2.5 Defibrillators and External Pacemakers

2.5 Defibrillators and External Pacemakers

Physiology of Defibrillation

The human heart relies on a coordinated electrical conduction system to contract and pump blood. Under abnormal conditions, this electrical system can degenerate into rapid, disorganized, and chaotic activity known as fibrillation. The two most common lethal cardiac arrhythmias are Ventricular Fibrillation (VF) and pulseless Ventricular Tachycardia (VT). During VF, the ventricles quiver uselessly and fail to pump blood, leading to sudden cardiac arrest and brain death within minutes if untreated. Defibrillation is the therapeutic delivery of a brief, high-energy electrical shock to the heart muscle. The purpose of the shock is not to "jump-start" the heart, but rather to depolarize a critical mass of the myocardium simultaneously. This temporary depolarization halts all chaotic electrical activity, allowing the heart's natural pacemaker, the sinoatrial (SA) node, to re-establish a normal sinus rhythm.

Monophasic vs. Biphasic Waveforms

The efficacy of a defibrillator shock depends heavily on the shape of the electrical pulse delivered to the patient. Historically, defibrillators utilized monophasic waveforms, but modern devices exclusively use biphasic waveforms.

Monophasic Waveforms

In a monophasic waveform, the electrical current flows in a single direction from one electrode pad to the other. To achieve high clinical success, monophasic defibrillators require high energy levels, typically delivering up to 360 Joules of energy. The downside of monophasic shocks is the high peak current required, which can cause post-shock myocardial damage (stunning of the heart muscle) and localized skin burns at the pad sites. Monophasic technology is now obsolete but may still be encountered on older equipment.

Biphasic Waveforms

In a biphasic waveform, the electrical current flows in a positive direction for a specified duration, then reverses and flows in a negative direction for the remainder of the pulse. Biphasic defibrillators are significantly more efficient than monophasic systems. Because the current reverses direction, it can depolarize the myocardial cells at much lower peak currents and total energy levels. Standard biphasic energy protocols typically range from 120 to 200 Joules for adults. There are two primary types of biphasic waveforms used by manufacturers:

• Biphasic Truncated Exponential (BTE): Adjusts the duration of the phase cycles based on patient impedance to ensure constant energy delivery.
• Rectilinear Biphasic (RBL): Maintains a constant, flat-topped current profile during the phases, regardless of patient impedance. Biphasic waveforms have been clinically proven to achieve higher rates of successful defibrillation on the first shock, with less energy and minimal myocardial stun.

Energy Delivery and Calculations

Defibrillators store electrical energy in a high-voltage capacitor. The energy ($E$) stored in a capacitor is calculated using the physical formula: $E = \frac{1}{2} C V^2$ Where:

• $E$ is the energy in Joules (J) or Watt-seconds ($\text{W}\cdot\text{s}$).
• $C$ is the capacitance of the internal capacitor in Farads (F).
• $V$ is the voltage across the capacitor in Volts (V).

For example, if a defibrillator utilizes a 150 $\mu\text{F}$ capacitor charged to 1,600 Volts, the stored energy is: $E = \frac{1}{2} (150 \times 10^{-6} \text{ F}) \times (1600 \text{ V})^2 = 0.5 \times 0.000150 \times 2,560,000 = 192 \text{ Joules}$

Clinical energy guidelines differ depending on the patient and the procedure:

• Adult Defibrillation: Typically 120 to 200 Joules for biphasic waveforms, and 360 Joules for monophasic.
• Pediatric Defibrillation: Dosed based on body weight, typically starting at 2 Joules per kilogram (J/kg) up to a maximum of 10 J/kg or the adult limit.
• Synchronized Cardioversion: Typically requires lower energy levels, starting at 50 to 100 Joules, depending on the specific atrial or ventricular arrhythmia.

Synchronized Cardioversion and the R-Wave

Not all cardiac arrhythmias require an unsynchronized high-energy shock. Rhythms like atrial fibrillation, atrial flutter, or stable ventricular tachycardia with a pulse are organized but hemodynamically unstable. Treating these conditions requires synchronized cardioversion.

The R-on-T Phenomenon

During the cardiac cycle, the ventricles undergo depolarization (represented by the QRS complex) and repolarization (represented by the T-wave). During the T-wave repolarization phase, the cardiac cells are in a vulnerable state. If an electrical shock is delivered during the T-wave, it can trigger disorganized ventricular contractions, inducing Ventricular Fibrillation. This lethal complication is known as the R-on-T phenomenon.

