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EP Procedures - Baseline Measurements

Baseline Measurements

One of the first procedures during an EP study is collection of the baseline measurements of the patient's rhythm. These measurements will be taken regardless of the rhythm that the patient is in, though ideally comparison of the patient's intrinsic rhythm pre and post procedure is preferred. The standard set of measurements taken are listed below.

From the surface 12 Lead

  • P-P:  Measure from the onset of one surface P wave to the onset of the next consecutive P wave. Note that in a normal rhythm, this value will be nearly identical to the R-R interval. In some abnormal rhythms such as atrial flutter, the P-P will be a factor of the R-R. This occurs when there is block between the atrium and ventricle. In other rhythms such as AVNRT, it may be impossible to measure the P-P.
  • R-R:  Measure from the onset or tip of one QRS to the onset of the next consecutive QRS.
  • PR 120-200ms:  Measure from the onset of a P wave associated with a QRS to the onset of that QRS. A value of 120ms to 200ms is considered to be normal. Values above 200ms are considered to be prolonged.
  • QRS 80-120ms:  Measure from the onset to the end of the QRS interval. Normal QRS intervals are between 80ms to 120ms. Values greater than 120ms indicate conduction delay through the right bundle, the left bundle or both.
  • QT:  Measure from the onset of the QRS to the end of the T wave. This value varies depending upon the heart rate. The higher the rate, the shorter the QT. In order to determine if a measured QT is normal, it must first be corrected.
  • QTc:  There are a few formulas that are used for correction of the QT interval.
    • One of the more commonly used is the Bazett formula.(1) This formula calculates the corrected QT by dividing the measured QT by the square root of the measured R to R interval.
    • Another commonly used formula is the Fridericia formula(2) which divides the QT interval by (the measured R to R interval multiplied by 0.33.)
    • The corrected values for men should be under 450ms. For women the upper end of the normal range is 470ms.

From the Intracardiac Electrograms

  • A-A:  This is measured from the onset or peak of one atrial signal to the same aspect of the next consecutive signal. If you measure from onset, use the onset of both electrograms. Remember to be consistent in your measurements.
  • R-R:  This is measured from the onset or peak of one ventricular signal to the same location on the next consecutive ventricular signal. Remember that if you use onset for the first measurement, use onset for the second. Always keep consistent in your measurment processes.
  • PA 20-50:  The PA time is the interval from the onset of the P wave to the onset of the atrial intracardiac signal from the High Right Atrial catheter. The clinical relevance of the PA interval has not been strongly documented and in one study (3) showed minimal clinical relevance over long term follow up. For this reason, this value is often excluded for the baseline measurements.
  • AH 55-130 / 50 - 140:  The AH interval shows the conduction time through the AV Node. This measurement is obtained by evaluating the time between the Atrial and His bundle electrograms on one of the His catheter recordings.
  • HV 35-55 measured H to first V (surface or IC):  The HV interval displays the conduction time from the His bundle which is located just below the AV node, to the first identifiable onset of ventricular activation. See below.

To measure the AH and HV intervals, you must first locate the HIS channel with the clearest signals. Notice that there are two HIS channels on this recording. The first HIS channel is the proximal recording. This shows clear A, H and V signals. The distal HIS channel has a clear V signal, though the A and H are barely discernible.

Once you have selected a channel to use for your measurements, place the electronic calipers at the start of the Atrial signal. Click to mark the start of your measured interval. Advance the calipers to the start of this His signal. Mark the end of your measurement. Here we see the AH interval to be 63ms.

The HV interval is measured from the start of the HIS signal to the beginning of the first ventricular signal. The first V signal may be on one of the surface leads, or it could be from one of the intracardiac leads. Note that in patients with preexcitation from WPW, the HV interval should be measured from the start of the HIS signal to the initial upslope of the delta wave. In these patients, the HV interval is very short and may be a negative value.


(1) Bazett HC. (1920). "An analysis of the time-relations of electrocardiograms". Heart (7): 353–370.

(2) Fridericia LS (1920). "The duration of systole in the electrocardiogram of normal subjects and of patients with heart disease". Acta Medica Scandinavica (53): 469–486.

(3) P-A interval: lack of clinical, electrocardiographic and electrophysiologic correlations. Wyndham CR, Shantha N, Dhingra RC, Wu D, Denes P, Rosen KM.

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