Transcutaneous pacing modes

Transcutaneous Pacing (TCP) is a temporary means of pacing a patient’s heart during an emergency and stabilizing the patient until a more permanent means of pacing is achieved.

It is accomplished by delivering pulses of electric current through the patient’s chest, stimulating the heart to contract. The most common indication for TCP is symptomatic bradycardia, most commonly resulting from acute MI, sinus node dysfunction, and complete heart block. During TCP, pads are placed on the patient’s chest either in anterolateral position or anterior-posterior (AP) position. The AP position is preferred because it minimizes transthoracic electrical impedance by sandwiching the heart between the two pads. Current is applied until electrical capture (characterized by a wide QRS complex since the SA node-AV node conducting pathway is bypassed, with tall, broad T-waves on the EKG) occurs. In addition to synchronized TCP, there is an option for asynchronous TCP in cases of VF, VT, complete heart block. Overdrive pacing is used to stop symptomatic tachydysrhythmias.

Finally, do not be fooled by the monitor into believing that the appearance of QRS complexes means that the patient’s heart has been captured and is delivering a sustainable blood pressure! You need to have some way to tell that the heart is being paced and generating a blood pressure; have a pulse oximeter or arterial line waveform for confirmation of the monitor’s electrical activity.

Indications: Hemodynamically significant (hypotension, chest pain, pulmonary edema, altered mental status) bradydysrhythmias unresponsive to atropine, asystolic cardiac arrest (more likely to be successful when initiated early after a witnessed arrest–unwitnessed arrest seldom responds to transcutaneous pacing), failed intrinsic pacemaker. When considering institution of transcutaneous pacing, always think about alternate causes for acute dysrhythmia, e.g. trauma, hypoxia, drug overdose, electrolyte imbalances and hypothermia. Treat underlying cause.

Technique: Ideal pacer pad placement “sandwiches” the heart between the pacing pads and mimics the heart’s normal electrical axis. Optimal placement for pads varies by manufacturer, but is generally anterior-posterior or anterior-lateral, with the former being most common. Begin at 10 milliamps and increase by increments of 10 until capture is noted. Target rate is generally 60-80 bpm. Strongly consider sedation, as external pacing can be quite uncomfortable. Most patients cannot tolerate currents of 50 milliamps and higher without sedation. Often 50-100 mA are required. Ideal current is 1.25x what was required for capture.

Mechanical capture of the ventricles is evidenced by signs of improved cardiac output, including a palpable pulse, rise in blood pressure, improved level of consciousness, improved skin color and temperature. Both electrical and mechanical capture must occur to benefit the patient. Pulses are difficult to palpate due to excessive muscular response.

It is safe to touch patients (e.g. to perform CPR) during pacing.

Pitfalls

• Skeletal muscle contraction occurs at current levels as low as 10 milliamps, and does NOT suggest electrical or mechanical capture.
• Most common reason for not obtaining capture is not adequately increasing the current. Current should be increased as much as necessary for electrical capture.
• Undersensing: when a pacemaker fails to detect intrinsic activity, and therefore delivers a pace pulse.
• Oversensing is inappropriate inhibition of the pacemaker due to detection of signals other than R waves (e.g. muscle artifact).
• Pacing thresholds may change without warning and capture can readily be lost