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(Chest. 2002;121:1947-1955.)
© 2002 American College of Chest Physicians

Variations in the Measurement of Weaning Parameters*

A Survey of Respiratory Therapists

Guy W. Soo Hoo, MD, MPH, FCCP and Louis Park, MD

* From the Pulmonary and Critical Care Section, West Los Angeles Veterans Affairs Medical Center, and Department of Medicine, UCLA School of Medicine, Los Angeles, CA.

Correspondence to: Guy W. Soo Hoo, MD, MPH, FCCP, West Los Angeles VAMC, Pulmonary and Critical Care (111Q), 11301 Wilshire Blvd, Los Angeles, CA 90073;


   Abstract
TOP
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 Appendix 1
 References
 
Objectives: Respiratory therapists differ in the methods used to obtain weaning parameters. A questionnaire survey was conducted to better characterize those differences.

Design: A questionnaire survey was conducted among respiratory therapists from nine hospitals in the Los Angeles area. The four-page, 32-question instrument was self-administered and anonymous. Responses were tabulated for analysis.

Setting: Respondents from nine hospitals, three hospitals with residency training programs and six community hospitals without training programs in the Los Angeles area.

Participants: One hundred two respiratory therapists.

Results: There was no universally acknowledged group of weaning parameters, although four parameters were named by > 90%. There was wide variation in methods used to obtaining weaning parameters. Almost all (91%) obtained measurements with the patients breathing their current fraction of inspired oxygen, but there was great variability in the ventilator mode used to collect these parameters (T-tube, continuous positive airway pressure, pressure support), with an equally wide range of pressures added to each mode (0 to 10 cm H2O). There was great variation in the time (< 1 to > 15 min) before recording weaning parameters. Measurement of parameters was done either with bedside instruments or read from the ventilator display. The maximal inspiratory pressure had great variation in the duration of airway occlusion (< 1 to 20 s), with the most frequent time frame being 2 to 4 s. Differences were noted between therapists from the same hospital as well as between hospitals.

Conclusions: There is great variation among respiratory therapists when obtaining weaning parameters. This calls for further standardization of the measurement of weaning parameters.

Key Words: mechanical ventilation • respiratory therapists • weaning parameters


   Introduction
TOP
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 Appendix 1
 References
 
The timing of extubation or discontinuation of mechanical ventilation remains one of the more challenging aspects of critical care management. A variety of strategies have been proposed, beginning with measurements collectively referred to as "weaning parameters," followed by trials of spontaneous breathing, partially supported breathing, or a more gradual reduction in ventilator support.1 These weaning parameters usually consist of measurements of respiratory rate (RR), tidal volume (VT), vital capacity, minute ventilation (E), and maximum inspiratory pressure (MIP). Other parameters include the frequency/tidal volume ratio (Fb/VT), dead space ventilation, inspiratory pressure at 0.1 s, work of breathing, compliance, or an index (eg, thoracic compliance, RR, arterial oxygenation, MIP index) derived from the aforementioned variables.2 3 4 5 6 7 8 9

None of the parameters have perfect positive or negative predictive capability, but in unison provide insight about a patient’s status. They may serve as predictors of the ability to tolerate unassisted ventilation as well as the need for continued ventilator support. It is imperative that these measurements are reproducible and accurate. Uniformity in techniques and data acquisition would be obvious requirements. This is self-evident and has led several investigators to call for the standardization of weaning parameters.10 11 12

Current clinical guidelines primarily list threshold values.13 Full descriptions of weaning parameters and techniques are not provided, but are available as references. These references usually refer to experimental and/or clinical studies with conditions and equipment that may not be universally available or applicable. Other sources may provide a reasonably detailed description of the technique,14 15 but are lacking in details that may be considered routine (fraction of inspired oxygen [FIO2], ventilator mode, duration before measurement).

The extent of variation in measurement of weaning parameters is unknown. Observation at our institution suggested significant variation, but this may not be reflective of general practice. Better characterization would not only provide support for standardization, but also identify areas in which to focus educational efforts. This led to the following questionnaire survey on the weaning practices of respiratory therapists in the Los Angeles area.


   Materials and Methods
TOP
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 Appendix 1
 References
 
A self-administered four-page, 32-item questionnaire was developed and piloted among respiratory therapists at our institution (see Appendix). The items specifically addressed the methods by which weaning parameters were obtained, and were based on published descriptions of the techniques incorporating input from the therapists. The questionnaires were anonymous and required about 10 min to complete. Some questions allowed multiple responses, and each response was tabulated for analysis.

