Transesophageal atrial pacing is a reliable and minimally invasive procedure that has been used as a diagnostic and therapeutic tool.1 However, the ability to achieve TAP can be limited by patient discomfort and a small anatomic stimulation window (ie, ZOC). Human patients describe discomfort as a retrosternal burning sensation, and IV administration of sedatives is occasionally required to perform this procedure.2–4 The development of discomfort has been linked to PTs > 15 mA.3–5 Additionally, it has been reported that unintentional EMS caused by transesophageal pacing stimuli may also contribute to patient discomfort.6 In 1 report,7 investigators proposed TAP as a method for temporary emergency cardiac pacing, and as such, a wide ZOC would logically facilitate rapid, successful initiation of TAP.
Multiple independent variables of TAP (including stimulus pulse width and certain physical characteristics of the pacing catheter [ES and IS]) have been evaluated to determine their effects on PT and ZOC. It has been clearly determined that a wide pulse width results in a lower PT, with the optimal recommended pulse width being 8 to 10 milliseconds.3,4 Studies4,6,8 conducted to investigate the relationship between IS and PT have revealed that the lowest PT can be achieved by use of a pacing catheter with an IS between 15 and 29 mm. However, the authors are not aware of any studies performed to evaluate the impact of a catheter with < 10 mm between the pacing electrodes. A single study8 was conducted to evaluate the impact of various sizes of electrodes on TAP by use of electrodes with total surface areas of 22, 44, and 66 mm2, and no significant impact of the size of the electrodes was identified. However, that study8 involved the use of 3 electrically connected 2.4-mm poles, which were separated by 0.5 mm, to simulate single larger poles. As such, physical separation of the poles may have impacted the results because an electrical field created by multiple separated poles may not simulate an electrical field created by a single large pole.
Shape of the pacing catheter may also impact transesophageal pacing. A recent study9 of dogs in which investigators compared 2 pacing catheters with differing catheter characteristics (differences in catheter shape and ES) revealed significant differences in pacing outcomes. Investigators of that study9 did not isolate the effects of catheter characteristics on PT and ZOC, but results suggested that the size of the electrodes and catheter shape (curved vs straight) may have a substantial effect on PT.
We hypothesized that a curved catheter with an IS of 15 mm and ES of 4 mm will result in the lowest PT, widest ZOC, and minimal EMS in dogs undergoing TAP. Therefore, the purpose of the study reported here was to determine the effect of IS, ES, and catheter shape on overall PT, ZOC, and the degree of EMS during TAP in dogs.
Materials and Methods
Animals
Ten healthy purpose-bred dogs were used in the study. Body weight ranged from 10.5 to 31 kg (mean, 18.8 kg). There were 6 Beagles and 4 mixed-breed dogs. One of the Beagles was a sexually intact female; the other 9 dogs were sexually intact males. No cardiac conduction disturbances were detected for any dog by use of surface ECG. The study protocol was reviewed and approved by the Institutional Animal Care and Use Committee of Michigan State University.
Each dog was premedicated with acepromazine (0.02 mg/kg, IM). Anesthesia was induced with propofol (2 to 3 mg/kg, IV) and maintained with isoflurane in oxygen delivered via a precision vaporizer in a standard rebreathing system. Each anesthetized dog was placed in left lateral recumbency and monitored by use of surface ECG throughout the study.
Catheter shape
A modified transesophageal pacing cathetera with 5-mm olive-shaped electrodes and a stylet (0.40 mm in diameter) was used to evaluate the effects of catheter shape on pacing outcomes during TAP. This malleable catheter was naturally straight; however, the wire stylet allowed us to manipulate the catheter to create a bend or curve. Pacing was attempted without the use of the stylet and then repeated with the stylet wire creating a curved catheter. Distance between the electrodes was 15 mm.
