Pacing

What is a Pacemaker?  

A pacemaker is an electronic device that provides an electrical signal to make the heartbeat when its anatomical pacemakers fail. The anatomical pacemakers drive what is called the “intrinsic” rhythm and can be disrupted by various conditions such as symptomatic bradycardia, heart blocks, ischemia, an MI, or post-cardiac surgery inflammation around the sinoatrial (SA) node or atrioventricular (AV) node. 

What Does “Intrinsic” Mean?

 Intrinsic means “built-in.” In this situation, it means, coming from the patient’s natural pacemakers, the SA or AV nodes; or sometimes from lower down in the ventricles. 

How do Pacemakers Work on the Heart?

 A pacemaker essentially does two things: it senses the patient's rhythm using a “sensing circuit”, and it sends out electrical signals using an “output circuit.” If the patient’s intrinsic rhythm becomes too slow or goes away completely, the electronic pacemaker senses that, and starts sending out signals along wires leading from the pacemaker control box to the heart muscle. If these signals are “capturing” properly, they provide a regular electrical stimulus making the heart contract at a regular rate fast enough to maintain the patient’s blood pressure. The signal from a pacemaker generates a visual signal on the EKG that looks like, and is called a “spike”. 

Pacemakers can work in two ways.

  • In asynchronous mode, they deliver a stimulus at a predetermined fixed rate regardless of the patient’s intrinsic rhythm. This has a downside in that it can deliver a stimulus during a T-wave, a vulnerable period for the heart resulting in what is known as an “R on T” putting the patient into ventricular tachycardia (VT) or ventricular fibrillation (VF). 

  • In synchronous mode (a.k.a sense or demand mode) the pacemaker senses the patient’s intrinsic P waves and QRS complexes (avoiding R on T) sending impulses at a predetermined rate only if the patient’s rhythm is less than the pacemaker's set rate. Sensitivity to the patient’s intrinsic rate is adjusted on the pacemaker. 

What are the Parts of a Pacemaker? 

The pacemaker box itself is called a “pulse generator”. The pacemaker box senses and sends an electrical signal or impulse to the heart muscle through wires. 

Are There Different Kinds of Pacemakers?

Pacemakers can be either temporary or permanent. 

  • Permanent pacemakers are internal or implanted and come in several varieties and are implanted in the chest under the skin with wires threaded through the subclavian vein leading to the heart. Nowadays they are very small, but can still Often be seen or felt on the chest. A pacing system that paces both the RA and the RV Is called an “atrioventricular” pacer and paces both the right heart chambers in sequence. The signal affects the left side chambers and stimulates them to contract as well.

    • How can an implanted pacemaker be reprogrammed? - Yes, there is a machine that the physician uses to communicate with an implant a pacemaker. This is why it’s important to know what brand pacemaker the patient has.

    • What is an AICD? - A I CD stands for automatic, implantable, cardioverter-defibrillator. This is a variety of a permanent implanted pacemaker that has a sensing circuit and an output circuit, but instead of acting only as a pacemaker, it also spends its time waiting for the onset of arrhythmias like VF, VT, or SVT. When it senses this it shocks the patient to hopefully correct the arrhythmia. They will also sometimes try to overdrive pace the patient out of a rapid rhythm.

  • Temporary pacemakers are made up of a control box and can be either a single or dual-chamber. Single chamber pacemakers have wires leading to the inner wall of the right ventricle that’s called a right ventricular wire or V- wire, And only provide a stimulus to the ventricles. In a dual-chamber pacemaker, they are separate wires for the RV and the right atrium(RA) are called the A- wire.

What is transvenous, transthoracic, and transcutaneous pacing?

Transvenous means that the pacing wire is threaded through an introducer into a vein, typically the jugular vein. And introducer like a MAC  is put in first and the wire is passed through it (often of pace port Swan-Ganz line that has a dedicated port on it is used) until it makes contact with the inner wall of the RV. Then the wire is attached to a Pacer box for pacing.

Transthoracic pacing means using epicardial wires inserted during cardiac surgery. These small wires sit on the outer wall of the heart and pass through the chest for connection to a pacemaker box. Typically they are paired as 2 A wires and 2 V wires or sometimes there will be one A wire one V wire and a ground wire. 

Transcutaneous pacing uses external pads placed on the patient’s chest and connected to a device like this all defibrillator. These are the same sticky pads used for defibrillation. They are applied to the patient and either an anterior-posterior or anterior-anterior placement And connected to the Zoll out with a cable. Diesel delivers electricity to the pads capturing and pacing the heart. 

How does a pacemaker box work?

The pacemaker box consists of a small computerized controller that’s run by a battery. The box senses and paces through the set of wires that lead to the endocardium.

