ECG and Electrolytes Flashcards
Normal QT Interval
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.
Corrected QT
The corrected QT interval (QTc) estimates the QT interval at a 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.
There are multiple formulas used to estimate QTc (see below). It is not clear which formula is the most useful.
Bazett’s formula: QTC = QT / √ RR
Fredericia’s formula: QTC = QT / RR 1/3
Framingham formula: QTC = QT + 0.154 (1 – RR)
Hodges formula: QTC = QT + 1.75 (heart rate – 60)
NB. The RR interval is given in seconds (RR interval = 60 / heart rate).
Bazett’s formula is the most commonly used due to its simplicity. It over-corrects at heart rates > 100 bpm and under-corrects at heart rates < 60 bpm, but provides an adequate correction for heart rates ranging from 60 – 100 bpm.
At heart rates outside of the 60 – 100 bpm range, the Fredericia or Framingham corrections are more accurate and should be used instead.
If an ECG is fortuitously captured while the patient’s heart rate is 60 bpm, the absolute QT interval should be used instead!
There are now multiple i-phone apps that will calculate QTc for you (e.g. MedCalc), and the website MDCalc.com has a quick and easy QTc calculator that is free to use.
Normal QTc values
QTc is prolonged if > 440ms (>11 small squares/2.2 big squares) in men or > 460ms (>11.5 small squares/2.3 big squares) in women
QTc > 500 (> 12.5 small squares/2.5 big squares) is associated with increased risk of torsades de pointes
QTc is abnormally short if < 350ms (<8.75 small squares/1.75 big squares)
A useful rule of thumb is that a normal QT is less than half the preceding RR interval
ECG: Hypokalaemia - U Wave
The U wave is a small (0.5 mm) deflection immediately following the T wave
U wave is usually in the same direction as the T wave.
U wave is best seen in leads V2 and V3.
U waves are prominent if >1-2mm or 25% of the height of the T wave.
The most common cause of prominent U waves is bradycardia.
Abnormally prominent U waves are characteristically seen in severe hypokalaemia.
Normal PR Interval
The PR interval is the time from the onset of the P wave to the start of the QRS complex.
It reflects conduction through the AV node.
The normal PR interval is between 120 – 200 ms duration (three to five small squares/0.6-1 big square)
If the PR interval is > 200 ms/1 big square, first degree heart block is said to be present.
PR interval < 120ms/ 3 small squares suggests pre-excitation (the presence of an accessory pathway between the atria and ventricles) or AV nodal (junctional) rhythm.
ECG: Hypomagnesaemia
Normal serum magnesium = 0.8 – 1.0 mmol/L.
Hypomagnesaemia = <0.8 mmol/L
The primary ECG abnormality seen with hypomagnesaemia is a prolonged QTc.
Atrial and ventricular ectopy, atrial tachyarrhythmias and torsades de pointes are seen in the context of hypomagnesaemia, although whether this is a specific effect of low serum magnesium or due to concurrent hypokalaemia is uncertain.
Nevertheless, correction of serum magnesium to >1.0 mmol/L (with concurrent correction of serum potassium to >4.0 mmol/L) is often effective in suppressing ectopy and supraventricular tachyarrhythmias, while a rapid IV bolus of magnesium 2g is a standard emergency treatment for torsades de pointes.
ECG: Hypercalcaemia
The main ECG abnormality seen with hypercalcaemia is shortening of the QT interval
In severe hypercalcaemia, Osborn waves (J waves) may be seen
Too much calcium is a short QTJ
Ventricular irritability and VF arrest has been reported with extreme hypercalcaemia
The Osborn wave (J wave) is a positive deflection at the J point (negative in aVR and V1)
The J point is the the junction between the termination of the QRS complex and the beginning of the ST segment.
It is usually most prominent in the precordial leads
Characteristically seen in hypothermia (typically T<30C), but they are not pathognomonic.
J waves may be seen in a number of other conditions:
Normal variant
Hypercalcaemia
Medications
Neurological insults such as intracranial hypertension, severe head injury and subarachnoid haemorrhage
Le syndrome d’Haïssaguerre (idiopathic VF)
ECG: Hypocalcaemia
Hypocalcaemia causes QTc prolongation primarily by prolonging the ST segment.
The T wave is typically left unchanged.
Dysrhythmias are uncommon, although atrial fibrillation has been reported.
Torsades de pointes may occur, but is much less common than with hypokalaemia or hypomagnesaemia.
ECG: Hypokalaemia
ECG changes when K+ < 2.7 mmol/l
Increased amplitude and width of the P wave
Prolongation of the PR interval
T wave flattening and inversion
ST depression
Prominent U waves (best seen in the precordial leads)
Apparent long QT interval due to fusion of the T and U waves (= long QU interval)
With worsening hypokalaemia…
Frequent supraventricular and ventricular ectopics
Supraventricular tachyarrhythmias: AF, atrial flutter, atrial tachycardia
Potential to develop life-threatening ventricular arrhythmias, e.g. VT, VF and Torsades de Pointes
Electrolyte Abnormalities causing LONG QT
Hypokalaemia
Hypomagnesaemia
Hypocalcaemia
ECG: Hyperkalaemia
Serum potassium > 5.5 mEq/L is associated with repolarization abnormalities:
Peaked T waves (usually the earliest sign of hyperkalaemia)
Serum potassium > 6.5 mEq/L is associated with progressive paralysis of the atria:
P wave widens and flattens
PR segment lengthens
P waves eventually disappear
Serum potassium > 7.0 mEq/L is associated with conduction abnormalities and bradycardia:
Prolonged QRS interval with bizarre QRS morphology
High-grade AV block with slow junctional and ventricular escape rhythms
Any kind of conduction block (bundle branch blocks, fascicular blocks)
Sinus bradycardia or slow AF
Development of a sine wave appearance (a pre-terminal rhythm)
Serum potassium level of > 9.0 mEq/L causes cardiac arrest due to:
Asystole
Ventricular fibrillation
PEA with bizarre, wide complex rhythm
(Warning! In individual patients, the serum potassium level may not correlate closely with the ECG changes. Patients with relatively normal ECGs may still experience sudden hyperkalaemic cardiac arrest.)
