Overdrive Pacing

Article Author:
Michael Self
Article Editor:
Christopher Tainter
Updated:
11/1/2019 7:31:50 AM
PubMed Link:
Overdrive Pacing

Introduction

Temporary cardiac pacing (TCP) is a type of exogenous cardiac pacing in which an external energy source that delivers electrical impulses to stimulate the heart to contract faster than its native rate. This intervention can be used to over-ride a malignant tachydysrhythmia or compensate for symptomatic bradycardia. TCP is typically used for dysrhythmias refractory to pharmacological therapies or cardioversion. Temporary cardiac pacing is not a new intervention; it was first attempted in 1952 when Paul Zoll used hypodermic needles in the chest wall to deliver a pulsating external current for two patients with asystole.[1] Today, TCP is available in a broad spectrum of critical care settings, from pre-hospital to the intensive care unit, delivered via a variety of modalities including transcutaneous, transvenous, epicardial, and transesophageal.[2] This activity will focus on temporary cardiac pacing in critical care settings.

Anatomy

Exogenous cardiac pacing utilizes an external power source to deliver electrical impulses to the myocardium, directly stimulating myocardial depolarization and ventricular (or atrial) contraction and allowing the physician to manipulate the electromechanical conductance of the heart.

In normal physiology, an electrical impulse is generated at the sinoatrial (SA) node, is transmitted through the atrioventricular (AV) node, and then down the His-Purkinje system leading to sequential ventricular depolarization and contraction. In bradydysrhythmias, a variety of pathologic processes can affect any point in the conduction system, leading to an insufficient heart rate and, therefore, cardiac output. When used for bradydysrhythmias, exogenous pacing generates extrinsic electrical impulses that bypass the affected conduction system causing direct ventricular depolarization. The physician can then stimulate ventricular contraction at a rate greater than the native ventricular or junctional rate, increasing cardiac output to meet demand.[2][3]

In contrast, refractory tachydysrhythmias have varied pathophysiology. In supraventricular tachydysrhythmias, there is often a re-entrant pathway that bypasses the AV node or an ectopic and unregulated pacemaker. Anti-tachycardia pacing (ATP) attempts to correct this by disrupting the re-entrant circuit or over-riding the ectopic pacemaker with an external electrical impulse at a rate of 10 to 20 BPM greater than the native rate, changing the pattern of repolarization. In refractory monomorphic ventricular tachycardia, ATP prevents abnormal ventricular automaticity by changing the pattern of ventricular depolarization and creating more uniform repolarization. In refractory polymorphic ventricular tachycardia, increasing the rate of ventricular depolarization decreases the ventricular refractory period, thereby reducing susceptibility to the R on T phenomenon.[3][4] Using ATP to override a native dysrhythmia is also called overdrive pacing (ODP).

Temporary transcutaneous pacing (TTCP) uses external pads to transmit an electrical impulse through the skin, subcutaneous soft tissue and chest wall to stimulate ventricular depolarization. Adherent cutaneous pads are used in either the anterior-posterior or anterior-lateral positions, with the former being preferred.[1]

Temporary transvenous pacing (TTVP) uses central venous access, typically via the right internal jugular or left subclavian vein, to pass an electrode into the right ventricle (RV). Electrical impulses are then delivered to the RV endocardium, depolarizing it first, thus resulting in a left bundle branch block (LBBB) pattern on an electrocardiograph (ECG).

Epicardial pacing is most often deployed intraoperatively during cardiac surgery. Pacing electrodes get placed on the epicardium at the right atrium (RA) and RV, or RA and both ventricles.[2]

Transesophageal pacing occurs through placing an electrode in either the mid-esophagus, stimulating the RA, or in the gastric fundus, stimulating the RV through the diaphragm. Transesophageal pacing is not common due to difficult lead placement and patient discomfort.[1]

Indications

Temporary cardiac pacing may be indicated for any symptomatic bradydysrhythmia when permanent cardiac pacing is not immediately indicated, unavailable, or too risky, such as severe hemodynamic instability. The most common indication is symptomatic bradycardia due to AV nodal block. There are a variety of reversible causes of bradycardia for which TCP may be indicated, including acute myocardial infarction, electrolyte disturbances, drug toxicity, damage to the intrinsic conduction system during cardiac surgery or valve replacement/repair, cardiac trauma, cardiac abscess, myocarditis, heart transplant, and others. TCP may also be indicated for refractory ventricular tachycardia, electrical storm, or refractory polymorphic ventricular tachycardia. Prophylactically, TCP may be an option for preventing tachydysrhythmias such as atrial fibrillation and atrial flutter following cardiac surgery.[1][2][3][4][5]

