Criteria abstracted from The Users' Guide to Medical Literature, from the Health Information Research Unit and Clinical Epidemiology and Biostatistics, McMaster University

Highlighted lines and questions below provide links to the pertinent description of criteria in The EBM User's Guide, now available at the Canadian Centres for Health Evidence

Specific questions in this review are based on Jaeschke RZ, Meade MO, Guyatt GH, Keenan SP, Cook DJ. How to use diagnostic test articles in the intensive care unit: diagnosing weanability using f/Vt. Crit Care Med. 1997;25(9):1514-2. [abstract]

Article Reviewed:

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Clinical Examination Reliably Detects Intrinsic Positive End-Expiratory Pressure in Critically Ill, Mechanically Ventilated Patients

Kress JP, O'Connor MF, Schmidt GA.

Am J Respir Crit Care Med 1999; 159: 290-294. [abstract] [Full-text for subscribers]

Reviewed by Renan Orellana MD, Mona McPherson MD, Pediatric Critical Care Section, Department of Pediatrics, Baylor College of Medicine, Houston, Texas

Review posted July 3, 2001

I. What is being studied?

Study objective:

This study assessed the diagnostic value of bedside clinical exams to predict intrinsic positive end-expiratory in pressure in mechanically ventilated patients.

Study design:

This is a prospective, blinded, comparative study performed in a single institution. The authors recorded 503 observations made by clinicians at different levels of training. Clinicians ranged from fourth-year medical students to fellows to attending physicians. They evaluated the clinician's ability to detect the presence or absence of intrinsic PEEP. Independent observers at the bedside recorded physician results and ventilator waveform readings. We assume that any interaction between the examiners and observer was avoided.

The patients investigated:

The investigators used adult patients with various causes of respiratory failure commonly encountered in the ICU: acute hypoxemic respiratory failure, ventilatory failure, postoperative respiratory failure, respiratory failure associated with sepsis/shock and intubation for airway protection. There were no exclusion criteria.

The interventions compared:

The presence of intrinsic PEEP in mechanically ventilated patients detected by inspection/palpation and auscultation during the expiratory phase was compared to a expiratory waveform on a monitor connected to a ventilator. The study focused on the ability of the physical exam to detect intrinsic PEEP compared to the "gold standard" of ventilator waveforms.

II. Are the results of the study valid?

Primary questions:

1. Was there an independent, blind comparison with a reference standard?

Yes. The physicians performing the physical exam were blinded to the ventilator waveforms. This blinding excludes bias. There was also an independent observer recording data at the same time of the physical exams. It is unclear whether the physician interpreting the waveforms was blinded to the clinical exam results. There was potential for bias in the waveform interpretation if the observer was privy to the exam findings. We can only assume that any interaction between the examiners and observer was avoided.

Another important issue is whether the ventilator waveform can be considered the "gold standard" by which to detect intrinsic PEEP. One could argue that exact tests like esophageal pressure monitoring or expiratory hold measurements may be more reliable. These tests are more invasive and also have limitations. For the purposes of this study we can accept the ventilator waveform display as the least invasive and most reasonable method (in terms of complexity/invasiveness of the procedure) to detect intrinsic PEEP.

2. Did the patient sample include an appropriate spectrum of patients to whom the diagnostic test will be applied in clinical practice?

Yes. The patient sample included all major causes of respiratory failure as outlined above. That permits a close resemblance to the clinical practice (in adults).

Secondary questions:

3. Did the results of the test being evaluated influence the decision to perform the reference standard?

No. The ventilator waveform analysis was performed regardless of whether the physical exam detected intrinsic PEEP. There was no verification bias.

4. Were the methods for performing the test described in sufficient detail to permit replication?

Yes. One of the merits of the study is the simplicity of its design. The maneuvers to determine intrinsic PEEP could be applied in the daily practice and the exam components were clearly described. They included simultaneous inspection and palpation of the patient's chest during exhalation looking for continuous chest wall incursion until the next inspiration and auscultation of the chest looking for expiratory sounds that persist until the next breath occurred.

III. What are the results?

1. Are the test's sensitivity, specificity, and likelihood ratios presented (or are the data necessary for their calculation provided)?

Yes. The investigators provided the calculations and presented the likelihood ratios in a very organized manner. While not all of the basic data was published, much of it can be inferred from the data provided. For reasons of space we present only our calculations for a likelihood ratios obtained from the overall results:

	SENSITIVITY	SPECIFICITY	PPV	NPV
CLINICAL EXAM OVERALL	0.72	0.91	0.95	0.58

LR (+) = SENSITIVITY/(1- SPECIFICITY)= 0.72/(1-0.91)= 8: Intrinsic PEEP by physical exam is 8 times more likely to be present in patients who actually have intrinsic PEEP, as opposed to patients without intrinsic PEEP.

LR (-) = (1- SENSITIVITY)/SPECIFICITY= (1-0.72)/0.91= 0.3: When intrinsic PEEP is ruled out by physical exam, it is 0.3 times more likely to be absent in patients without intrinsic PEEP, as opposed to patients with intrinsic PEEP.

Pre-test probability of intrinsic PEEP (from waveform measurements): 69.8% (We will use the approximation of ~70% for the ease of calculations.)

Pre-test odds = pretest probability/(1 - pretest probability) = 70/30= 2.3

(+) Posttest odds = pretest odds x LR = (2.3)(8)= 18.4 (this varies from the study because of our rounding to the nearest decimal approximation)

(+) Posttest probability= posttest odds/(1 + posttest odds) = 18.4/(18.4 + 1) = 0.95: 95% probability of having intrinsic PEEP when the physical exam detects intrinsic PEEP.

(-) Post test odds: (2.3)(0.35)= 0.81 (-) Post test probability: 0.81/(0.81 + 1) = 0.45: 45% chance that intrinsic PEEP is absent when the physical exam rules out intrinsic PEEP. This number is even lower than the NPV expected before the test.

In summary, when the physical exam detects intrinsic PEEP, the patient likely has intrinsic PEEP. When the physical exam does not detect intrinsic PEEP, one is less sure that PEEP is truly not present.

IV. Will the results help me in caring for my patients?

1. Will the reproducibility of the test result and its interpretation be satifactory in my setting?

Yes. Clinical exams should be universal in method and are easy to perform. Clinical exams should be interpreted the same in pediatric patients as adults. The study was strengthened by having physicians of different levels of experience and training participate.

2. Are the results applicable to my patients?

Yes. We see the same causes of respiratory failure in children. These adult patients seemed to be comparable to the average PICU patient except for age and size. It would be easy and interesting to repeat this study in children to determine if the accuracy of clinical exams is maintained.

3. Will the results change my management?

Maybe. It implies that the detection of intrinsic PEEP does not require the use of expensive technology. The results of the study may increase reliance on physical examination as a detection tool, thereby reducing costs by avoiding unnecessary use of expensive waveform monitors. This might allow better distribution of the resources, since waveform monitors then be used in patients don't appear on physical exam to have intrinsic PEEP. In either case, iPEEP should be quantified with an expiratory hold maneuver, if possible.

4. Will patients be better off as a result of the test?

Yes. The detection of intrinsic PEEP by clinical exam may promote different ventilator management. Improved confidence in the detection of intrinsic PEEP by physical exam may be especially useful in ICUs without readily available waveform graphics.

 

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