Synchronization Mechanism

To prevent the R-on-T phenomenon, the defibrillator must be placed in sync mode. In sync mode, the device monitors the patient's ECG waveform and identifies the peak of the R-wave (ventricular depolarization). When the clinician presses and holds the shock button, the defibrillator does not discharge immediately. Instead, it waits for the next detected R-wave and discharges the energy precisely on the R-wave peak (or within 20 to 30 milliseconds of it), safely away from the vulnerable T-wave. BMETs must test the synchronization delay using a simulator. The delay between the peak of the simulator's R-wave and the defibrillator discharge must be verified to be under 60 milliseconds (with many standards requiring less than 30 ms).

Electrode Pad Placement and Skin Impedance

Proper placement of defibrillation pads (or paddles) is critical to ensure that the maximum amount of current passes directly through the heart muscle. The two standard pad placements are:

1. Anterior-Lateral (Apex-Sternum): One pad is placed on the upper right chest (sternum) just below the clavicle, and the other pad is placed on the lower left chest (apex) over the ribs. This creates a current path that cuts diagonally through the heart.
2. Anterior-Posterior: One pad is placed on the front of the chest over the heart (precordium), and the other pad is placed on the patient's back between the shoulder blades. This is commonly used in cardiac pacing and pediatric defibrillation.

Skin Impedance and Preparation

The human thorax presents electrical resistance, known as thoracic impedance (typically ranging from 50 to 150 $\Omega$). If impedance is too high, current flow is reduced, making the shock ineffective. High impedance also causes electrical energy to dissipate as heat at the skin-pad interface, leading to severe burns. To minimize impedance, pads are pre-gelled with conductive polymer. BMETs must educate clinical staff on proper skin preparation: shaving excessive chest hair, drying sweat or water, and avoiding placement of pads directly over implanted pacemakers or medication patches.

External Non-Invasive Pacemakers

Many clinical defibrillators include a built-in external transcutaneous pacemaker. Temporary pacing is used to treat hemodynamically unstable bradycardia (abnormally slow heart rate). Transcutaneous pacing (TCP) delivers electrical pulses through large chest pads to stimulate ventricular contraction. BMETs must understand the key pacemaker parameters:

1. Pacing Rate: Set in beats per minute (bpm), typically ranging from 30 to 180 bpm.
2. Pacing Current (Output): Measured in milliamperes (mA), ranging from 0 to 200 mA. The current is increased until electrical capture (a pacing spike followed immediately by a wide QRS complex on the ECG) and corresponding mechanical capture (a palpable pulse matching the pacing rate) are achieved.
3. Pacing Modes:
   • Demand Mode: The pacemaker senses the patient's intrinsic heart rate. If the patient's heart rate falls below the set pacing rate, the device delivers pacing pulses. If the patient's rate is above the threshold, the pacemaker is inhibited. This prevents competitive pacing and reduces the risk of R-on-T shocks.
   • Asynchronous (Fixed-Rate) Mode: The device delivers pacing pulses at the set rate regardless of the patient's intrinsic heart rate. This mode is used only in emergency situations when sensing is unreliable or absent.

Performance Testing and Preventive Maintenance

Defibrillators are emergency life-support equipment and require rigorous PM testing (typically semi-annually). A specialized defibrillator analyzer is used for these tests.

50-Ohm Dummy Load Testing

A defibrillator analyzer contains a precision 50-ohm dummy load resistor, which simulates the nominal thoracic impedance of a human chest. During testing, the defibrillator pads are connected to the analyzer's test posts. The BMET charges the defibrillator to various energy levels (e.g., 5 J, 50 J, 100 J, 200 J) and discharges the shock into the analyzer. The analyzer measures the actual delivered energy. According to standards (like AAMI DF80), the delivered energy must be within $\pm 15%$ (or $\pm 4$ Joules, whichever is greater) of the selected energy.

Charge Time Verification

The BMET must measure the time it takes for the defibrillator to charge to its maximum energy level (typically 200 J or 360 J) while running on battery power. The charge time should not exceed 10 seconds. Excessive charge times indicate a degraded battery or a failing charging circuit.

Pacemaker Calibration Verification

Using the analyzer, the BMET verifies the pacemaker output:

• Rate Accuracy: The pacing rate (bpm) is measured and must be within $\pm 5%$ of the set value.
• Current Output: The output current (mA) is measured across the dummy load and verified against the set current.
• Pulse Width: The duration of each pacing pulse (typically 20 to 40 milliseconds) is verified to ensure it meets specifications.