Contraindications

There are no absolute contraindications to temporary cardiac pacing. However, TCP should be avoided or used with caution in certain scenarios. The most common reason to avoid any kind of temporary cardiac pacing is hemodynamically stable bradydysrhythmias with rare or tolerable symptoms. Clinicians should avoid cardiac pacing in hypothermia (unnecessary) and prolonged bradyasystolic arrest (futile). Clinicians should also avoid ransvenous pacing in patients with a prosthetic tricuspid valve, as catheter placement may damage the valve or the catheter may become stuck in the valve, and patients with an excessive risk of bleeding, including those with acute myocardial infarction receiving thrombolytics, anticoagulation and antiplatelet agents.[1][2][3]

Equipment

Temporary transcutaneous pacing requires a pulse generator and monitoring unit with standard defibrillation pads. These are ubiquitous in most medical settings. The pulse generator must have a pacing function. ECG electrodes, non-invasive or invasive blood pressure monitoring, and pulse oximetry are strong recommendations.[1][6]

Temporary transvenous pacing requires central venous access, typically with a 6 French (Fr) venous percutaneous introducer sheath, a transvenous pacing catheter, and an external pulse generator. Ultrasound guidance for central venous access is also a strong recommendation. A 12 lead-capable ECG machine, cardiac monitor, non-invasive or invasive blood pressure monitoring and pulse oximetry should be available. This procedure is sterile, and standard sterile technique is necessary.[1][2][3]

The transvenous pacing catheter is mostly bipolar, 3 Fr to 5 Fr in diameter and 100 cm long. Lines are typically present at 10 cm intervals to estimate catheter depth. Catheters may be flexible, semi-floating, or rigid. For most emergent indications, when temporary pacing would be in order, a semi-floating catheter with a balloon is used. The balloon holds 1.5 ccs of air and requires testing before insertion. The leading end of the catheter has two electrodes, of which the negative is most distal. Adapters allow the electrodes to be attached to the pulse generator.[2][3]

The external pulse generator delivers an electrical current through the pacing catheter, measured in milliamperes (mA). Generators share the same basic features, including electrical output and cardiac sensing components. These are present as dials on the generator’s face. Output control regulates the current delivered, functionally controlling the ability to obtain electrical capture. Rate control selects the pacing rate. Sensitivity control establishes the threshold at which a sensed intrinsically generated current inhibits the pacemaker from firing. This setting is the choice for demand (synchronous) pacing. For most emergent indications, the sensitivity control is turned to the lowest setting, providing asynchronous (fixed rate) pacing.[2][3]

Epicardial pacing typically requires an external pulse generator and surgically placed epicardial pacing electrodes. The electrode location requires confirmation with the surgical team.[2][7]

Technique

Temporary transcutaneous pacing:

  • Depending on the clinical scenario, consider analgesia and/or sedation before, or immediately after, initiating transcutaneous pacing as the electrical current needed for capture is painful.
  • Place the pacing pads in an anterior-posterior (preferred) or anterior-lateral configuration.
  • Attach the pads to the pulse generator, most often a defibrillator with pacing capabilities. Some pulse generators require the attachment of cardiac monitoring leads for proper functioning.
  • Select the pacing function on the defibrillator.
  • Select the desired rate per indication, typically 60 to 80 BPM or 10 to 30 BPM greater than the intrinsic rate.
  • Starting at 70 milliamps (mA), increase the output by 5 to 10 mA until the initiation of capture, indicated by a wide-complex QRS following every pacer spike (electrical capture) and signs of improved perfusion (mechanical capture).
  • It is critically important to confirm ventricular contraction (mechanical capture), as it is not difficult to be misled by electrical activity representing a QRS complex, which is possible by confirming a pulse which matches the set pacemaker rate, by palpation, echocardiogram, pulse oximetry, or arterial waveform.
  • The current at which capture is obtained is called the threshold current.
  • Once capture is confirmed, set the current at 5 to 10 mA higher than the threshold current to prevent loss of capture.
  • If the pacing rate does not get captured at greater than 120 mA, re-position the pads and repeat the above steps.[1][6]

Temporary transvenous pacing:

  • Ensure that the pulse generator is in good working condition and all equipment is at the bedside.
  • Obtain central venous access using an appropriately sized percutaneous venous introducer sheath, typically 6 Fr. Inappropriately sized sheaths may be unable to pass the pacing catheter, or may leak around it. The right internal jugular vein and left subclavian veins are the preferred vessels because of a more direct path to the right ventricle. Ultrasound guidance for placement is highly recommended. Detailed instruction on obtaining central venous access is beyond the scope of this article.