IS
A modified transesophageal pacing cathetera with 5-mm olive-shaped electrodes and a curved stylet (0.40 mm in diameter) was used to determine the effects of the space between electrodes on pacing outcomes during TAP. The IS was defined as the distance from the distal end of the proximal electrode to the proximal end of the distal electrode. Interelectrode spaces of 5, 15, and 25 mm were evaluated. There was variation in electrode spacing for the commercial catheter; therefore, the distal electrode was removed so that the space between electrodes was 5 mm.
ES
Three modified curved 6F electrophysiologic cathetersb with electrodes of 2, 4, and 6 mm were used to evaluate the effects of ES on outcomes during TAP. Modification involved applying electrically conductive epoxy to the standard 2-mm electrodes and affixing a 0.31-mm-thick sheet of copper cut in lengths of 2, 4, and 6 mm to create the new ES. Catheter modifications were necessary because there were no commercially available catheters that met the specifications required for the study. Total surface area of the electrodes was 12.56, 31.41, and 43.98 mm2 for 2, 4, and 6 mm, respectively. Impedance was tested for conduction integrity. The IS was 20 mm for each electrode.
Pacing
Each catheter was passed transorally into the distal portion of the esophagus. A semiridged hollow silicone tubec (6 mm in diameter) was used as a guide because without it, the flexible pacing catheter would fold on itself within the oropharynx and esophagus. A location 7.6 cm caudal to the dorsal border of the scapula (with the limb in a neutral standing position) was used as an external anatomic landmark for initial placement of the distal pole of the pacing catheter. Previous experiments performed by our research group revealed this location was reliably caudal to the heart in all dogs. Once the pacing catheter was in place within the esophagus, the silicone guide was removed. Bipolar atrial pacing was then attempted with the distal electrode as the anode and proximal electrode as the cathode. A transesophageal pulse generatord was used with each catheter configuration. Pacing was initially attempted by use of a pulse amplitude of 40 mA, pulse width of 10 milliseconds, and pulse frequency 20 beats/min greater than the intrinsic heart rate. The catheter was then slowly withdrawn until atrial capture was achieved as evident on the ECG. The PT, defined as the minimum pulse amplitude required to pace the atria throughout the respiratory cycle, was determined and recorded at all sites of atrial capture. For each dog, the degree of EMS was subjectively scored by a single investigator (EHC). Scoring was performed on a scale of 0 to 3 as follows: 0 = no appreciable EMS; 1 = cutaneous twitches but procedure could continue unaffected; 2 = stimulation of the diaphragm (respiration rate matched pacing rate) or forelimbs, which caused body movement; and 3 = movement of the entire body.
After the initial site of capture was determined, the catheter was withdrawn in increments of 5 mm. The PT was determined at each site until atrial capture could not be achieved with a pulse amplitude of 40 mA. The distance over which atrial capture was achieved was termed the ZOC.
Statistical analysis
Statistical analysis was performed by use of a statistical program.e Normality of the data was evaluated by use of the Shapiro-Wilk test. Differences in PT, ZOC, and EMS for each catheter configuration were analyzed by use of Student paired t tests. Simple linear regression analysis was used to evaluate the relationship between ZOC and IS. The coefficient of determination (ie, R2) and associated P values were reported; assumptions of the linear regression were verified by examination of the residual plots. A Bonferroni correction was used, when appropriate. Values of P < 0.05 were considered significant.
Results
Pacing
Pacing was achieved in all dogs for all catheter configurations (Table 1). The effect of catheter shape, ES, and IS on PT, ZOC, and EMS was plotted (Figure 1).
Box-and-whiskers plots depicting the effect of catheter shape (straight [S] or curved [C]), ES (2, 4, or 6 mm), and IS (5, 15, or 25 mm) on PT (A), ZOC (B), and EMS (C) during TAP in 10 dogs. Scoring of EMS was on a scale of 0 to 3 as follows: 0 = no appreciable EMS; 1 = cutaneous twitches but procedure could continue unaffected; 2 = stimulation of the diaphragm (respiration rate matched pacing rate) or forelimbs, which caused body movement; and 3 = movement of the entire body. Each box represents the first and third quartiles, the horizontal line in each box represents the median, the whiskers represent the 10th and 90th percentiles, and the circles represent outliers. *Median score was 0, and the first and third quartiles were 0. a,bWithin a characteristic, values with different letters differ significantly (P < 0.05).
