My Variables Only Have 6 Letters: August 2010

Being out of school only recently, I'm often asked "book" questions which are likely to be fresh in my mind. One of these that had me stumped was simply, "what is the appropriate current settings for pediatric transcutaneous pacing?"

I had no answer.

Honestly I had no idea, but assumed it would be weight based, and along the lines of the PALS guidelines for defibrillation. However, when I researched this topic in my PALS book I found there were no answers for pediatric pacing [1]. In fact, there was little mention of TCP whatsoever! Going over to ACLS I found no answers for current settings in adults, just when it was indicated [2].

However, in Paramedic school we had been taught the appropriate current ranges for TCP in adults, which ranged from 20-200 mA. Zoll et al found that most adults responded to TCP in the range of 40-70 mA, however, some required currents up to the device maximum of 140 mA [3]. After a few hours of searching for guidelines specific to pediatrics (including the Philips, Physio-Control, and Zoll websites), I came across a study on TCP in pediatrics which focused on the current required for different electrode sizes. Much to my amazement, the current settings required for external transcutaneous pacing of pediatrics are the same as for an adult!

A total of 56 pacing trials were conducted, 53 of which were successful in obtaining capture. A mean output of 63 +/- 14 mA (range, 42-98) at threshold using the large electrodes was comparable to published adult requirements. Béland MJ et al [4]

How could this be, wouldn't a smaller heart need less energy?

It seemed paradoxical at first, but reviewing the anatomy and physiology of a myocyte with an emphasis on the physics aspect puts it into perspective [5]. Each myocyte in the heart is a part of what amounts to an big electromechanical pump. Given a sufficient input stimulus a myocyte contracts and forwards a stimulus to its neighbors, which follow suit, leading to the eventual coordination of systole and diastole.

The goal of any artificial cardiac pacemaker, whether internal or external, is to act as the primary input stimulus by applying a current to an area of the heart which exceeds the stimulation threshold, i.e. the current required to cause a response from the myocardium.

Therefore, transcutaneous cardiac pacemakers attempt to exceed the stimulation threshold of a single area. It would be hard to achieve coordinated ventricular activity if the current was too high, instead you would have defibrillation. It stands to reason that if the only threshold required to overcome is the stimulation threshold of a single area of the myocardium, the weight of the heart--generalized as the weight of the patient--would be irrelevant.

In contrast, the goal of defibrillation is to bring all electrical activity in the heart to a halt. Defibrillation is not successful unless the all of the reentrant activations of ventricular fibrillation are stopped. Therefore the therapeutic energy levels are going to be proportional to the amount of myocardium you are acting on. Hence, pediatric defibrillation energy dosages are weight based.

So what seemed counterintuitive at first, is actually fairly logical. Pediatric transcutaneous cardiac pacing has the same energy requirements as adults because myocardium has the same stimulation threshold regardless of age. This deduction is supported in the literature as well:

No correlation has been defined between transcutaneous pacing threshholds and age, body weight, body surface area, chest diameter, cardiac drug therapy, or etiology of underlying heart disease. [6]

So there we have it, transcutaneous cardiac pacing current setting ranges are universal amongst our patient population. Below is a guideline I've created as a supplement to the material contained within PALS:

Pediatric Transcutaneous Cardiac Pacing
Symptomatic bradycardia in the pediatric population is most often related to hypoxia secondary to respiratory etiologies. In rare situations it may exist in spite of adequate ventilation and oxygenation. Given the presence of a high degree heart blocks, or symptomatic bradycardia refractory to aggressive BLS and ALS treatments, transcutaneous cardiac pacing should be initiated without delay.

Indications

High degree heart blocks

Symptomatic bradycardia refractory to ventilation, oxygenation, chest compressions, and pharmacological treatments

Contraindications
The only contraindication of TCP is an inability to place the pads on the patient without overlap or sufficient distance between them.

Side Effects
The side effects of TCP are most frequently muscle activation and associated pain. These are dose dependent effects which are a combination of the current delivered, size of the pads, location of the pads, and width (time) of the delivered pulse [7].

To minimize these side effects use the largest available pads, placing them in an Apical-Posterior fashion. While larger pads require higher current outputs, there is a decrease in the current delivered per surface area reducing the side effects associated with TCP.

Often, management of these side effects is achieved through concurrent pharmacological treatment with analgesics and/or sedatives.

Dose
Pediatric transcutaneous cardiac pacing (TCP) is defined by two dosing parameters: output current and rate. This guideline assumes the pacemaker is in fixed mode.

Output Current
As with adult patients, the output current for pediatric transcutaneous cardiac pacing should begin at 20 mA (or the lowest setting available) and increase in 5-10 mA increments until electromechanical capture is obtained. Additionally, the current may be increased an additional 5-10 mA above the determined threshold to ensure continued capture. If the device maximum output current is reached and no electromechanical capture exists, discontinue TCP and troubleshoot. Attempt an alternative pad placement (anterio-apical or anterior-posterior) and ensure the negative pad is on the anterior aspect of the chest. If capture is still not obtained, resume CPR and obtain expert consultation.

Output Rate
In contrast to adult patients, the output rate for pediatric transcutaneous cardiac pacing is age based. The final output rate should be titrated to an adequate systolic blood pressure to resolve perfusion problems, e.g. an improvement in mental status. Care should be taken to avoid tachycardic rates or hypertension. Consult a length-based resuscitation tape (e.g. Broselow™ tape) for appropriate starting output rates and systolic blood pressure. An example table is given below, adapted from the North Carolina 2009 EMS Standards [8]:

Age Rate (bpm) Systolic BP (mmHg)

0-3 mo 120-150 85 (+/-25)

3-6 mo 120-130 90 (+/-30)

7-10 mo 120 96 (+/-25)

11-18 mo 110-120 100 (+/-30)

19-35 mo 110-120 100 (+/-20)

3-4 yr 100-110 100 (+/-20)

5-6 yr 100 100 (+/-15)

7-9 yr 90-100 105 (+/-15)

10-12 yr 80-90 115 (+/-20)

>12 yr 70-80 120 (+/-20)

Age	Rate (bpm)	Systolic BP (mmHg)
0-3 mo	120-150	85 (+/-25)
3-6 mo	120-130	90 (+/-30)
7-10 mo	120	96 (+/-25)
11-18 mo	110-120	100 (+/-30)
19-35 mo	110-120	100 (+/-20)
3-4 yr	100-110	100 (+/-20)
5-6 yr	100	100 (+/-15)
7-9 yr	90-100	105 (+/-15)
10-12 yr	80-90	115 (+/-20)
>12 yr	70-80	120 (+/-20)

References

My Variables Only Have 6 Letters

Pages

Monday, August 23, 2010

Pediatric Transcutaneous Pacing