Placement without ECG guidance:

  • Connect the pacing electrodes to the pulse generator.
  • Insert the catheter into the introducer sheath so that the balloon and electrodes are past the distal end of the introducer sheath (approximately at the 20cm mark on the catheter). Note: do not inflate the balloon while it is within the introducer sheath.
  • Turn on the pulse generator. Set the pacing generator to the desired rate depending on the indication, typically 60 to 80 BMP or at least 10 BMP greater than the native rate. The initial output requires setting to 2 to 5 mA. Decrease the sensitivity to the lowest level.
  • Inflate the balloon and advance the catheter slowly. The cardiac monitor will typically show pacer spikes. When the catheter passes the right ventricle and contacts the endocardium, a wide QRS complex with an LBBB pattern will follow every pacer spike (electrical capture). It is also reasonable to intermittently deflate the balloon to check for capture.
  • Deflate the balloon, secure the catheter in place, and make a note of catheter depth.
  • Ensure mechanical capture by evaluating: signs of perfusion, peripheral/central pulse rate equals pulse generator set rate, pulse oximetry waveform, arterial line waveform.
  • Decrease the output slowly until capture is lost. Increase the output to regain capture; this is the threshold current, typically less than 1 mA. Increase the output to approximately 2.5 times the threshold current, typically 2 to 3 mA.
  • If demand pacing is the goal, adjust the sensitivity such that native cardiac electrical impulses inhibit the pacemaker. See the section on epicardial pacing below for a more in-depth discussion of demand pacing.

Placement with ECG guidance:

  • Connect the negative (distal) electrode to ECG lead V1 using an alligator clip.
  • Insert the catheter into the introducer sheath so that the balloon and electrodes are past the distal end of the introducer sheath (approximately at the 20 cm mark on the catheter). Note: do not inflate the balloon while it is within the introducer sheath.
  • Inflate the balloon and slowly advance the catheter. Closely monitor the ECG. ECG morphology should change predictably with electrode location.
    • In the high RA, there will be a large negative p-wave, typically greater than the QRS complex, followed by a negative QRS complex.
    • As the catheter passes through the RA, the p-wave becomes biphasic and then positive.
    • When the catheter passes into the RV, the p-wave becomes smaller and negative, followed by a deeply negative QRS complex.
    • Note: the duration of the QRS complex will depend on the native cardiac activity. For example, an AV nodal escape rhythm will have a narrow QRS complex, while a ventricular escape rhythm will have a wide QRS complex.
    • When the catheter makes contact with the right ventricular endocardium, an injury pattern will result, with a deep, negative QRS complex followed by marked ST elevation.
  • Advance the catheter until observing the RV pattern or RV endocardial pattern.
  • Deflate the balloon, secure the catheter in place, and make a note of catheter depth.
  • Ensure mechanical capture by evaluating: signs of perfusion, peripheral/central pulse rate equals pulse generator set rate, pulse oximetry waveform, arterial line waveform.
  • Decrease the output slowly until capture is lost. Increase the output to regain capture; this is the threshold current, typically less than 1 mA. Increase the output to approximately 2.5 times the threshold current, typically 2 to 3 mA.
  • If demand pacing is the desired goal, adjust the sensitivity such that native cardiac electrical impulses inhibit the pacemaker. See the section on epicardial pacing for a more in-depth discussion of demand pacing. [1] [2] [3]

Epicardial pacing:

  • Select the desired parameters on the pulse generator, depending on the clinical scenario. For the prevention or treatment of tachyarrhythmias, this may be asynchronous. For the prevention or treatment of bradydysrhythmias, this may be asynchronous or synchronous (demand) pacing.
  • If demand pacing is the objective, adjust the sensitivity such that native cardiac electrical impulses inhibit the pacemaker. Then set the pacemaker rate. In demand pacing, this represents the backup rate, and the pacemaker will deliver an impulse if it does not sense a native electrical impulse at a rate greater than the backup rate. 
  • Adjust the pacemaker output and evaluate for signs of mechanical capture as described above.[2][7]

