Approach to a patient with QTc interval abnormality in ECG

Fundamentals of ECG
Approach to a patient with abnormalities of
QT interval in ECG
Dr. Md.Toufiqur Rahman
MBBS, FCPS, MD, FACC, FESC, FRCP, FSCAI,
FCCP,FAPSC, FAPSIC, FAHA,FACP
Professor & head of Cardiology
CMMC, Manikganj
Ex professor of cardiology,
NICVD, Dhaka

drtoufiq1971@gmail.comProfessor Dr Md Toufiqur Rahman
Fundamentals of ECG QT segment

Fundamentals of ECG
Conduction system of the heart

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,
effectively the period of ventricular systole
from ventricular isovolumetric contraction
to isovolumetric relaxation.
What is the Q-T Interval of an ECG?

What is the Q-T Interval of an ECG?
Case-1: A 23 years old gentleman presented with generalized weakness and fatigue ,
2 hours after several bouts of vomiting and diarrhea. His serum potassium was 2.3
mEq/L and he had the following ECG.
Figure: ECG showing Apparent QTc 500ms – prominent U waves in precordial leads ,
Hypokalaemia causes apparent QTc prolongation in the limb leads (due to T-U
fusion) with prominent U waves in the precordial leads.

Case : A 65 years old lady presented with generalized weakness and exertional dyspnea for last
5 years. She was hypertensive and was being treated with beta blockers and diuretics. On
color Doppler study , asymmetrical septal hypertrophy with systolic anterior motion(SAM) of
AML of mitral valve was found with LVOT gradient 89 mm Hg. She was diagnosed as
Hypertrophic Obstructive cardiomyopathy( HOCM). Her serum potassium and magnesium
level was low. She had the following ECG.

Case-2: A 21 years old gentleman presented with generalized weakness and fatigue,
muscle cramps, tingling and numbness of lower limbs for 1 day. He was a diagnosed
case of dilated cardiomyopathy with severe LV systolic dysfunction and taking
diuretics for last 6 months. His serum potassium was 2.9 mEq/L and serum
magnesium was 1.4 mg/dl. He had the following ECG.
Figure: ECG showing prolonged QTc (>490 msec).

Case: A 34 years old gentleman presented with severe upper abdominal pain with
vomiting for 4 days. He was diagnosed as a case of acute pancreatitis. His serum
calcium level was 2.0 mmol/L. He had the following ECG.
Figure: ECG showing prolonged QTc (460 msec).

Case: A 56 years old gentleman presented with chest discomfort on exertion and
dizziness occasionally. He was diabetic, hypertensive and smoker. 2 years back he
was underwent coronary angiogram and repot revealed critical triple vessel disease.
He was advised to do coronary revascularization but patient refused. Now the
patient has the following ECG.
Figure: ECG showing LBBB with complete heart block with left axis
deviation with prolonged QT interval ( QT- 657 msec; QTc = 773 msec)

Case: A 19 years old gentleman presented with shortness of breath for last 5 years.
He was getting diuretics prescribed by local doctors and now referred for specialist
consultation. His color Doppler showed large atrial septal defect (ASD) with
pulmonary hypertension( PASP- 96 mm Hg) Her serum potassium and magnesium
level were below the normal. He had the following ECG.
Figure: ECG showing RBBB with short PR interval and prolonged QTc
interval( QT 496 msec; QTC 627 msec) .

The QT interval is inversely proportional to heart rate:
 The QT interval shortens at faster heart rates
 The QT interval lengthens at slower heart rates
 An abnormally prolonged QT is associated with an
increased risk of ventricular arrhythmias,
especially Torsades de Pointes.
 Congenital short QT syndrome has been found to
be associated with an increased risk of
paroxysmal atrial and ventricular fibrillation and
sudden cardiac death.
What is the relation of QT interval to heart rate?

What is Corrected QT interval (QTc)?
The corrected QT interval (QTc) estimates the QT interval at a
standard heart rate of 60 bpm.
This allows comparison of QT values over time at different heart
rates and improves detection of patients at increased risk of
arrhythmias.
What are the formulas to measure corrected QT interval?
There are multiple formulas used to estimate QTc (see below). It
is not clear which formula is the most useful.
Bazett formula: QTC = QT / √ RR
Fridericia formula: QTC = QT / RR 1/3
Framingham formula: QTC = QT + 0.154 (1 – RR)
Hodges formula: QTC = QT + 1.75 (heart rate – 60)
The RR interval is given in seconds (RR interval = 60 / heart rate).

What is Normal QTc values?
QTc is prolonged if > 440ms in men or >
460ms in women
QTc > 500 is associated with increased risk
of torsades de pointes
QTc is abnormally short if < 350ms
A useful rule of thumb is that a normal QT
is less than half the preceding RR interval

What are the causes of a prolonged QTc (>440ms)?
 Hypokalaemia
 Hypomagnesaemia
 Hypocalcaemia
 Hypothermia
 Myocardial ischemia
 ROSC Post-cardiac arrest
 Raised intracranial pressure
 Congenital long QT syndrome
 Medications/Drugs

Causes of a short QTc (<350ms)
Hypercalcaemia
Congenital short QT syndrome
Digoxin effect
Short QTc

Figure: J wave / Osborn waves in severe hypercalcaemia (4.1 mmol/L).
Short QTc

Figure: Marked shortening of the QTc (260ms) due to hypercalcaemia;
Hypercalcaemia leads to shortening of the ST segment and may be associated with
the appearance of Osborne waves
Short QTc

Figure: QT interval due to digoxin (QT 260 ms, QTc 320ms approx)
Short QTc

Approach to a patient with QTc interval abnormality in ECG

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