Nine hospitals in the Los Angeles area participated in the survey. Three of the hospitals have active residency training programs in all the major disciplines (medicine, surgery, etc.), while the other six were community hospitals without training programs. All of the hospitals have > 100 inpatient beds, with two of the teaching hospitals having > 500 inpatient beds. All of the hospitals have from 20 to 50 ICU beds, except one of the training hospitals with > 50 ICU beds. After obtaining permission for administering the questionnaire from the director of each respiratory therapy department, the questionnaires were distributed to members at the individual hospitals. When completed, the questionnaires were either retrieved by one of the investigators or mailed back to our institution.

The responses were tabulated using an Excel spreadsheet (Microsoft; Redmond, WA). Data were examined both in aggregate pooling all respondents as well as focusing on respondents from the same hospital. Statistical analysis with comparison of proportions was performed using Primer for Biostatistics software (Version 4.0; McGraw-Hill; New York, NY).


   Results
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 Appendix 1
 References
 
Of a total of 166 questionnaires distributed, 102 questionnaires (61%) were completed and returned for analysis. The majority of respondents (75%) report > 5 years of experience, and over half are certified respiratory therapists. Of the 102 respondents, 46 persons (45%) work in teaching institutions. Ventilator management is primarily directed by specialty internists (pulmonary/critical care) [41%], but the therapists also receive directions from others (general internists, surgeons, specialty surgeons, anesthesiologists, and physicians in training).

Most of the respondents were responsible for 1 to 9 patients receiving ventilation per shift, with two respondents (2%) responsible for > 10 patients receiving ventilation. Most of the respondents (69%) noted no specific time for obtaining weaning parameters, but 32 respondents (31%) responded that weaning parameters were obtained only during daytime hours (before 3 PM). The majority of respondents (68%) noted that patients were extubated at any time, with 32% noting that extubation occurred primarily during daytime hours and never after 8 PM. About one half of the therapists (49%) responded that they did not require a specific order for weaning parameters. When analyzed based on the type of institution, 30 therapists (65%) from teaching institutions, compared to 22 therapists (39%) at a nonteaching institution, responded that weaning parameters required an order. This difference (p = 0.016, z = 2.414) likely reflects the increased availability of physicians at a teaching institution. The majority of respondents (79%) believed that weaning parameters consisted of a predesignated group of variables.

Most of the therapists (95%) required patients to be arousable before performing weaning parameters, but approximately one half have obtained these measurements in unconscious patients (47%). Most of the therapists (91%) obtained weaning parameters with the patient breathing supplemental oxygen (current FIO2 setting), with a minority having the patient breathe room air.

Table 1 shows the ventilator mode in which weaning parameters were obtained. Multiple responses were permissible and responses included combinations of T-tube, continuous positive airway pressure (CPAP), and pressure support (PS). Only three respondents exclusively obtained weaning parameters with the patient breathing off the ventilator and through a T-tube. The remainder utilized some combination of T-tube or CPAP (97%), with or without added pressure, and with or without added PS. Twenty-three respondents (23%) obtained weaning parameters using PS, but without added CPAP pressure. Nine of the respondents used some combination of CPAP pressure and added support with PS. Figure 1 provides a breakdown by hospital on the modes used to measure weaning parameters. In two hospitals (hospital A and hospital H), weaning parameters were obtained only in the CPAP mode, but even in those two hospitals, there were differences among respondents whether measurements were performed with a CPAP of 0 cm H2O or 5 cm H2O. In the remaining hospitals, CPAP was the most common mode. In hospital F, hospital G, and hospital I, all of the respondents reported no additional pressure in the CPAP mode (0 cm H2O). In the others that reported measurement with added pressure, the pressure was variable, with the most common response of 3 to 5 cm H2O. This variability was also noted when PS was used, with the range of responses spanning 3 to 12 cm H2O.


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  		Table 1. Ventilator Mode in Obtaining Weaning Parameters

 


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  		Figure 1. Breakdown by hospital and representing the percentage of responses from each hospital of the ventilator mode used to measure weaning parameters.

 
Table 2 provides further information on the frequency, time to recording, and types of weaning parameters. Most respondents (68%) reported obtaining weaning parameters one to five times per week. Two respondents reported that they measured zero weaning parameters in an average week. With respect to the time, a small number (6%) reported values for weaning parameters almost immediately (< 1 min), while others recorded values after > 15 min (5%). The most frequent time frame was between 1 min and 5 min (68%).


