurements, respiratory tract and oropharyngeal secretions were suctioned. The patients then were placed in a semi-recumbent position at 45°. None of the ventilator parameters were changed during the measurement period. The patient was disconnected from the ventilator and was attached to the aneroid manometer. According to a computer-generated random table, either MIPuni or MIPsta was used for the first three measurements, after which the investigator performed another three measurements using the other method (t1). Then, 20 min later, patients were reconnected to the ventilator with the same parameters, and the same investigator repeated the two series of three measurements using the same methods in the same order as during the first time period (t2).

MIPuni was measured by connecting patients to the manometer during a 20-s period and recording the maximal value. After maximal expiration, MIPsta was recorded with the free end of the T tube manually occluded. The peak negative pressure, if maintained for at least 1 s, was recorded during maximal inspiration. During both procedures, all patients were encouraged to make maximal efforts. The protocol was conducted in accordance with the ethical standards of our institution's committee on human experimentation.

Data Analysis
To determine which method would display the MIPbest, we analyzed the maximal value of each set of triplicate measurements. Comparisons between the means of the maximal values of MIP in t1 and t2 as well as the study of the residual effect of one method over the other, were performed through one-way analysis of variance (ANOVA) with Bonferroni adjustment for multiple comparisons. The coefficients of variation (variance/mean) for each method were compared using a paired Student's t test.


   Results
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 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
The population characteristics are shown in Table 1 . The majority of the patients studied were surgical patients undergoing short-term mechanical ventilation. Of the largest values, most were obtained using the unidirectional valve (76% in the first setting and 81.5% in the second) and < 10% were obtained using the "static" method (Fig 1 ). Because in each set of measurements (t1 and t2) the methods were performed sequentially with a short time interval, we first analyzed whether there was any residual effect of one method on the other. We found this not to be the case (ANOVA, p > 0.05). The averages of the MIPbest values resulting from both methods and in both periods differed significantly (ANOVA, p < 0.00001 with Bonferroni adjustment). During t1, the average (± SD) MIPuni value was 64.39 ± 20.45 cm H2O, and the average MIPsta was 50.33 ± 16.93 cm H2O (ANOVA, p < 0.001 with Bonferroni adjustment). During t2, the average MIPuni was 65.39 ± 20.50 cm H2O, and the average MIPsta was 50.76 ± 17.44 cm H2O (ANOVA, p < 0.0001 with Bonferroni adjustment). The average MIPbest in each method was the same in t1 and t2 (p > 0.79 for MIPsta and p > 0.54 for MIPuni). In t1 and t2, the MIPbest values recorded by MIPuni were approximately 28% higher than those recorded by MIPsta (Fig 2 ).


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  		Table 1. Population Characteristics

 


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  		Figure 1. Graph showing that MIPuni > MIPsta in 75.9% of the measurements in t1 and in 81.5% of the measurements in t2; MIPuni = MIPsta in 14.8% of the measurements in t1 and in 9.3% of the measurements in t2; and MIPuni < MIPsta in 9.3% of the measurements in t1 and in 9.2% of the measurements in t2..

 


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  		Figure 2. Graph showing the average MIPbest using both methods in t1 and t2. In t1, MIPuni = 64.39 and MIPsta = 50.33 (ANOVA, p < 0.001with Bonferroni adjustment). In t2, MIPuni = 65.39 and MIPsta = 50.76 (ANOVA, p < 0.0001 with Bonferroni adjustment).

 
The effort-to-effort coefficient of variation (three consecutive measurements) during t1 was 13.03 ± 9.39% for MIPuni and 13.76 ± 9.65% for MIPsta (Student's t test, p > 0.2). During t2, the effort-to-effort coefficient of variation of MIPuni was 11.81 ± 9.85% and 10.66± 7.36% for MIPsta (Student's t test, p > 0.8). The coefficient of variation for the maximal value of MIPuni between t1 and t2 was 10.88 ± 9.27%. The value for MIPsta was 11.59 ± 9.61% (p > 0.62), showing good reproducibility of measurements at short intervals (Table 2 ).


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  		Table 2. Effort-to-Effort Coefficient of Variation and Coefficient of Variation After 20 min*

 

   Discussion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
We designed this study to determine which of the two methods is the best way to measure MIP in a mechanically ventilated patient. To accomplish this, we compared both the absolute values obtained in each method and the reproducibility of these values over short time intervals and with consecutive measurements. Two studies16 ,17 compared MIPuni with a modified MIPsta (occlusion was maintained for 20 s). Although both demonstrated larger values for MIPuni, reproducibility was not studied, and they used a manometer with a maximum measurable pressure of 60 cm H2O, which was less than the mean observed during our study. Also, in one of the studies, patients were not studied at the moment of extubation.17

