Pacemakers

The Conduction System

• Electrical impulses from the myocardium cause the heart to beat (contract)
• The electrical signal starts in the sinoatrial (SA) node, which sits at the top of the right atrium
• The SA node is the heart's "natural pacemaker."
• When the natural pacemaker emits an electrical impulse, the atria contract
• The signal then goes through the atrioventricular (AV) node
• The AV node assesses the impulse and sends it down the bundle of His after a short transmission delay
• The bundle of His divides into right and left bundle branches, leading to the ventricles
• Heart rate varies depending on physical demands, stress, or hormonal factors, even though the SA node sends electrical impulses at a certain rate
• An impulse being blocked as it travels through the bundle branches results in bundle branch block

The Electrocardiogram

• During each cardiac cycle, the waves of depolarization moving through the heart produce electrical currents
• These currents spread through the body's interstitial fluids and up to the body's surface
• Recording electrodes on the skin's surface, on opposite sides of the heart, detect such electrical potentials
• Signals are filtered, amplified, and registered as a measure of the heart's electrical activity
• The resulting record from the procedure is an electrocardiogram (ECG or EKG)
• An ECG helps determine if any cardiac malfunction has an electrical cause
• A typical ECG has a P wave, a QRS complex, and a T wave
• Electrical currents from atrial muscle cells depolarizing before contraction result in the P wave
• The QRS complex follows as currents from the ventricles depolarize prior to their contraction
• After ventricular contraction, ventricle muscle cells repolarize and reversing electrical currents produce the T wave
• The T wave typically happens 0.25–0.35 s after ventricular depolarization

Cardiac Pacemaker Components

• An artificial cardiac pacemaker is an implanted device generating an excitatory wave at a heart site
• Pacemakers can be placed:
• At the SA node to replace a failing node and establish a normal sinus rhythm
• At the AV node if there is a malfunction
• Within one or both ventricles for excitation beyond a partial or total heart block
• A cardiac pacemaker consists of a pulse generator, leads, a battery system, and a programmable timing circuit
• Leads are wires connecting the pulse generator to cardiac tissues
• The insertion site of the device is in the upper chest, just under the skin
• The pacing lead, connecting to the pulse generator header, includes a flexible insulated wire

Cardiac Pacemaker Electrodes

• The tip that can be inserted through a vein into the heart delivers impulses from the pulse generator for stimulating the heart to beat
• Leads inserted via a vein into the heart feature tines or a screw end for threading into the cardiac muscle

Asynchronous Pacemaker

• The electric stimulus of an asynchronous pacemaker appears at a uniform rate, independent of the body
• It provides a fixed heart rate
• It was the first type of pacemaker, developed in the mid-twentieth century
• The power supply sends energy to the pacemaker circuit, and uses primary battery sources
• The oscillator sets the pulse rate for the pacemaker
• This controls the pulse output circuit, which provides the stimulating pulse to the heart
• The pulse is conducted along wires to the cardiac electrodes
• Every block is important in pacemaker construction, so each must be highly reliably
• Constant-voltage or constant-current amplitude pulses are the usual types of output stimuli
• Constant-voltage amplitude pulses have a range of 5 to 5.5 V and a duration of 500 to 600 μs
• Constant-current amplitude pulses range from 8 to 10 mA and last 1 to 1.2 ms
• Asynchronous pacemakers range from 70 to 90 beats/min
• Lead wires, must be mechanically strong and electrically conductive, since their distal ends must withstand heart beats
• Another must is to be well insulated
• Lead wires consist of interwound helical coils of spring-wire alloy molded in silicone rubber
• The helical coiling of the wire minimizes stress on it
• Multiple strands insure against failure following rupture of a single wire

Synchronous – Demand Pacemaker

• Often patients require cardiac pacing intermittently and can establish a normal cardiac rhythm between period of block
• The demand pacemaker consists of a timing circuit, an output circuit, electrodes, and a feedback loop
• The timing circuit runs at a fixed rate, usually 60 to 80 beats/min
• After each stimulus, the timing circuit resets, waits, and generates the next pulse.
• However if a natural beat occurs in the ventricle, the feedback circuit detects and amplifies the ECG signal
• This signal then resets the timing circuit to await before producing the next stimulus
• The timing circuit resets and the process repeats if the heart beats before a stimulus
• With a normal heart rate exceeding the timing circuit rate, the pacemaker is in standby, while the heart operates on its own
• If temporary heart block happens, the pacemaker takes over at the timing circuit's fixed rate.

Atrial-Synchronous Pacemaker

• The atrial-synchronous pacemaker has a circuit to replace a blocked conduction system
• The pacemaker detects electric signal (P wave of the ECG) by an atrial electrode and uses delays to activate a stimulus pulse
• Voltage v1 detected by the atrial electrodes is a pulse
• The atrial signal is amplified and passed through a gate to a monostable multivibrator, pulse v2 of 120 ms
• Another monostable multivibrator creates pulse duration of 500 ms
• V4 blocks signals from the atrial electrodes for a period of 500 ms
• This eliminates any artifact caused by ventricular contractions
• The falling edge of the 120 ms pulse triggers a monostable multivibrator of 2 ms
• Pulse v₂ acts as a delay, so ventricular stimulus happens 120 ms after atrial contraction
• Then v3 controls an output circuit applying stimulus to ventricular electrodes

Rate-Responsive Pacing

• Synchronous pacemakers can meet rate and cardiac output demands, but don't function for a physiologically intact individual
• The demands of stressful activities like exercise cannot be fully met
• A sensor is used to convert a physiological variable to an electric signal as input to the timing circuit
• This part of the pacemaker controls the heart rate based on a sensed physiological variable
• The sensor may be located within the pacemaker, or at some other point in the body
• Many variables can control rate responsive pacemakers
• An accelerometer is an example of sensor to control the pacing rate depending on patient activity
• The signal from the accelerometer is bandpass filtered and integrated for activity estimate
• A control algorithm for the timing circuit later determines pacing rate based on activity