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  		Table 2. Weaning Parameters

 
Table 2 also lists the most commonly reported weaning parameters. No measurement was named by all respondents. The most frequently reported parameters (> 90%) were the MIP, VT, RR, and E. The Fb/VT ratio was specifically reported by < 20%. The combination of RR and VT was reported by 94 of respondents (92%), and the Fb/VT ratio can be easily calculated from these two values. This variable (Fb/VT) was more often specifically reported by therapists from the teaching hospitals (16 of 46 respondents, 35%) as compared to community hospitals (3 of 56 respondents, 5%; z = 3.62; p < 0.01). No other differences were noted between therapists based on their hospital of practice in terms of methods by which they measured or reported weaning parameters (T-tube vs CPAP vs PS).

Table 3 provides more information on RR, VT, and E. These parameters can be directly measured using a bedside spirometer, but can also be recorded from the ventilator display or bedside monitor. About 70% reported values using the ventilator display. About 30% reported making direct measurements of these parameters, except for RR, which was directly observed by almost 75% of respondents. This parameter (RR) also had the greatest number of dual responses, with approximately 40% using both methods. This dual response was noted in < 10% for VT or E.


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  		Table 3. Weaning Practices

 
Table 4 is devoted to the MIP. In contrast to the other variables outlined in Table 3 , the MIP was obtained with a manometer by almost 90% of respondents as opposed to using the ventilator software package. Only 25% responded using both the manometer or ventilator software package. There was a wide range of occlusion times before reporting the MIP, with the most frequent time between 2 s and 10 s (88%). Most respondents (68%) obtained three values, and most reported the highest value (75%). The occlusion time was not uniform among therapists in any of the hospitals surveyed. Figure 2 depicts the percent of therapists responding for each time frame at each of the surveyed hospitals. In hospital I, almost 90% of the therapists occlude the airway for 2 to 4 s to obtain a MIP. The most diversity can be seen in hospital B, where no single time frame was marked by > 30% of the respondents.


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  		Table 4. Weaning Parameters

 


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  		Figure 2. Breakdown by hospital and representing the percentage of responses from each hospital of the occlusion time used when measuring the MIP. sec = second.

 

   Discussion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
Appendix 1
 References
 
Despite several decades of use, the constellation of bedside measures collectively referred to as "weaning parameters" have never quite met expectations. Witness the large number of variables that are considered weaning parameters and the continued search for bedside measurements that more accurately predict successful discontinuation of mechanical ventilation.1 2 3 4 5 6 7 8 9 15 These physiologic markers may provide insight about a patient’s ability to tolerate discontinuation of mechanical ventilation, but they do not address other factors that may prevent successful extubation (secretion control or clearance, upper-airway obstruction, comorbidities). Their predictive value may worsen, as patients may be very different from the original reference group (longer duration of ventilation, age, gender, different measurement methodology, etc.). Trials of spontaneous breathing have gained enthusiasm, but they can also be viewed another type of weaning parameter, albeit more dynamic and more pragmatic,16 but with the same limitations as noted earlier.

Irrespective of the measurement or the manner in which the information is used, weaning parameters can only be useful if they accurately reflect the patient’s condition. Respiratory therapists play a crucial role, as they are delegated the task of obtaining and reporting weaning parameters. It follows that accuracy is related to measurement techniques, with uniform techniques likely to produce the most reliable information. Available guidelines are general and lack explicit details. This in turn allows some latitude in these measurements.13 14

The results of this survey confirm the wide diversity in measurement techniques of weaning parameters among respiratory therapists. The differences involve nearly every facet of measurement, and occur among therapists from different hospitals, as well as among therapists in the same hospital.

There is no "consensus" as to which measures constitute weaning parameters. No parameter was named by all of the respondents, although four were noted in > 90% (MIP, VT, E, and RR) and are often considered the core group of weaning parameters. Although the Fb/VT ratio has gained support as the single most predictive measurement in weaning,2 it was specifically noted by < 20%. However, the ratio is derived from RR and VT, and therefore, if not specifically reported, easily calculated. It was also marked more often by therapists from teaching hospitals, suggesting that the Fb/VT ratio may not be embraced as widely in community hospitals as teaching institutions.

Among the many variations in measurement, two areas are especially noteworthy. First, there is considerable variation in the ventilator mode used to record these variables. Most therapists (85%) made measurements in the CPAP mode, with a minority of respondents using a T-tube. In addition, there is no uniformity in the amount of supplemental pressure present during measurement.