Citation: American Journal of Veterinary Research 77, 3; 10.2460/ajvr.77.3.275
Mean ± SD (range) values for effects of catheter characteristics during TAP in 10 dogs.
| Characteristic | Category | PT (mA) | ZOC (cm) | EMS |
|---|---|---|---|---|
| Catheter shape | Straight | 21.00 ± 7.56a (10.0–32.5) | 3.90 ± 2.17a (2.0–8.5) | 1.49 ± 0.77a (0.56–2.17) |
| Curved | 13.75 ± 5.03b (7.5–22.5) | 4.94 ± 1.78b (3.0–9.0) | 0.84 ± 0.89b (0.03–1.43) | |
| ES | 2 mm | 12.00 ± 5.63 (5.0–22.5) | 5.05 ± 2.02 (2.0–8.5) | 0.54 ± 0.38a (0–1.17) |
| 4 mm | 11.75 ± 6.13 (5.0–20.0) | 5.70 ± 2.77 (2.5–12.0) | 0.82 ± 0.70a (0–1.67) | |
| 6 mm | 14.00 ± 7.09 (7.5–22.5) | 5.10 ± 2.67 (1.0–11.0) | 1.24 ± 0.90b (0.33–2.29) | |
| IS | 5 mm | 21.75 ± 8.74a (12.5–32.5) | 2.50 ± 1.13a (1.0–4.5) | 0.10 ± 0.32a (0–1.00) |
| 15 mm | 13.75 ± 5.03b (7.5–22.5) | 4.85 ± 1.78b (3.0–9.0) | 0.91 ± 0.88a,b (0.10–1.43) | |
| 25 mm | 14.50 ± 5.37b (7.5–25.0) | 6.45 ± 2.30b (I.5–9.0) | 1.31 ± 0.44b (0.71–2.00) |
Scoring of EMS was on a scale of 0 to 3 as follows: 0 = no appreciable EMS; 1 = cutaneous twitches but procedure could continue unaffected; 2 = stimulation of the diaphragm (respiration rate matched pacing rate) or forelimbs, which caused body movement; and 3 = movement of the entire body.
Within a column within a characteristic, values with different superscript letters differ significantly (P < 0.05).
Catheter shape
Use of the curved catheter resulted in a significantly lower PT than did use of the straight catheter. Significantly less EMS was detected with use of the curved catheter, compared with results for the straight catheter. Transesophageal pacing by use of the curved catheter resulted in a significantly wider ZOC than did pacing by use of the straight catheter.
IS
Transesophageal pacing with an IS of 5 mm resulted in a significantly higher PT than did pacing with an IS of 15 or 25 mm. No significant difference in PT was detected between an IS of 15 and 25 mm. Transesophageal pacing with an IS of 5 mm resulted in significantly less EMS than did pacing with an IS of 25 mm. A significant increase in width of the ZOC was detected as the space between the electrodes increased from 5 to 15 mm and from 5 to 25 mm. No significant difference in ZOC was detected when pacing was performed with an IS of 15 mm, compared with results for pacing with an IS of 25 mm. Additionally, regression analysis revealed a positive linear relationship (R2 = 0.47) between IS and ZOC.
ES
Use of an ES of 6 mm yielded significantly more EMS than did use of an ES of 2 or 4 mm. The ES had no significant effect on PT or width of the ZOC.
Discussion
Transesophageal atrial pacing is a simple, noninvasive method for control of heart rate in dogs without substantial atrioventricular conduction disturbances. In humans, many studies have been performed to identify ideal catheter characteristics. To our knowledge, only 1 study9 has been conducted to investigate the effect of differences in catheter shape on the ability to perform TAP in dogs. That study9 was limited and did not separately assess the effect of catheter shape, ES, and IS. Thus, the optimal characteristics of a transesophageal pacing catheter for use in dogs have not yet been identified. To the authors’ knowledge, the study reported here was the first in which investigators determined the effect of ES, IS, and catheter shape on PT, ZOC, and EMS in dogs.
