Abstract
BACKGROUND: The measurement of the maximal inspiratory pressure (PImax) is of great importance in choosing the time for the start of weaning. OBJECTIVE: To measure the inspiratory pressure in mechanically ventilated patients suitable for weaning to determine the point at which the PImax is achieved within 60 seconds of observation, and analyze factors associated with PImax values.
METHODS: Measurement of PImax was accomplished with a digital vacuometer with a unidirectional valve, which allows only exhalation (PImaxUV). With this technique, values are registered and stored, remaining accessible whenever necessary. All patients were on mechanical ventilation, and met the criteria recommended by the American Thoracic Society/European Respiratory Society in 2007 to undergo weaning trial.
RESULTS: Eighty-four from the 87 enrolled patients completed the test. No patients reached the PImaxUV in the first 20 seconds of observation. PImaxUV was achieved between 20.1 and 40 seconds in 12 patients (14.0%), and between 40.1 and 60 seconds in 72 cases (86.0%). In a multivariate analysis model in which age, sex, days of mechanical ventilation, APACHE score, and respiratory drive (measured as airway-occlusion pressure 0.1 s after the start of inspiratory flow [P0.1]) were included, only age (P = .006) and P0.1 (P = .003) were significantly associated with the values of PImaxUV.
CONCLUSIONS: Within an observation period of 60 seconds, the majority of patients reached the maximal inspiratory peak between 40.1 and 60 seconds. Older patients were found to have lower PImaxUV values, whereas higher values for P0.1 strongly correlated with higher PImaxUV values. These findings are potentially useful to improve successful weaning prediction in the future, but further studies are needed to better clarify this issue.
Introduction
Maximal inspiratory pressure (PImax) can be defined as the maximum pressure generated during inspiration against an occluded airway. Measurement of PImax has been used in the clinical setting to assess inspiratory muscle strength for a long time,1,2 but the subject is still a matter of controversy. Although some ventilator-dependent patients may display poor cooperation when executing voluntary maneuvers, values of PImax have been found to be useful for predicting successful weaning in some studies.3,4
Marini and co-workers suggested an approach for standardization of the operational procedure to measure PImax in such a setting. They used a unidirectional valve to permit exhalation while inhalation was blocked, thereby allowing patients to perform the maximal inspiratory effort at a lung volume approaching residual volume. Values of PImax with use of the unidirectional valve (PImaxUV) are expected to be maximal.5
Despite that, a subsequent study employing such a standardized approach has shown that the reproducibility of PImax values in ventilator-dependent patients is poor. “True” PImax in intensive care unit patients is often significantly underestimated and dependent on both patient and investigator. Even highly reproducible PImax measurements at any one sitting do not reliably reflect maximal efforts.6–9 A clear disadvantage of this approach, which employs analogical vacuometer, is the manual computation of the PImax values, which lacks accuracy due to the inherent investigator dependence.6,8,9 Another matter of controversy is the duration of observational period to yield reliable PImax values, which ranges from 20 to 60 seconds in different reports.6–13
This study aims to measure the inspiratory pressure within 60 seconds of observation on mechanically ventilated patients to determine the time needed to achieve the PImaxUV and to study the factors associated with such values. To circumvent previous limitations, we resort to a digital vacuometer, which automatically records the test results.
QUICK LOOK
Current knowledge
The measurement of maximal inspiratory pressure is an important indicator of respiratory muscle strength.
What this paper contributes to our knowledge
In mechanically ventilated patients the maximal inspiratory pressure value is achieved between 40 and 60 seconds. Older patients have lower maximal inspiratory pressure values.
Methods and Patients
Patients who were clinically stable and ready to undergo a weaning trial by their primary physicians were enrolled in this prospective observational study. Patients were selected from 2 critical care units in Niterói, Rio de Janeiro, Brazil. The study was approved by the ethics committee of the Medical School/Hospital Universitário Antônio Pedro of Universidade Federal Fluminense, under the number 259/09, and informed consent was obtained from each patient, whenever possible, or from the patient's next of kin.
To be included, subjects had to be older than 18 years and on mechanical ventilation for at least 24 hours. Patients were accepted for enrollment if they met the guidelines of the American Thoracic Society/European Respiratory Society (2007) to start weaning trials.14 Exclusion criteria were as follow: tracheal stenosis, intracranial pressure > 20 mm Hg, sedation, post-abdominal surgery risk evisceration, overt cardiac failure or hemodynamic instability, and signs of infection/reinfection. Ventilators used were the 840 (Covidien-Nellcor and Puritan Bennett, Boulder, Colorado) and Servo-s (Maquet, Rastatt, Germany), and endotracheal tube internal diameters varied from 7.5 mm to 8.5 mm.
