Quiz #13 Answer: Statistics - Z-score

A newborn (weight 3 kg, Length 50 cm) has aortic valve annulus of 6.0 mm. One published report states the following values for aortic valve annulus in newborns of the same body size: Mean 7.3 mm & Range of 6.38 – 8.22 mm. Range is reported as 2 standard deviations from the mean.

Question 1: What is the Z-score of the aortic valve in our patient with 6.0 mm diameter?

Answer: Z-score is (- 2.8)

Bonus Question: Please provide the steps of deriving the Z-score
Answer: Z-score is the represents the observed value as a the number of standard deviations away from the population mean. So, to calculate the Z-score for a given observed value, one has to know the population mean and SD.

Here the mean is 7.3 mm.
We need the SD. The given range is 2 SD on either sides of the mean. Therefore, 8.22 - 7.3 = 0.92 (which is equal to 2 SD).
SD = (0.92 ÷ 2) = 0.46

Given value for aortic valve diameter is 6.0 mm.
6.0 - mean = deviation from the mean.
i.e. 6.0 - 7.3 = (-1.3)

Deviation from mean ÷ SD = Z-score
i.e. (-1.3) ÷ 0.46 = (-2.8)
Z-score is (-2.8).

Quiz #11 - Answer: CVP Trace

Question: The two snap shots were taken 1 minute apart from a postoperative patient.What is the difference between the CVP traces in each of the panels?

Answer: A white arrow has been added to each panel to help identify the "a" wave in each trace.
Top panel (Clock time 13:44 Hrs) - Recorded with A-pacing at 130 bpm. There is Atrio-ventricular synchrony. "a" wave is small (& is smaller than the "v" wave). Mean CVP is 7 mmHg. Arterial BP 92/46 (mean 64) mmHg.

Bottom panel (Clock time 13:45 Hrs) Again, the white arrow identifies the "a" wave. Comparatively, these are "giant" a waves. A-pacing is stopped. Patient's native rhythm is accelerated junctional rhythm at 104 bpm. AV synchrony is lost - atria are contracting against a closed AV valve causing "giant" a waves. Now, the CVP is 9 mmHg (higher than when there was AV synchrony) and arterial BP is lower (73/33, mean 46 mmHg) - emphasizing the importance of AV synchrony in this patient.

Note: "Cannon" a waves are occasional giant a waves seen in patients with complete AV block. In this situation, a waves are normal or close to normal during several beats when there is apparent AV synchrony by shear coincidence of a sequential atrial and ventricular contractions. But, there will be occasional sudden appearance of one giant a wave at variable intervals when apparent AV synchrony is lost.

Quiz 10 Answer: Clinical Examination

Answers:
What is the sign shown in figure 1?
Answer: Wrist sign

What is the sign shown in figure 2?
Answer: Thumb sign

If a patient has both signs, he/she gets 3 points in the "systems score" - according to the Revised Ghent Criteria for diagnosis of Marfan syndrome.
(Click here for System Score table)

Other links for information about Marfan syndrome:
http://www.ncbi.nlm.nih.gov/books/NBK1335/#marfan.Diagnosis
http://www.marfan.org/marfan/4265/Diagnostic-Criteria