Transcutaneous cardiac pacing (TCP) is a quick, non-invasive method used in emergency medical situations to support a patient’s dangerously slow heart rate. The TCP delivers electrical impulses through pads placed on the skin to stimulate the heart muscle. Determining the correct settings is a time-sensitive process, as the goal is to stabilize the patient’s circulation rapidly. The initial impulse setting involves both a rate (measured in beats per minute) and a current (measured in milliamperes), which are adjusted based on the patient’s physical response.
Understanding Transcutaneous Pacing
Transcutaneous pacing is employed as a temporary, external measure for severe, symptomatic bradycardia—a heart rate too slow to maintain adequate blood flow. This technique is often a bridge therapy, used until a more permanent or stable pacing method, such as transvenous pacing, can be established. Conditions like high-grade atrioventricular block or severe sinus node dysfunction necessitate this emergency external cardiac support.
The pacing device works by sending an electric current through the chest wall to the heart muscle, causing it to contract. This energy is delivered via large, adhesive electrode pads placed on the skin, typically in an anterior-posterior or anterior-lateral configuration. A severely slow heart rate can lead to symptoms of poor perfusion, including low blood pressure, altered mental status, and shortness of breath. Because TCP is non-invasive and initiated rapidly, it is a valuable tool for stabilizing patients in a cardiac emergency.
The Standard Initial Pacing Rate and Current
The initial setting for transcutaneous pacing involves selecting a rate and a current, which serve as starting points for the titration process. The pacing rate is generally set to adequately support circulation without overworking the heart. This initial rate is commonly set between 60 and 80 beats per minute (bpm), with 70 bpm or 80 bpm being frequently cited default settings.
The current, measured in milliamperes (mA), is the strength of the electrical impulse and the other half of the initial setting. Standard practice often sets the starting current to a minimal output, such as 0 mA, 5 mA, or 10 mA. Starting low protects the patient from unnecessary discomfort while the current is gradually increased to find the therapeutic level. Some protocols for unstable patients may recommend starting at a higher current, such as 40 mA or 70 mA, to decrease the time to capture. It is important to note that this starting current is usually far below the level required to stimulate the heart, necessitating immediate adjustment once pacing begins.
Determining Effective Pacing Capture
After setting the initial rate and minimal current, the next step is to gradually increase the current until effective “capture” is achieved. Capture is the point at which the electrical impulse successfully causes the heart muscle to depolarize and contract. This process of increasing the current in small increments, often 5 to 10 mA at a time, is called titration.
The first indication of successful stimulation is electrical capture, observed on the cardiac monitor as a wide QRS complex and T wave following each pacing spike. Electrical capture alone is insufficient; the clinician must also confirm mechanical capture, meaning the heart contraction produces a palpable pulse matching the set rate. The lowest current output achieving reliable electrical and mechanical capture is the pacing threshold, typically 40 to 80 mA. To ensure continuous, reliable pacing, the final therapeutic current is set 10% to 20% higher than this threshold.
What the Patient Feels
The electrical current required for transcutaneous pacing must be strong enough to pass through the skin, fat, and chest wall to reach the heart, making the process uncomfortable for a conscious patient. The high current often causes involuntary contractions of the skeletal muscles in the chest wall, which patients describe as a strong, painful thump or knocking sensation.
The large electrode pads are commonly placed in an anterior-posterior configuration to direct the current through the heart. Higher current levels needed for capture correlate with increased discomfort, even with modern pad designs. For conscious patients, sedation and pain management are necessary to help them tolerate the procedure, especially if pacing is prolonged. Medications are carefully chosen to minimize side effects that could worsen the patient’s compromised circulation.