Procedures
Measurements were planned to be performed for a 60 second period before spontaneous breathing trials. PImax was obtained using a unidirectional valve method and a digital vacuometer (MVD 300, Globalmed, Porto Alegre, Rio Grande do Sul, Brazil). The device has a scale of 300 cm H2O, with a 1 cm H2O interval, and is designed to perform measurements each 100 ms. Before testing, all patients were under pressure-support ventilation mode. Patients were not on sedatives and were allowed to breath spontaneously for 10 seconds before engaging in the test. They were positioned at 45°, had their airway cleared, and were pre-oxygenated for 2 minutes with an FIO2 of 1.0.9,12,15 The orotracheal tube cuff was hyperinflated to prevent air leakage during mensuration.9 Patients were then disconnected from mechanical ventilation and allowed to breathe spontaneously for 10 seconds before connection of their endotracheal tube to the digital vacuometer, when they were at functional residual capacity level.9,15 Patients were continuously monitored with pulse oximetry and electrocardiogram, and had continuous respiratory physiotherapist attention. Measurements were digitally recorded, allowing easy access to values of parameters whenever needed. Respiratory drive (measured as airway-occlusion pressure 0.1 s after the start of inspiratory flow [P0.1]) was calculated as the mean of the values of inspiratory pressure at 0.1 seconds from the first 3 respiratory cycles. Patients were not coached for the maneuver. A typical plot of the inspiratory pressures against the time obtained during the test is depicted in Figure 1.
Typical register showing that the maximal inspiratory peak (82 cm H2O) along 60 seconds of observation was achieved at 50.8 seconds.
Statistical Analysis
Results of continuous variables are expressed as mean ± SD, and those from categorical variables as frequencies. Analysis of variance for repeated measures, complemented by Scheffé test, was used to compare the continuous variables between the time ranges. Frequencies were compared using the chi-square test. Associations with PImax values were tested employing linear regression. An initial univariate analysis was followed by a multivariate model with backward elimination. Statistical analysis was performed employing SPSS package version 13.0 (SPSS, Chicago, Illinois). P values < .05 were considered significant.
Results
A total of 87 patients were examined. In 3 cases, tests were discontinued before the planned 60 seconds of airway occlusion: a 27-year-old black female with Guillian-Barré syndrome had bradycardia at 47 seconds; a 22-year-old black with a head trauma developed a hypertensive peak at 52 seconds; and a 79-year-old white female with abdominal sepsis had desaturation (SpO2 < 90%) at 52 seconds. They were all returned to mechanical ventilation and promptly recovered without any sequelae. Characteristics of the remaining 84 patients are in Table 1.
General Features of Studied Patients (n = 84)
The median value P0.1 was 2.7 cm H2O (2–6.3 cm H2O). Differences between the medians of inspiratory pressure peaks in the 3 time ranges were statistically significant: 27 cm H2O (7–118 cm H2O), 40 cm H2O (11–120 cm H2O), and 47 cm H2O (11–138 cm H2O), P < .001 for every comparison) (Fig. 2).
Distribution of maximal inspiratory pressure measured with a unidirectional valve (PImaxUV) in the 3 time ranges. * P < .001 versus 0–20 seconds. † P < .001 versus both 0–20 seconds and 20–40 seconds. The box and whisker plot shows the median, interquartile range, 5th percentile, and 95th percentile.
No patient reached a maximal inspiratory peak in the first 20 seconds of observation. Only 12 patients (14.0%) reached the maximal inspiratory peak between 20.1 and 40 seconds of observation. In 72 (86.0%), maximal inspiratory peak was found between 40.1 and 60 seconds.
In a univariate model of linear regression in which age, sex, days of mechanical ventilation, Acute Physiology and Chronic Health Evaluation (APACHE) score, and inspiratory drive (P0.1) were tested, age and P0.1 were the only variables significantly associated with values of PImaxUV (P = .009 and P = .004, respectively) (Table 2). In a second model, consisting of a multivariate analysis using backward elimination in which the same variables were included, age (P < .006) and P0.1 (P < .003) persisted significantly associated with the values of PImaxUV. In this model, the equation for PImaxUV calculation would be:
Univariate Linear Regression Analysis Using Inspiratory Pressure as the Dependent Variable
Discussion
Management of the difficult-to-wean patient is one of the most challenging problems in critical care medicine.16 The PImax achieved during a brief period of spontaneous breathing is a useful parameter to guide the decision to start the weaning process.17–25 In this study, we assessed the inspiratory pressure using a unidirectional valve method (PImaxUV) and a digital vacuometer within an observational period of 60 seconds to evaluate the time required to achieve maximal values and to analyze the factors associated with the values of PImaxUV. The employment of digital technology, which automatically records the test results, allowed us to prevent observer influence upon the data collection and to revisit the data whenever needed.
