Best Way to Measure MIP?
August 1, 1999 3 minutes read
Best Way to Measure MIP?
Abstract & Commentary
Synopsis: In a study of 54 patients comparing two methods of determining maximum negative inspiratory pressure (MIP), use of a unidirectional expiratory valve produced more negative values than those obtained by instructing the patient to exhale to residual volume and inhale maximally against a closed airway.
Source: Caruso P, et al. Chest 1999;115:1096-1101.
In this study evaluating different techniques for measuring maximum inspiratory pressure (MIP), 54 patients were evaluated while undergoing weaning from mechanical ventilation. They had been ventilated for an average of 22.3 hours, with a range of 4-152 hours. The patients’ average age was 63 years and most were recovering from surgery. About half the patients were male. Only about 10% of the patients suffered from acute lung injury. MIP was determined three times in each patient using one method, then after a 20-minute rest period, the other method was used for three determinations. The largest (most negative) value of the three attempts was used for comparison. The order in which the methods were applied in each patient was randomly determined.
The average MIP was about 64 mmHg using the unidirectional valve, and only about 51 mmHg with the single maximum effort breath method. Variation between the three attempts with each determination averaged 10-13% and was not significantly different between the two methods or the order they were applied. Either method was internally consistent, delivering reproducible results. However, the unidirectional valve produced significantly greater (more negative) values in most patients.
Comment by Charles G. Durbin, Jr., MD, FCCM
The MIP is one of a series of weaning parameters that may be useful in determining whether a patient is ready for extubation. This variable reflects diaphragm strength but is influenced substantially by patient cooperation, drug use, the measurement device used, and by the technique used to make the measurement. This study is important for several reasons. The superiority of the unidirectional valve method described by John Marini (J Crit Care 1986;1:32-38) over the single-effort breath is confirmed. The consistency and reproducibility of values obtained by this method are also convincingly demonstrated. A surprising degree of reproducibility was also seen with the single-effort-breath method. However, the almost 30% difference in absolute values obtained is both statistically significant and clinically relevant.
The reason for the difference may be related to the type of patients studied. Because these were primarily surgical patients, the presence of pain and the drugs used to treat it may have been contributing factors. Only three of the patients had MIPs less than 20 mmHg, often considered a critical value. The difference between the two methods may be less in more marginal patients, although information about this was not reported in this paper. Also, the outcome following extubation was not reported in this study.
Several patients had more negative values with the single maximum effort breath method than with the unidirectional valve. Were these the "weak" patients who were stressed to fatigue with the unidirectional valve method? It is not possible to tell from this report. One clinical observation with the unidirectional value technique is that if successive inhalation efforts generate a diminishing negative pressure, the measurement technique should be abandoned to avoid the potential for fatigue.
This report suggests that determination of MIP with a unidirectional exhalation valve produces more negative and, therefore, more accurate values. Studies reporting the use of MIP should be suspect if they did not obtain the value by using this technique. Whether the large discrepancy noted in this study of relatively strong patients persists in weaker, more marginal patients remains to be tested.
Factors increasing the inconsistency of determination of MIP include:
a. advanced patient age.
b. female sex.
c. presence of acute lung disease.
d. following surgery.
e. patient cooperation.
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