Pacemaker Types and Function

Pacemaker Types

• Pacemakers can be single chamber, dual chamber, or biventricular
• They can be temporary (external) or permanent (internal)

Pacemaker Modes

• Pacemakers operate in two modes: asynchronous or synchronous.

Asynchronous Mode

• Ignores the heart's intrinsic activity
• Does not sense intrinsic activity
• Paces at a fixed rate set

Synchronous Mode

• Senses the intrinsic heart rate and delivers voltage to cause depolarization
• Paces only when the intrinsic heart rate is less than the set rate
• WILL NOT fire if the heart rate is above the set rate

Pacemaker Function

• Paces the heart, measured in milliamperes (mA)
• Senses the heart, measured in millivolts (mV)

Wave Detection and Sensitivity

• R wave detection relies on a voltage level above the pacemaker's set level
• Sensitivity relates to the voltage level needing to be exceeded to detect an R or P wave

Threshold and Intervals

• Threshold denotes the minimum current needed to pace the heart
• AV interval is the duration between paced or sensed atrial depolarization and the subsequent ventricular stimulus
• VA interval is the duration between sensed or paced ventricular depolarization and the next atrial stimulus

Effective Refractory Periods

• The amount of time in which a cell is unable to respond to a new stimulus
• Protects against arrhythmias

Temporary Pacemaker Indications

• Used for unstable bradyarrhythmias or tachyarrhythmias
• Employed in third-degree heart block
• Provides support post open-heart surgery
• Use ends upon resolution of the problem or permanent pacemaker implantation

Permanent Pacing Indications

• Sinus node dysfunction
• Symptomatic impulse formation diseases
• Sinus bradycardia, sinus pause, SA block
• AV node disease
• Atrial fibrillation, using dual-site pacing as a means to decrease intra-atrial conduction time

Pacemaker Code

• Indicates the chamber paced, chamber sensed, and response to sensing

Sensing Response

• Triggered (T) means that a sensed intrinsic depolarization results in pacemaker firing
• Heart will respond to a sensed intrinsic activity by delivering a pacing stimulus
• Inhibited (I) means the device DOES NOT PACE
• Pacemaker will withhold a pacing stimulus on sensing the heart’s electrical activity
• Prevents competition with the heart’s intrinsic rhythm and reduces the risk of inducing arrhythmias
• The Pacemaker will fire if the heart fails to beat on its own
• Dual (D) defines the dual inhibition of atrial and ventricular pacing in response to intrinsic ventricular depolarization
• The pacemaker inhibits both atrial and ventricular pacing if the heart is functioning normally
• Pacemaker will fire in the needed chamber (atria or ventricle) if intrinsic heart activity is missing or delayed
• None (O) Utilized for asynchronous pacing, regardless of intrinsic activity
• Useful when electrocautery, which interferes with pacemakers, is being used
• Placing a magnet on the patient’s chest puts the pacemaker in asynchronous pacing mode, such as D00 at 99 HR
• Example DDD signifies both atrium and ventricle paced, both atrium and ventricle depolarizations sensed, inhibition in response to depolarization, and can trigger a response to depolarization
• Example: VVI indicates ventricle paced, ventricle sensed, and that a ventricle event outside of the refractory period will be inhibited

Chamber Pacemakers

• Lead placement in the right ventricle is most common
• Paces or senses in one chamber only, for example, AAI, VVI

Dual Chamber Pacemakers

• Leads placed in the right atrium (RA) and right ventricle (RV)
• DDD can pace and sense in both the atrium and ventricle
• Features 2 timing intervals, the AV interval and VA interval

Leadless Permanent Pacemakers

• Indicated for AV block
• Implanted under fluoroscopy into the RV septum
• Contains 4 tines instead of leads
• Utilizes internal atrial sensing algorithms to adjust pacing which coordinates with the atria

Temporary Pacemakers

• Transvenous pacing is invasive; wire is placed from the internal jugular (IJ) to the RV
• Epicardial pacing is invasive, with most surgeons placing 2 atrial and 2 ventricular wires, with some surgeons placing two ventricular wires only
• Transcutaneous pacing is non-invasive
• Esophageal pacing is invasive

Pacing Wires and Cable Connectors

• Polarity of wires, black to negative, red to positive.
• Leads connect from the wire insertion to the controller

Checking Pacemaker Function

• Confirming all connections are secure
• Settings must be confirmed before starting (mode, rate, output, sensitivity, AV interval)
• Check in Demand mode only, which prevents firing and causing R on T

R on T Phenomenon

• Ventricular extrasystole caused by ventricular depolarization superimposing on a previous beat’s repolarization
• Causes include undersensing of the pacemaker
• Pacemaker starting in asynchronous mode, allowing an erroneous beat to fall on the T wave
• Avoided if the pacemaker is in DDD or VVI mode, because can sense the heart’s intrinsic rhythm
• If the pacemaker is in DOO or VOO mode, and the intrinsic heart rate is lower than the paced rate, It will speed up the HR
• Alternatively, if the intrinsic heart rate is higher than the paced rate, the pacer will pace on a T wave, and VT or VF may occur