When extension of the period of tracheal occlusion is considered, patient discomfort and safety emerge as natural concerns. Discomfort is intrinsically associated with the strategy to measure PImaxUV. Previous attempts to shorten the period to achieve PImax by coaching were not encouraging.5,8,9 We could not complete the protocol of 60 seconds of airway occlusion in 3 out of 87 patients, who were returned to mechanical ventilation without any sequelae. Therefore, the strategy of extension of the tracheal occlusion period did not offer any substantial risk for patients. Although we believe that repeat measurements could increase the precision of data collection, this approach could unnecessarily expose patients to an intrinsic discomfort inherent to the procedure. Future studies correlating the data derived from one measurement with weaning outcome could shed light to this issue.
Our results showed that, as a whole, values of PImaxUV progressively increased along the observation period. As previously suggested5,8,9 progressive reduction of the residual volume and incremental stimulation of the respiratory center can possibly account for this finding. Progressive recruitment of fast-type myofibers may also have contributed to the phenomenon.26
No patient reached a maximal inspiratory peak in the first 20 seconds of observation. More importantly, PImaxUV values were found between 40.1 and 60 seconds in 85.8% of the cases, confirming our hypothesis that longer periods of observation would generate higher inspiratory pressures. Several studies have been published measuring inspiratory pressure employing unidirectional valve. When the observation period was restricted to 20 seconds, PImaxUV values were generally reached at 15–20 seconds of observation.5 At least 3 studies have extended the observation period to more than 20 seconds.9,12,13 Consistent with our findings, PImax values were all found after the first 20 seconds of observation. Indeed, the highest values were found close to 40 seconds of observation, suggesting that the commonly used 20 seconds of airway occlusion may be insufficient to an adequate measurement of the PImax. It should be pointed out that 29 patients (35%) reached the PImax in the last 5 seconds of observation in this study, allowing us to think that they did not reach a plateau along the test.
We resorted to a linear regression analysis to search for factors that could affect PImaxUV values. Included independent variables consisted of age, sex, days of mechanical ventilation, APACHE score, and inspiratory drive. In both, univariate and multivariate analysis, age and inspiratory drive emerged as the only significant determinants among those tested. In this model, age and inspiratory drive together could explain around 20% of the PImaxUV (R2 = 0.19, P = .002). The obtained equation suggested that each year of increase in age would reduce the PImaxUV value 0.34 cm H2O, and each cm H2O of increase in P0.1 would increase the PImaxUV value 7.1 cm H2O. In healthy adults, age and sex are major predictors of PImax values.1 Our findings, extracted from mechanically ventilated patients, confirmed age but not sex as a significant determinant of PImax values, but the negative effect of sex could be related to the small sample size. Data also pointed out that P0.1, a parameter commonly used to guide ventilator weaning, has a strong correlation with PImaxUV values.
We are not sure that measurements need to be performed until 60 seconds. It is interesting to note that the standard deviation of values at up to 20 seconds is not much different than the ones at 40 seconds and up to 60 seconds, and so reliability seems similar. The definite answer to that question, however, can only come from subsequent studies correlating PImax collected at different times with weaning outcome.
Although PImax is presently viewed by some authors as an important parameter in the judgment to start weaning,14,27–31 the value of PImax in isolation may not have a great impact in the weaning outcome. A more precise measurement of this parameter, however, does represent a technical improvement and the use of this refinement plus the ratio of frequency to tidal volume (f/VT) on a daily basis may help monitoring of patients on prolonged ventilation and perhaps improve the clinical result and survival in the intensive care unit.
Conclusions
In conclusion, this study shows that PImax values, assessed in standardized conditions, employing modern technology, vary substantially within a 60 second period of observation, with the majority of peak values sitting between 40.1 and 60 seconds. Older patients were found to have lower PImaxUV values, whereas higher values for P0.1 strongly correlated with higher PImaxUV values. Our study should be viewed as preliminary, since we measured PImaxUV values without following the patients for their weaning outcomes.
ACKNOWLEDGMENTS
The authors thank the respiratory physiotherapists and physicians of the Intensive Care Unit of the Hospital e Clínica São Gonçalo and Hospital Estadual Azevedo Lima, for their collaboration and dedication in our study.
Footnotes
- Correspondence: Leonardo Cordeiro de Souza PT MSc, Medical Science Postgraduation Course, Universidade Federal Fluminense, Rua Marques de Paraná 303, 2° Andar Niterói 24033-900, Rio de Janeiro, Brazil. E-mail: leonardo.uti{at}gmail.com.
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The authors have disclosed no conflicts of interest.
- © 2012 by Daedalus Enterprises Inc.