In our study, MIPuni displayed significantly larger values than MIPsta. There are some possible explanations for this finding. These include an increase in the respiratory drive during the maneuver and an improved effectiveness of the respiratory system. An increase in the respiratory drive could have been caused by our blocking the patient's inspiration and adjusting the stimulation to a higher level after the previous "inefficient" inspiration. Indeed, when facing an increased mechanical load, the respiratory drive increases to maintain adequate alveolar ventilation.18 In normal persons, the chemical stimulus to breathe increases exponentially as the duration of apnea increases,19 and the duration of breath-holding is shortest at low lung volumes.20 In MIPsta, in contrast, the drive was more dependent on the patient's collaboration than on the patient's physiologic response. Another possible reason for the increased MIP is that MIPuni was probably performed at a pulmonary volume closer to the RV, which is, as previously described, a factor in increasing inspiratory pressure.21 During MIPuni, patients should have used a smaller pulmonary volume because we used a valve that blocked inspiration, and the maneuver lasted 20 s, thereby forcing patients to progressively reduce pulmonary volumes. Lack of coordination between patient and observer is an additional difficulty in using MIPsta. Notwithstanding these possible explanations, we cannot draw a definite conclusion about the reason that MIPuni showed significantly higher values than MIPsta because we have not measured the pulmonary volumes and respiratory drive.

The usual number of measurements for MIP estimation is three, although more maneuvers could be necessary to achieve maximal results; usually 10 or more are necessary.8 ,22 Despite the fact that there are studies showing that 10 or more maneuvers are necessary, which is in part due to a "learning effect,"8 these maneuvers were conducted with ambulatory patients and without a unidirectional valve. Because mechanically ventilated patients are more susceptible to fatigue, respiratory distress, and poor cooperation, the results in ambulatory patients might not be applicable to ICU patients.

Other factors limiting the depth of a maximal inspiration are the antagonistic actions of abdominal muscles, glottic closure, and respiratory weakness preventing the patient from reaching the RV.23 ,24 However, glottic closure did not occur in the present study because patients had an endotracheal tube or tracheostomy when the study was conducted.

Because MIPuni involves less coordination between patient and observer, encompasses a physiologic response (increased drive after a previous inefficient inspiration) thereby demanding less patient cooperation, and is closer to MIP, we expected that this method would be more capable of consecutively reproducible results than MIPsta; however, this was not the case. Consecutive measurements showed good reproducibility, both when MIPsta was used in previous studies10 ,11 and when both methods were used in our study, with coefficients of variation that averaged 10 to 13% when the study was performed by a single observer. However, using different observers25 and patients that are more ill does not guarantee good reproducibility. Aldrich and Spiro9 showed that maximal and submaximal efforts using MIPsta have the same reproducibility. This is most likely due to the discomfort evoked by the muscular activity required for MIP efforts having a certain reproducible threshold. Nevertheless, both methods increased their reproducibility after 20 min, although this was not statistically significant (Table 2 ).

We used a time period of 20 min between tests because the measurement of MIP is a brief maneuver that requires strength and often causes high-frequency fatigue, and that is usually followed by rapid recovery within 10 min.26 All patients tolerated both procedures well, and no side effects, particularly negative pressure pulmonary edema, were observed during the study.

Is MIP a useful measurement in the ICU? It can be used as an index in weaning patients from mechanical ventilation or as a diagnostic tool. Some studies3 reported MIP to be a good index with which to decide when to wean patients from mechanical ventilation, whereas other studies not only failed to confirm this, but also showed that MIP was less accurate in predicting success than other indexes.7 ,27 However, when muscular weakness or neuromuscular disease was the only or principal cause of respiratory failure (eg, Guillain-Barré syndrome and polymiositis), MIP could be used as the principal weaning index.2 MIP is also useful for quantifying the degree of respiratory muscle strength in patients with dyspnea, respiratory failure, or weaning failure.12 A problem in using MIPuni as a diagnostic tool is the lack of normal values because most studies performed with healthy volunteers used MIPsta.1 ,13 ,14 However, this is less important when MIPuni is used as a weaning index because its described power is derived from statistical methods, generally, receiver operating characteristics curves.7 ,28

The major limitation of the present study is the type of patient included. The majority were surgical patients undergoing short-term mechanical ventilation. Only three patients presented with an MIP < -20 cm H2O, a value recognized as borderline for successful weaning. This only occurred when we used MIPsta. Therefore, extrapolations from our results must be done with caution.

In conclusion, MIPuni displays values that are 30% higher than those of MIPsta; hence, it discloses values probably near the real MIP. The reproducibility and safety of both methods are good and are equal to consecutive and short time interval measurements. Based on our findings, the use of MIPsta as a diagnostic component or as a predictive index must be re-evaluated.


   Footnotes
 
Correpondence to: Pedro Caruso, Alameda Anapurus 631, apto 131, São Paulo, SP, Brasil CEP 04087 001; e-mail: pchold@usp.br

Abbreviations: ANOVA = analysis of variance; MIP = maximal inspiratory pressure; MIPbest = maximal value of inspiratory pressure obtained after three measurements; MIPsta = maximal inspiratory pressure measured when a negative pressure was maintained for at least 1 s after a forceful expiration to residual volume; MIPuni = maximal inspiratory pressure measured with a unidirectional expiratory valve; RV = residual volume.

Received for publication April 22, 1999. Accepted for publication October 14, 1998.


   References
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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