Pacemaker Components

• Leads
• Box
• Battery (with a life of 7-10 years)
• Electrodes
• Barbs/screws

Magnets with Pacemakers

• Usage can cause a device-specific rate change or a percentage decrease
• Useful with DDD-VVI
• A magnet would be used on a pacemaker-dependent patient during electrocautery
• Magnets are placed over the pacemaker any time EMI is expected
• Magnet pacing rates may differ from the set fixed pacemaker rate

Pacemaker Malfunctions

• Failure to pace
• Failure to capture
• Pacing at abnormal rates
• Failure to sense (undersensing) leads to inappropriate pacing
• Undersensing causes cardiac activity to go undetected
• The device mistakenly believes the heart is not beating, so an unnecessary unneeded impulse is fired
• Main clinical risk from this includes competitive pacing, leading to arrhythmias like R on T
• Oversensing causes inhibition of pacing
• The pacemaker detects extraneous signals as intrinsic activity
• The device is inhibited from pacing
• Clinical risk from this includes inappropriate pauses or bradycardia
• Oversensing and malfunction is a unique issue for dual-chamber devices
• Malfunction can be caused by drugs and certain conditions
• Drugs such as procainamide may cause failure to capture
• Quinidine use may increase the threshold.
• Flecainide (Na+ channel blocker) raises the threshold
• Electrical interference, hypokalemia, hyperkalemia, myocardial ischemia, and issues with pacemaker placement, leads, or the generator are potential conditions

Anesthetic Considerations

• Requires knowledge of external or existing pacemaker
• Determine if patient is pacemaker dependent
• Permanent pacemaker insertion may involve MAC (monitored anesthesia care) in an EP (electrophysiology) lab or cath lab
• NC (nasal cannula) is preferred for oxygenation to avoid potential fire hazards associated with masks
• Location of the pacemaker
• Verification of pacemaker function, typically by a device representative
• Magnet availability

Electrocautery and Electrosurgery

• Electrocautery passes through alternating current through a resistant metal wire electrode for coagulation only
• Electrosurgery involves direct heating of tissue for cutting and coagulation
• Monopolar electrosurgery directs current from the electrode, through the body, to a grounding pad, and back to the generator
• Bipolar electrosurgery restricts the current to a small tissue volume for better control
• Advantages of bipolar include reduced damage to sensitive tissues near the instrument
• Reduced chance of current arcs directly to the surrounding surface

Electrocautery Considerations for Permanent Pacemakers

• Bovie grounding pad placement should be away from the generator
• Return electron should be opposite the surgical site
• Bipolar, short bursts, and the lowest effective current should be used by the surgeon

Complications of Permanent Pacemaker Placement

• Lead dislodgement
• Pneumothorax
• Infection
• Bleeding

Pacemaker Adjustments for Surgical Procedures

• Lithotripsy is safe if the generator is protected, and the beam is not directed towards the device
• TURP/hysteroscopy: the pacemaker may need to be asynchronous because the surgeon will use monopolar cautery
• ECT (electroconvulsive therapy) can be performed with the pacemaker in asynchronous mode
• MRI considerations
• TENS/SSEP/MEP are generally safe

Insertable Cardiac Loop Recorders (ICLR)

• Implanted in the left pectoral region within subcutaneous fat
• Alternative to the Holter cardiac monitor
• No absolute contraindications for implantation
• Absence of wiring between the device and the heart
• Device activation is triggered by a patient using a magnet during a presyncopal or syncope episode
• Used for diagnosis and management of various cardiac arrhythmias and symptoms

ICLRs Anesthetic Considerations

• Need a cardiologist's progress notes
• Interrogate the device before surgery
• Assess recordings of recent life-threatening arrhythmias
• Made with a titanium shell patients may feel a pulling during MRI
• Electrocautery is safe but data loss can occur if the device is reset
• Focal point of the beam should not be set on the device as it can cause damage during lithotripsy

ICDs

• The size of a pacemaker
• Two types: transvenous or subcutaneous
• Transvenous leads sense, pace, and deliver biphasic shocks
• Can be single, dual, or biventricular (cardiac resynchronization therapy)
• Pulse generator and leads come standard on all types

ICD Functions

• Able to sense ventricular depolarization
• Provide VF detection
• Provide tachycardia detection and discrimination
• Tiered therapy options
• Biphasic shocks use lower energy requirements

ICDs Anesthetic Considerations

• GA vs MAC (most devices are placed under MAC)
• Avoid Lidocaine, as it obscures checking; also avoid procainamide (both affect defibrillation energy requirements)
• Maintenance: technique and caution with EF