Added pressure during measurement adds another layer of complexity to this issue. Minimal amounts of pressure are based on the premise that the pressure eliminates the resistance of the endotracheal tube, providing a more accurate assessment of a patient’s status.17 It is unclear if this is a valid assumption. There is no consensus as to the exact pressure required to compensate for the endotracheal tube, and there may not be an attainable consensus.18 19 A patient’s work of breathing can be reduced with small amounts of CPAP or PS.20 It follows that measurement over the range of pressures (1 to 10 cm H2O) reported in this survey could also change a patient’s weaning parameters.

The effect of added pressure on one parameter (Fb/VT ratio) alters its ability to predict successful extubation. The original report identified a threshold value of 105, obtained with patients breathing through a T-tube.2 Other investigators have examined the ratio with patients receiving partial support (CPAP or PS) or combinations of both modes with different levels of pressure or have followed the ratio over time.21 22 23 They have either reported a decline in its predictive capability or a need to adjust the ratio higher when using added pressure.

The time to record values represents the other major area of variation in measurement. The responses (< 1 s to 15 min) in Table 2 outline this variation. A better appreciation of the importance of time can be gained by reviewing the MIP. Early investigations utilized 30 to 50 s of airway occlusion in this measurement.24 25 Marini and colleagues26 demonstrated continued increasing negative pressures that plateau after 20 s. Branson and colleagues27 found maximal pressures required a minimum of 12 s, and recommend 15 s of airway occlusion. It follows that the MIP should be recorded after 15 to 25 s of airway occlusion.14 In this survey, only 5% reported airway occlusion for 16 to 20 s. None responded with > 20 s. The most frequent response of 2 to 4 s is an inadequate occlusion time, and would likely underestimate this parameter.

Almost 40% report using the ventilator software package. This might minimize concerns with standardization, but these packages also underestimate the MIP.28 29 This becomes evident when examining the MIP measurement of one ventilator. In the Puritan Bennett 7200a ventilator (Puritan Bennett; Carlsbad, CA), activation of the maneuver initiates a 10-s time frame for measurement.30 Inspiration closes the proportional solenoid valves of the ventilator and occludes the airway for 3 s. Exhalation ends the maneuver, and the maximum negative pressure generated is reported as the MIP. The 3-s period is much less than the recommended time frame for airway occlusion, and only one breath is reported. Other software packages may have different algorithms, but it is not surprising that this package underestimates the MIP.

Based on this survey, the vast majority of patients undergoing MIP measurements do not have the airway occluded for the recommended amount of time. Other areas that may lead to variations in the MIP include issues with reproducibility, maximal effort, and test-to-test variation.31 32 If decision making is based on this parameter, numerous management issues can be affected, including the timing of "weaning trials," extubation, and tracheostomy.

Although there is much debate on the clinical utility of weaning parameters, it is self-evident that they must be obtained in a standardized fashion. Only then can values be considered reflective of a patient’s status and confidently incorporated into clinical decision making. It is acknowledged that the role of weaning parameters has decreased over the past decade. However, one third of therapists in this survey report making measurements more than six times a week, and only two respondents reported zero parameters a week. This suggests that physicians still use these measurements in patient management.

Based on responses to this survey, it is conceivable that weaning parameters obtained in the same hospital on the same day by different therapists on the same patient could be very different because of differences in measurement technique. Patients with borderline weaning parameters would be most affected by this lack of standardization, but all patients risk misclassification. This study was designed to assess the measurement of weaning parameters by the respiratory therapists of one community. Although limited in scope, it represents both private and public hospitals and is likely representative of the practice in other communities.

These results also raise questions about other practices, specifically trials of spontaneous breathing. Although this was not specifically addressed by the questionnaire, there is likely a similar degree of variation with respect to the conduct of the trials using T-tube, CPAP, or CPAP plus PS. The results of this survey reinforce the need for standardization of all techniques, as well as continued study of the parameters that may better identify a patient’s ability to tolerate extubation.


   Appendix 1
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Appendix 1
References
 
Weaning Parameters Survey Questionnaire

Thank your for your cooperation. We would appreciate any other comments.


   Footnotes
 
Abbreviations: CPAP = continuous positive airway pressure; Fb/VT = frequency/tidal volume ratio; FIO2 = fraction of inspired oxygen; MIP = maximum inspiratory pressure; PS = pressure support; RR = respiratory rate; E = minute ventilation; VT = tidal volume

Received for publication April 17, 2001. Accepted for publication January 4, 2002.


   References
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Appendix 1
 References
 

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