Catheter Ablation of Ventricular Arrhythmias Using Half-Normal Saline

Introduction

Catheter ablation for ventricular arrhythmias (VA) has become increasingly common with the advent of more sophisticated three-dimensional mapping systems and irrigated-tip radiofrequency (RF) catheters with contact force sensing ability. Although success rates remain high for catheter ablation of outflow tract VA in the endocardium, VA arising from intramural sites in the left ventricular summit, papillary muscles, or interventricular septum can pose a challenge due to inadequate lesion depth.¹ The inability of a standard RF ablation to produce durable ablation lesions in these sites is associated with unsuccessful ablation procedures and recurrent VA. Several techniques have been developed to overcome this challenge, including simultaneous two-catheter unipolar RF ablation,² bipolar RF ablation,³ transcoronary ethanol ablation,⁴ and needle RF catheter ablation.⁵ Open-irrigated RF catheter ablation using half-normal saline irrigation, as opposed to standard irrigation with 0.9% sodium chloride, has been associated with increased lesion size and depth in myocardial tissue, with several recent studies attesting to its safety and efficacy.^6-10

In this article, we describe a ventricular tachycardia (VT) ablation procedure in a patient treated successfully with half-normal saline irrigation.

Case Description

A 54-year-old male with no prior history of cardiovascular disease presented to the primary care clinic with an elevated pulse. A 12-lead electrocardiogram revealed frequent and unifocal premature ventricular complexes (PVCs) in the form of ventricular bigeminy and couplets (Figure 1). Ambulatory cardiac monitoring showed frequent PVCs with couplets, bigeminy and trigeminy, in addition to runs of VT of up to 1 minute in duration. Overall, 48-hour Holter monitoring recorded over 71,000 ventricular ectopic beats (29% VA burden). Symptoms included exertional dyspnea and chest discomfort. He denied syncope or near syncope. Echocardiography demonstrated normal left ventricular function (ejection fraction 55-60%) with mild chamber enlargement (end-diastolic dimension of 6.1 cm) and normal valvular function. Coronary arteriography did not reveal significant coronary artery disease. Treatment with beta-blockers was ineffective. 

He was taken to the electrophysiology laboratory for three-dimensional electroanatomic mapping with intracardiac echocardiography guidance (CARTOSOUND, Biosense Webster, Inc., a Johnson & Johnson company) under light sedation with remifentanil. Left ventricular mapping was performed with a contact force sensing irrigated-tip RF ablation catheter (THERMOCOOL SMARTTOUCH, Biosense Webster, Inc.) via the retrograde aortic approach, with intermittent boluses of unfractionated heparin administered to maintain an activated clotting time >300 seconds. Bipolar voltage in the left ventricle was normal. The patient continued to have spontaneous PVCs, couplets, and short runs of VT at baseline. Pace and activation mapping techniques were used to localize the VA to the posteromedial papillary muscle, confirmed with real-time intracardiac echocardiography (Figures 2 and 3). The earliest electrograms recorded during VA had an early far-field component (Figure 4), indicative of a deeper focus within the papillary muscle.¹¹ Standard RF ablation to this area at 40-45 watts for several minutes significantly reduced the patient’s spontaneous VA burden. He no longer had runs of VT, but continued to have frequent PVCs. We then switched the irrigated catheter solution from 0.9% to 0.45% sodium chloride, with 1,000 units/liter unfractionated heparin. We remapped the region around the posteromedial papillary muscle and consolidated our lesions in the area of earliest activation during VA, starting ablation at 25 watts and titrating up to 35 watts to target an impedance drop of 10% with 30- to 60-second lesions delivered at each location. An electrophysiology study was performed using programmed ventricular stimulation, burst pacing, and boluses of intravenous epinephrine to confirm non-inducibility of VA post ablation.

The patient returned to clinic one week post ablation, and a repeat 12-lead ECG (Figure 5) and ambulatory cardiac monitor showed maintenance of sinus rhythm. 

Discussion

We describe a case of a patient with a papillary muscle VA who was successfully treated by irrigated RF catheter ablation using half-normal saline. Both in vivo and ex vivo studies have recently demonstrated open-irrigated ablation using a solution with a lower ionic concentration. Thus, higher impedance can create larger and deeper lesions. It has been proposed that the higher impedance surrounding the catheter tip with half-normal saline (~180 ohms), relative to normal saline (~90 ohms), decreases dissipation of RF energy into the irrigant and allows for more current delivery into the myocardium.^1,9 

In a multicenter study of 94 PVC/VT ablation cases refractory to standard RF ablation, ablation with half-normal saline was successful in 83% of patients with a mean follow-up of over 6 months. Additionally, there were no complications related to half-normal saline use, although steam pops were observed in 12.6% of patients.⁹ The rate of steam pops is significantly higher than that seen with standard RF ablation (~1.5%), and is related to the higher tissue temperatures achieved with the higher amount of current delivery and resistive heating seen with half-normal saline. Although no cases of cardiac perforation or tamponade were reported, discretion is advised with the use of half-normal saline in areas of thinner tissue — for example, in left ventricular aneurysms, on the right ventricular free wall, in atrial tissue, or in the coronary venous system. Furthermore, the vast majority of study patients with VAs treated with half-normal saline had structural heart disease (71% with ejection fraction <50%, and largely non-ischemic cardiomyopathy). An accompanying editorial cautioned against extending the study findings to catheter ablation of VA in structurally normal hearts, without the establishment of safe parameters for energy delivery, warning that “steam pops in these scenarios are not as likely to be benign.”⁸ In our patient, care was taken to titrate energy delivery to target steady impedance drops of ~10% from baseline, avoiding excessively rapid rises or falls. Also, by using intracardiac echocardiography, one can directly visualize the catheter-tissue interface for changes in tissue echogenicity and microbubble formation to titrate RF power and duration accordingly.^12,13

There is another benefit to irrigated RF with half-normal saline in patients with structural heart disease. The lower ionic concentration translates to a sodium chloride load of 4.5 grams per liter bag, versus 9 grams with normal saline irrigant. This theoretically has the potential to reduce the need for loop diuretic administration and the incidence of post-operative heart failure exacerbation. Furthermore, the administration of hypotonic fluid did not contribute to hyponatremia or hypokalemia. In the multicenter study, there was no significant difference in post-operative serum electrolytes concentrations (sodium, potassium, chloride) with half-normal saline usage.

Conclusion

Standard mapping and catheter ablation techniques are highly successful for the majority of VAs arising from a normal endocardium, but the results of catheter ablation for VA arising from thicker structures or intramural foci have been suboptimal. One technique developed to address this challenge is irrigated RF ablation using half-normal saline, as opposed to 0.9% sodium chloride. The ability to deliver deeper, larger, and more durable RF lesions can translate into more effective outcomes when ablating on the interventricular septum, left ventricular summit, or papillary muscles. Small studies have suggested ablation with half-normal saline is safe, although the higher incidence of steam pops deserves circumspection when ablating in structurally normal hearts or thin tissue. Additional studies are needed to define the optimal parameters for power, contact force, and lesion duration for half-normal saline RF ablation before this technique becomes more broadly adopted into clinical practice. 

Disclosures: The author has no conflicts of interest to report regarding the content herein. 

Erratum: Figure 5 in the print edition of the article "Catheter Ablation of Ventricular Arrhythmias Using Half-Normal Saline" was incorrect (Figure 1 was inadvertently placed). The correct Figure 5 is included here in the online version of this article. We sincerely regret the error.   

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