Complications

Complications of TTCP include pain, failure to obtain capture, loss of capture, and rarely cutaneous burns. Complications of TTVP are more numerous, and many are related to central venous access, including infection, bleeding, damage to nearby structures, vein thrombosis, air embolism, pneumothorax, and others. Additionally, TTVP can sometimes induce VT or ventricular fibrillation.[1][2][3][8][9]

Clinical Significance

Select clinical pearls regarding TCP include:

  • Careful patient selection is paramount as the risks associated with certain types of TCP outweigh the benefits.
  • TCP should be used when maximum medical therapy, including cardioversion when appropriate, has failed.
  • Assuming that electrical capture is equivalent to ventricular or mechanical capture is one of the most significant pitfalls in TCP. Electrical activity in the form of a QRS complex is sometimes present without subsequent ventricular contractions. TTCP is especially prone to this due to the chest wall impedance and greater depth of the ventricles from the pacing pads. Ensure mechanical capture is present by confirming a pulse that matches the set pacemaker rate, by palpation, echocardiogram, pulse oximetry, or arterial waveform. Check for signs of mechanical capture as mentioned above and re-evaluate them frequently, especially after transfers, procedures, or repositioning.[6]
  • Another pitfall related to TTCP is basing capture on the ECG waveform of a separate cardiac monitor. Many defibrillators/pulse generators require placement of their own ECG electrodes, and any decision about electrical capture should have their basis in ECG waveforms on the defibrillator/pulse generator itself, rather than a separate cardiac monitor.
  • TTVP catheter placement should be confirmed by a chest X-ray. Continuous echocardiography may also be useful to confirm lead placement.[10][11]

Enhancing Healthcare Team Outcomes

Temporary cardiac pacing is a potentially life-saving procedure for refractory dysrhythmias that can bridge patients to a destination therapy. However, TCP is not without risks, and a team-based, an interprofessional approach can improve outcomes, decrease adverse events, and increase patient safety. All members of the healthcare team should be aware of the indications for TCP to improve patient selection, therefore reducing the exposure of patients to the risks associated with this procedure unnecessarily. Fostering a culture of open, bidirectional communication may allow all members of the healthcare team to voice their concerns, potentially preventing adverse events.

During TTCP, a team-based approach involving physicians, nurses, and emergency medical technicians (EMTs) is essential for timely deployment. A coordinated approach to defibrillation pad and ECG electrode placement, including rolling the patient for anterior-posterior pad placement, is necessary. After initiating TTCP, nurses should frequently evaluate the patient for pain and communicate their concerns to the provider if additional analgesia or sedation is required. The entire healthcare team should know how to assess for signs of mechanical capture and alert the appropriate provider if a loss of capture is suspected.

Temporary transvenous pacing also requires an interprofessional team-based approach. An interprofessional approach to central venous catheter placement and maintenance has been shown to decrease the complication rate.[12] Similar to TTCP, all members of the healthcare team should frequently evaluate for signs of a loss of capture and alert the appropriate providers if suspected.

Temporary cardiac pacing is a bridge from a variety of diseases to definitive therapy and often requires a an interprofessional approach. Cardiology and electrophysiology specialists should be involved early to prepare for permanent pacemaker placement, percutaneous coronary intervention, or other definitive care. In suspected overdoses, toxicology consultation can guide the initial treatments, provide information on the expected course, and the suitability of a particular poisoning for TCP. In post-cardiac surgery patients, intensive care management by intensivists and cardiothoracic surgeons has been shown to improve outcomes and decrease adverse events.[13] Coordination between the intraoperative and intensive care unit teams regarding epicardial lead placement and intraoperative pacing requirements is essential for a smooth transition of care. Checklists may decrease adverse events related to the transfer of care.[14] With the interprofessional approach, patients can achieve optimal results while experiencing minimal adverse events. [Level 5]

Nursing, Allied Health, and Interprofessional Team Interventions

Nurses working in the emergency department, cardiology or the cardiac surgery ward should be familiar with external pacing and indications.


References

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[12] Zingg W,Cartier V,Inan C,Touveneau S,Theriault M,Gayet-Ageron A,Clergue F,Pittet D,Walder B, Hospital-wide multidisciplinary, multimodal intervention programme to reduce central venous catheter-associated bloodstream infection. PloS one. 2014;     [PubMed PMID: 24714418]
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