cardiac pacemakers part ii

Cardiac Pace-Makers 

Pacemaker Malfunctions, Part II

Salah Atta, MDProfessor of Cardiology,Cardiology Department

Outlines• Understanding Pacemaker Cardiac Cycle.

• Pacemaker follow Up.

• Programmable parameters

• PM Rhythm strip interpretation.

• Pacemaker malfunctions and Pseudomalfunctions.

Sequence of events in a PPM cardiac cycle

• Each cycle begins with Sensing• For Each sensing, the PM responds by two actions: pacing or inhibition is the 1st and starting a new timing interval is the 2nd.

• Each Interval ends by re-sensing in the same or the other chamber according to the type of PPM and so-on.

Sequence of events in one cycle in a single chamber PM (VVI)

• Sensing: • If there is no detected intrinsic activity:  pacing will be delivered to RV, produce LBBB morphology QRS.

•  then a period equal to the programmed lower heart rate (pacing interval) is waited then resensing.

• If there is intrinsic activity detected, no pacing is delivered and the system waits for an interval called the  escape interval and re-sense and so on.

Sequence of events in one cycle in a dual chamber PM

• Sensing in the atrium>> no sensed event >> atrial pacing and start a paced AV delay interval and an atrial pacing interval (Lower Rate).

• If an atrial event is sensed >> no atrial pacing and start a sensed AV delay interval and an atrial escape interval.

• At end of AV delay, sensing in the ventricle occurs>> no sensed ventricular event >> pacing in the ventricle >> start a V pacing interval according to programmed lower rate and a Post ventricular atrial refractory period (PVARP), after it atrial sensing starts again.

• On the contrary, if a sensed ventricular event >> no V pacing and start a new V escape interval (Lower Rate), at its end V sensing occurs again and a PVARP.

Sooo • Maximum Rate cycle length = A-V delay + PVARP.

DDD pacing (atrial tracking)

DDD Pacemaker

A DDD pacemaker puts in the part of the beat that’s missing in order 

to maintain AV synchrony

Pacemaker Follow-up

• Aim– Verify appropriate pacemaker operation– Optimize pacemaker functions

– Respond to pt’s requirements

– To detect ERI or indications of lead repositioning.

• Interval– Immediate and 7 days Post.op.– Postoperative 1,3, 6,then every 6-12 month till near the elective replacement or the battery voltage is ↓ 2.7 volt, then more frequent follow ups.

Interrogate

Measure Impedance

Check Diagnostics

Measure Threshold

Follow-up Check

Check Sensing

Electrical Characteristics

• Battery voltage/Current drain• Longevity.• Lead Impedance(s)• Lead pacing threshold(s)• Sensing threshold(s)• Magnet effect.• Programmable values.

• Device longevity estimate appears on the initial interrogation.

• Based on remaining battery voltage / battery impedance and percent pacing

Pacemaker DiagnosticsDevice Longevity Estimate

Lead Impedance Measurement

• Impedance measurementImpedance measurement : :

• Lead Maturity occurs in about 8 weeks.300 - 1000 ohms.... calculated / displayed300 - 1000 ohms.... calculated / displayed• Low Lead Impedance

– Partial lead/insulation break – Fluid/blood in header

• High Lead Impedance– Open Circuit e.g. lead not connected to device (Set

Screw)– Lead fracture

An Insulation Break Around the Lead Wire Can Cause Impedance Values to

Fall• Insulation breaks expose

the wire to body fluids which have a low resistance and cause impedance values to fall

• Current drains through the insulation break into the body which depletes the battery

• An insulation break can cause impedance values to fall below 300 ohms

A Wire Fracture Within the Insulating Sheath May Cause Impedance Values

to Rise• Impedance values across a break in the wire will increase

• Current flow may be too low to be effective

• Impedance values may exceed 3,000 ohms

Sensing and capture tests

• Check sensed P wave…

• Check sensed R wave…

• Pacing threshold test using either amplitude or pulse width

Setting sensitivity is like creating a wall that blocks out signals. You build thewall to a certain height defined in mV. If you build a wall that is 5 mV tall, the only

signals that the device can “see” are those that are taller than 5 mV.

Sensitivity is adjusted at least one half to one third of the sensed threshold: it is an order to the PM to get inhibited by any sensed electrogram above the programmed value e.g 1.5 mv and to ignore it if it is less than this.

Ventricular Pacing Threshold TestSensing and Capture testing should be performed to insure adequate safety margins are programmed

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Pacing threshold: the least amount of current (mAmps) needed to evoke capture and get an impulse (<1 mAmp)

Accordingly the pacing output by the PM is adjusted at at least double this value (too high threshold means too much electericity and rapid Battery depletion.

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Programmabilityand programmable values

Capacity to noninvasively alter one of several aspects of the function of a pacer

Desirable since pacer requirements for a person change over time

Programmable Parameters in VVI

• Minimum or lower heart rate (Pacing interval) (Maximum length of time the Pacemaker will wait for intrinsic activity).

• Pacing output, lead polarity.• Sensitivity• Rate responsiveness if VVIR• Hysteresis (a longer escape interval than the

pacing interval to allow intrinsic activity even if a bit slower) so: Hysteresis= escape interval- pacing interval.

VVI with Hysteresis

Programmable Parameters in DDD

• Mode of pacing.• Minimum or lower heart rate (Pacing interval).• Maximum heart rate.• Pacing threshold. • Sensitivity.• Rate responsiveness if DDDR.• A-V delay.• Refractory periods to avoid conflicts.• Mode switch.

Refractory Periods• Refractory period =

a programmable interval occurring after the delivery of a pacing impulse or after a sensed intrinsic complex, during which the pacemaker can sense signals but chooses to ignore them

Atrial Refractory Period

AV delay PVARP

TARP

AVD PVARP

VRP

Atrial Channel

Ventricular Channel

DDD Mode: Refractory Periods

Dual Chamber Pacing behaviour

• Maximum Rate / Maximum Tracking Rate

The fastest rate that the Ventricular channel can track intrinsic P-waves

Pacemaker Wenckebach

Beyond Maximum Heart Rate

Mode Switchingfrom DDD to DDI or VVI

Permanent Pacemakers Follow Up

Rhythm Strip Interpretation

• Look for presence of an underlying rhythm

• Look closely for the presence of a pacer spike

• Look for the presence of P waves (intrinsic or paced) and their rate and relationship to the QRS complexes

• Examine QRS complexes to determine if they are paced or intrinsic

Permanent Pacemakers

Rhythm Strip Interpretation• If ventricular pacing, note whether each

beat is preceded by a P wave (paced or intrinsic) and whether the AV(PR) interval is constant before the ventricular paced beats

• Observe for fusion beats

• Are there pauses or spikes not followed by P or QRS.

Permanent Pacemakers

Rhythm Strip Interpretation• Unacceptable f indings denoting Malfunction:

-Any spike not followed by P or QRS.

-Any pause longer than Minimal pacing

interval (considering Hysteresis).

-Absent pacing if with slower heart rate than

the programmed or

-Spikes at unexpected sites.

Thank you