Rate:

Heart rate is set on the pacemaker. It controls the number of times a post generator will produce a pulse and directly affects the cardiac output. It can be turned down to a rate below the intrinsic rate to act as a back up in the event of sudden bradycardia or it can be turned up to increase the amount of forward blood flow. Rate is often adjusted in situations where cardiac ischemia is a rate dependent problem as lower rates may reduce myocardial oxygen demand

Stimulation threshold - how much electricity does a pacemaker use to pace the heart?

The output of the pacemaker is measured in two ways: signal amplitude, and pulse width. Signal amplitude means how much electricity the pacemaker puts out through the wire with every stimulus or pulse. Amplitude is measured in milliamperes (mA). There is a dowel to control the milliamperes (mA) on the front of the temporary pacemaker. The stimulation threshold is the minimum electrical stimulus needed to consistently elicit a cardiac depolarization and is controlled by the milliamperes (mA)setting. Pulse width means how long each pulse lasts. The electrical poles have to be strong enough and last long enough to capture the myocardium.

What is the Sensing threshold?

Pacemakers not only pace, but they also sense electrical activity. The sensitivity threshold is the minimum myocardial voltage, expressed as millivolts (mV) required to detect a P or R wave. The sensitivity of the pacemaker is also set on the pacer box. The lower the mV setting number is, the more sensitive the pacemaker. (Smaller number = bigger sensitivity).

To use maximal sensitivity settings could cause the pacemaker to mistake various random fluctuations of electrical activity for cardiac activity. This could lead to the pacemaker not firing, or firing constantly competing with the intrinsic rate.

Because pacing wires lose effectiveness over time it is also important to check the sensing threshold frequently. When the electrode wires are freshly inserted, the actual maximum sensitivity of the pacemaker is very high. It can potentially detect very subtle changes in local electrical activity. The general range of sensitivity for a normal pacemaker box is 0.4 to 10 mV for the atria and 0.8 to 20 mV for the ventricles.

Steps for finding the Stimulation Threshold

  • Increase pacemaker rate 10 beats higher than intrinsic rhythm (100% captured beats).

  • Gradually decrease the output by turning the knob counter-clockwise to a smaller output. Observe when the pacing stimulus is no longer capturing.

  • Increase output until consistent capture is obtained = Stimulation Threshold.

  • Set the output at two times the stimulation threshold

Steps for finding the Sensing Threshold

  • Decrease the rate of the pacemaker to the patient's intrinsic rate (no firing of the pacemaker).

  • Turn the mA to zero.

  • Set the sensitivity on demand (small number) and gradually turn the sensitivity dial counter-clockwise to a larger number.

  • Note when the sensitivity light stops flashing with each beat = Sensitivity Threshold.

  • Set the sensitivity at 1/2 the threshold

What do the letters VVI, DDD, AOO, etc mean?

When referring to modes of pacing, the NGB classification system is used. The first letter stands for the chamber that is paced, the second letter is for the chamber that is sensed, and the third letter stands for the response the pacer makes to sensed intrinsic beat.

1. First Letter = Chamber Paced

A = Atrium

V = Ventricle

D = Dual (Atrium & Ventricle)

O = None

2. Second Letter = Chamber Sensed

A = Atrium

V = Ventricle

D = Dual (Atrium & Ventricle)

O = None

3. Third Letter = Response to Sensing

I = Inhibit

T = Triggered

D = Dual (Triggered+Inhibited

O = None (Asynchronous)

For example, VVI - mode programmed pacemaker paces the RV, senses the RV, and is inhibited from firing if it senses an intrinsic beat. DDD programmed pacemakers pace both chambers (D stands for dual), they sense both chambers and each of the two wires is inhibited by an intrinsic beat.

A patient may generate their P waves but fail to conduct them to the ventricles. The A - wire will be inhibited by the patients’ P waves but the V -wire will sense and follow them. So the patient will be in sinus rhythm, with V -  pacing, and will be able to increase and decrease the heart rate in a normal way in response to the body’s need.

Pacemaker Troubleshooting 

Failure to Pace

  • No paces spike visible at appropriate time

  • Causes:

    • Dislodged lead

    • Battery failure

    • Fractured wire

    • Disconnected wire

    • Generator failure

    • Over sensing pacemaker misinterpreting other activity as a QRS complex usually muscular in nature

Failure to Capture

  • Pacer generated QRS is not visible

  • Causes:

    • Dislodge lead

    • Malpositioned lead

    • Battery failure

    • Fractured lead

    • Perforated ventricle

    • Faulty connections

    • Pacing at voltage less than capture threshold

Failure to Sense

  • Pacemaker may compete with patients on intrinsic rhythm

  • Causes:

    • Sensitivity set to high

    • Catheter lead malpositioned

    • Fractured lead

    • Pulse generator failure

    • Break in lead insulation

    • Battery failure

Cheryl