SA Node

To evaluate SA node automaticity in the EP lab, physicians will perform

• SNRT or CSNRT measurements

• Atrial ERP or RRP measurements

• AV conduction mapping

• Atrial conduction mapping

Answer explanation

The electrophysiology study can be used to asses the automaticity of the SA node. The measurement designed to to this is the sinus node recovery time (SNRT)... two indices are commonly used to relate SNRT to the BCL. The first is the corrected sinus node recovery time (CSNRT), which is calculated by subtracting the cycle length of the patient's sinus rhythm in msec from the SNRT. Thus: CSNRT=SNRT-BCL

The upper normal value of SCNRT is usually considered to be 525 msec... The second index commonly used for assessing SA nodal function is the ratio of SNRT to the BCL [(SNRT/BCL)x100%]. A ratio of greater than 16% is usually considered abnormal.

An EP test indicative of severe sinus node dysfunction is

• Atrial FRP > 200 msec

• Atrial ERP < 200 msec

• CSNRT > 525 msec

• CSNRT < 525 msec

Answer explanation

CSNRT > 525 msec. is abnormal. CSNRT=SNRT=BCL. The upper normal value of CSNRT is usually considered to be 525 msec.

What EP technique is used to measure SNRT?

• Overdrive suppression

• Programmed stimulation

• Parahisian pacing

• Evaluating antegrade AV conduction

Answer explanation

Measurement of the SNRT is based on the phenomenon of overdrive suppression, which is the pause induced in an automatic focus by a temporarily overdriving pacemaker. With an electrode in the high right atrium near the SA node, pacing is initiated at a rate slightly faster than the basic sinus rate. The recovery interval (the interval from the last paced atrial complex to the first spontaneous SA node depolarization) represents the degree of overdrive suppression induced by pacing. The SA node becomes fatigued by the fast rate and takes some time to recover - Sinus Recovery Time.

The technique to measure the SA node conduction time (SACT) uses

• Isuprel to increase the sinus rate

• A High RA extrastimuli to reset the sinus node

• A 30 second Hi-RA pulse train, just faster than the sinus rate

• The conduction time between Hi RA and Low RA electrodes

Answer explanation

The technique (measuring SACT) depends on resetting the sinus node with single extrastimuli delivered to the high atrium... sino atrial conduction time (SACT) is intended to detect delayed conduction between sinus node and adjacent atrial tissue.

Which patient below has normal electrogram measurements in milliseconds?

• AA 673, QRS 86, QT 401, AH 73, HV 48

• AA 673, QRS 401, AH 48, HV 73

• AA 673, QRS 105, QT 401, AH 130, HV 25

• AA 673, QRS 86, QT 401, AH 73, HV 105

Answer explanation

AA = Total Cycle Length

QRS = <100 msec normal

QT = approximately 400 msec

AH = 55-125 msec

HV = 35-55 msec

A patient's Sinus Node Recovery time was measured at 1175 msec, sinus cycle length 930 msec, QRS duration 86 msec, and HV of 52. What is the CSNRT?

Answer explanation

CSNRT (correct sinus node recovery time) is SNRT-BCL = 1175-930=245. SNRT is considered normal <1500 ms and CSNRT <550

The SNRT evaluates sinus node automaticity. CSNRT is "corrected" in an attempt to compare it to normal. What is the upper limit of normal for the CSNRT?

Answer explanation

A conservative upper limit of "normal" for the SNRT, above which most experts would agree that the SA node dysfunction is present, at 1500 msec... the corrected sinus node recover time (CSNRT), which is calculated simply by subtracting the patients BCL from the SNRT. Thus CNRT=SNRT-BCL. By convention, the upper limit of "normal" for the CSNRT is 525 msec. In other words, the sNRT should be no more than 525 msec longer than the BCL.

SACT measurement is used to evaluate a patient for

• SA node exit block

• Dual AV pathways

• SA node automaticity

• Retrograde SA node conduction

• Sick Sinus Syndrome

Answer explanation

SA node exit block is evaluated by the SACT (Sino Atrial Time). Prolonged SACT's are thought to indicate a propensity for SA nodal exit block (in which impulses are formed normally within the SA node but do not penetrate through the perinodal tissue to the atrial myocardium)... Although SNRT assesses SA nodal automaticity (its intrinsic ability to continue firing), the measurement of sinoatrial conduction time (SACT) was developed to assess SA conduction (the time it takes to conduct that impulse to the atrium). If the SA node fires normally, but is blocked getting out of the node area, that is the SA node exit block.

The SACT (Sino Atrial Conduction Time) is calculated by the formula

Answer explanation

To measure SACT, an electrode is placed in the high right atrium near the SA node and atrial pacing is used to depolarize and thus reset the SA node. The subsequent return interval (the time from the last paced atrial depolarization to the first spontaneous SA nodal depolarization) is assumed to reflect the sum of the time it takes for the paced impulse to penetrate into the SA node (at which time it resets the SA node), plus the basic sinus cycle length, plus the time it takes for the subsequent spontaneous beat to penetrate out of the SA node. Assuming the time of penetration into and out of the node are equivalent, then: Return interval=BCL+2SACT... A normal SACT is considered to be 50-125 msec.

SNRT and CSNRT should be measured at several cycle lengths. In the measurements below, what CSNRT should be reported?

• CL - 600 ms

  SNRT - 1100

  CSNRT - 500

• CL - 500

  SNRT - 1010

  CSNRT - 510

• CL - 450

  SNRT - 970

  CSNRT - 520

• CL - 400

  SNRT - 930

  CSNRT - 530

• CL - 350

  SNRT - 870

  CSNRT - 520

Answer explanation

The upper limit of normal is 525 msec. Here 530 is the highest number and is the only CSNRT outside of normal range. The elecrophysiology study can be used to assess the automaticity of the SA node. The measurement designed to do this is the sinus node recovery time (SNRT)... two indices are commonly used to relate SNRT to the Basic Cycle Length (BCL). The first is the corrected sinus node recovery time (CSNRT), which is calculated by subtracting the cycle length of the patient's sinus rhythm in msec from the SNRT. Thus CSNRT=SNRT-BCL. The upper normal value of CSNRT is usually considered to be 525 msec... The second index commonly used for assessing SA nodal function is the ratio of SNRT to the BCL [(SNRT/BCL)x100%]. A ratio of greater than 16% is usually considered abnormal.