“Junctional escape rate” could be what your text is referring to by the expression “escape de unión”. I believe the complete expression is “tasa de escape de unión”, not just “escape de union”, as per the second reference.

My confidence level is low though since the medical field does not belong to my main specialties.

I hope this helps.

https://cyberleninka.org/article/n/1112486
JSÈiL JOURNAL OF MEDICAL
«P?" CASE REPORTS
CASE REPORT Open Access
Autonomic etiology of heart block in amyotrophic lateral sclerosis: a case report

An electrocardiogram (ECG) was obtained which demonstrated hemodynamically stable new third degree atrioventricular (AV) block with a junctional escape rate of 50 beats per minute (Figure 1). A previously obtained ECG indicated normal sinus rhythm. On further review, he was not prescribed medications with AV nodal blocking properties while at home or during his hospitalization. On echocardiogram, a structurally normal heart was observed with normal left ventricular dimensions and function without significant valvular disease. He was offered a permanent dual chamber pacemaker for definitive treatment, however, he declined.

https://arribasalud.com/bloqueo-auriculoventricular/
Bloqueo Auroventicular (…)
La tasa de escape de unión es de (…)

https://www.sciencedirect.com/topics/biochemistry-genetics-a...
Cardiac Arrhythmias in the Fetus, Infants, Children, and Adolescents with Congenital Heart Disease
In Chou's Electrocardiography in Clinical Practice (Sixth Edition), 2008
AV Junctional Arrhythmias
Junctional escape beats and junctional escape rhythm are seen commonly in normal children, especially on ambulatory recordings during sleep, (…)