Electrode size has been evaluated in humans by use of a multipolar catheter with the poles electrically connected to mimic a single larger pole.8 Results of that study8 to evaluate 3 ESs were similar, with no significant effect of ES on pacing outcomes. However, TAP performed by use of 4-mm (31-mm2) poles required the lowest overall PT and had the widest ZOC.
In a 1982 study,4 it was reported that an IS of 30 mm was optimal for obtaining the minimum overall PT for TAP in humans. Investigators in subsequent studies3,8 found no significant differences in pacing outcomes with an IS of 15, 22, and 28 mm or 10, 17, and 24 mm, respectively. The findings of these studies suggest that the effect of IS on pacing outcomes (considering PT, ZOC, and EMS) is complex and that use of different distances between the pacing electrodes may be beneficial, depending on the pacing scenarios. A smaller IS would be ideal when any degree of EMS would be detrimental to the procedure being performed or to minimize patient discomfort. Alternatively, when TAP is performed in a situation that is more of an emergency for a patient with hemodynamically substantial bradycardia, in which a wider ZOC would be desired to allow the operator to establish pacing rapidly, a catheter with a greater distance between the poles would be ideal.
To the authors’ knowledge, the information reported here was the first provided on the effect of catheter shape on pacing outcomes in humans or domestic animals. The results clearly indicated that a curved TAP catheter significantly improved all TAP outcomes (ie, lower PT, wider ZOC, and less EMS), compared with results for a straight catheter. There may be multiple reasons for this finding. The curved shape may have caused deviation or depression of the electrodes (perhaps caused by the stylet used to create the shape of the pacing catheter), thus enhancing contact of the pacing catheter with the esophageal surface. Additionally, the curved shape of the catheter may have resulted in a smaller distance between the electrodes and atria. This should result in a lower PT because it has been theoretically determined that as the distance from the electrical impulse to the target tissue decreases, the required stimulus is reduced.10,11 Because the study reported here was conducted without fluoroscopic guidance, we could not assess the relationship between the catheter electrodes and atria. Further investigation is needed to determine the manner by which catheter shape reduced PT and whether other catheter shapes might provide additional improvements.
Most commercial human TAP catheters are straight, and because pain associated with TAP has been linked to PT, a curved catheter may also prove to be advantageous in human medicine. Therefore, further studies are warranted regarding the importance of catheter shape for TAP in humans.
The ideal pacing catheter would result in a low PT and wide ZOC with minimal EMS. On the basis of the results of the present study, we recommend use of a curved multipolar catheter to perform TAP in dogs. A curved multipolar catheter is recommended to allow clinicians the flexibility to adjust the IS on the basis of the clinical situation. Although ES did not have a significant effect on pacing outcomes, we suggest use of a curved pacing catheter with an ES of 4 mm because this ES resulted in less EMS than did an ES of 6 mm and because it also resulted in the widest ZOC.
Acknowledgments
Presented in abstract form at the American College of Veterinary Internal Medicine Forum, Seattle, June 2013.
ABBREVIATIONS
| EMS | Extraneous muscular stimulation |
| ES | Electrode size |
| IS | Interelectrode spacing |
| PT | Pacing threshold |
| TAP | Transesophageal atrial pacing |
| ZOC | Zone of capture |
Footnotes
Esoflex 10S transesophageal pacing catheter, FIAB, Vicchio, Italy.
6F CRD supreme quadripolar electrophysiologic catheter, St Jude Medical, Saint Paul, Minn.
18F disposable esophageal stethoscope and temperature probe, 400 series, Mindray, Duluth, Ga.
Transesophageal cardiac stimulator, model 7A, CardioCommand, Tampa, Fla.
Excel, Microsoft Office 2010, Microsoft Corp, Redmond, Wash.
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