QT interval

The QT interval is the time from the start of the Q wave to the end of the T wave.It represents the time taken for ventricular depolarisation and repolarisation.

QT shortens at faster heart rates

prolonged QT is associated with an increased risk of ventricular arrhythmias

MEASUREMENTS:

measured in either lead II or V5-6
Large U waves (> 1mm) that are fused to the T wave should be included

Smaller U waves and those that are separate from the T wave should be excluded

The maximum slope intercept method is used to define the end of the T wave 

Corrected QT

estimates the QT interval at a heart rate of 60 bpm.
improves detection of patients at increased risk of arrhythmias.

Bazett’s formula: QT_C = QT / √ RR 

QTcis prolonged if > 440ms in men or > 460ms in women

prolonged QTc 

Hypokalemia, hypocalcemia, hypomagnesemia, hypothermia, myocardial infarction, congenital long QT, drugs, raised ict

SHORT QT INTERVAL

Hypercalcemia, congenital short QT , digoxin effect

Gene therapy

D/D

Axial hypotonia with limb hypertonia: organic aciduria

d/d IMPRINTING DISORDERS:

1. Prader–Willi syndrome 

2.Angelman syndrome 

3.Beckwith–Wiedemann syndrome 

4.Silver Russell syndrome 

5. Cancers

6. Autism 

7. Pseudohypoparathyroidism type 1 

8.  Maternal & paternal UPD syndromes

9.Transient Neonatal diabetes mellitus •

d/d DIGENIC INHERITANCE:

1.Waardenberg syn: mutation in MITF and TYR genes

2. Nonsyndromic hearing loss: enlarged vestibular aqueduct & Pendred syndrome SLC26A4                                                   and KCNJ10 genes

                                                   also mutation in Cx 26, 30 , 31

3.Retinitis pigmentosa

d/d MACROCEPHALY

1. Hurler syndrome
2. MLC
3. Cardio-facio-cutaneous syndrome

d/d FLOPPY INFANT 
1. Down syndrome
2. Zellweger syndrome, look like down syndrome
3. Prader Willi synd: feeding problem
4. Congenital myotonic dystrophy: extremely floppy, need ventilatory support at birth
D/D Rhizomelic shortening of bones