Syllabi - Critical Care - Diagnosis Scenario

You admit a 77 year old female with community acquired pneumonia to the ICU. She was admitted from the emergency department with hypoxemia that is refractory to supplemental oxygen. She is intubated but this required fiber optic intubation because she could not be intubated by direct laryngoscopy. She was placed on antibiotics and required . She does not have other significant past medical history. Two days later, her fraction of inspired oxygen is 40% and she is off positive end expiratory pressure. You decide she is ready to be extubated but the critical care fellow expresses concern that she will fail extubation and will not be able to be reintubated. You state that you would like to use some diagnostic test to help predict if she will be successfully extubated. Your fellow states that the respiratory rate to tidal volume ratio can be used to predict successful extubation and her ratio is 50.

You from the question: In mechanically ventilated patients, can the respiratory rate to tidal volume ratio be used to predict successful extubation? You perform a MEDLINE search using the Mesh terms 'extubation' and 'sensitivity' and 'specificity' and find an article about predictors of successful extubation (NEJM 1991:324:1445-1450.)

Read the article and decide:

• Are the results of this diagnostic test valid?
• Are the results of this diagnostic test important?
• Can you apply this valid, important evidence about a diagnostic test in caring for your patient?

Completed Diagnosis Worksheet for Critical Care Medicine

Citation

Yang KL, Tobin MJ.
A prospective study of indexes predicting the outcome of trials of weaning from mechanical ventilation. N Engl J Med 1991; 324:1445-50.

Are the results of this diagnostic study valid?

Was there an independent, blind comparison with a reference ("gold") standard of diagnosis?

Yes.

Was the diagnostic test evaluated in an appropriate spectrum of patients (like those in whom it would be used in practice)?

Yes.

Was the reference standard applied regardless of the diagnostic test result?

Yes.

Are the valid results of this randomised trial important?

Sample calculations:

		Target Disorder (iron deficiency anaemia)		Totals
		Present	Absent
Diagnostic Test Result (serum ferritin)	Positive (< 65 mmol/L)	731 a	270 b	1001 a + b
	Negative (≥ 75 mmol/L)	78 c	1500 d	1578 c + d
Totals		a + c 809	b + d 1770	a + b + c + d 2579

Sensitivity = a/(a+c)
= 731/809
= 90%

Specificity = d/(b+d)
= 1500/1770
= 85%

Likelihood Ratio (positive result) (LR+) = sens/(1-spec)
= 90%/15%
= 6

Likelihood Ratio (negative results) (LR-) = (1-sens)/spec
= 10%/85%
= 0.12

Positive Predictive Value = a/(a+b)
= 731/1001
= 73%

Negative Predictive Value = d/(c+d)
= 1500/1578
= 95%

Pre-test Probability (prevalence) = (a+c)/(a+b+c+d)
= 809/2579
= 32%

Pre-test-odds = prevalence/(1-prevalence)
= 31%/69%
= 0.45

Post-test odds = Pre-test odds x Likelihood Ratio
Post-test Probability = Post-test odds/(Post-test odds + 1)

Your calculations:

		Successful extubation		Likelihood Ratio
		Present	Absent
Diagnostic Test Result	< 80	29	3
	> 80	7	25
Totals		36	28	64

Sensitivity = a/(a+c)
= 29/36
= 80%

Specificity = d/(b+d)
= 25/28
= 89%

Likelihood Ratio (positive result) (LR+) = sens/(1-spec)
= 7.5

Likelihood Ratio (negative results) (LR-) = (1-sens)/spec
= 0.22

Positive Predictive Value = a/(a+b)
= 90%

Negative Predictive Value = d/(c+d)
= 79%

Pre-test Probability (prevalence) = (a+c)/(a+b+c+d)
= 56%

Pre-test-odds = prevalence/(1-prevalence)
= 1.28

Post-test odds = Pre-test odds x Likelihood Ratio
= 9.6

Post-test Probability = Post-test odds/(Post-test odds + 1)
= 9.6/1

Can you apply this valid, important evidence about a diagnostic test in caring for your patient?

Is the diagnostic test available, affordable, accurate, and precise in your setting?

Yes, the authors gave a detailed description of how the test was performed?

Can you generate a clinically sensible estimate of your patient's pre-test probability (from practice data, from personal experience, from the report itself, or from clinical speculation)

Approximately 70%

Will the resulting post-test probabilities affect your management and help your patient? (Could it move you across a test-treatment threshold?; Would your patient be a willing partner in carrying it out?)

Her respiratory rate to tidal volume ratio of 50 gives her a post-test probability of 95% and this crosses my treatment threshold of 90%

Would the consequences of the test help your patient?

Yes, since she is a difficult intubation, we want to minimise our chance of a false positive (we state she could be successfully extubated but she fails).

Additional Notes

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; 25:1514-21. This article provides an excellent review of how to evaluate an article about a diagnostic test.

Successful Extubation: How to diagnose

Citation

Yang KL, Tobin MJ. A prospective study of indexes predicting the outcome of trials of weaning from mechanical ventilation. N Engl J Med 1991; 324:1445-50.

Clinical Question

In mechanically ventilated patients, can the respiratory rate to tidal volume ratio be used to predict successful extubation?

Search Terms

'extubation' and 'sensitivity' and 'specificity'

The Study

• Target disorder and gold standard; Successful extubation
• Study setting: mechanically ventilated patients in an an ICU

The Evidence

Respiratory rate to tidal volume ratio	Successful extubation	Failed extubation	Likelihood ratio
< 80	29/32	3/32	7.53
80-100	6/12	6/12	0.77
> 100	1/20	19/20	0.04
Totals	36	28

Comments

• Study designed to evaluate weaning but outcome was successful extubation.
• 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; 25:1514-21.

This article provides an excellent review of how to evaluate an article about a diagnostic test.