“Racing heart” is also called Tachycardia. Cort Johnson did an excellent post of Dr.Visser back in 2013. Checking PubMed, I found a relevant study from 2011 on POTS patients (which often overlaps with CFS) using Ivabradine (only approved in the US in 2015). I have known some CFS patients that have been admitted into hospital because of this.
“Twenty-four ME patients with orthostatic intolerance underwent a conventional 10-min active standing test and echocardiography both on a “good day” and a “bad day”, defined according to the severity of their symptoms. The mean heart rate at rest was significantly higher on the “bad days” than on the “good days”. During the standing test on a “bad day”, 5 patients (21 %) failed to maintain an upright posture for 10 min, whereas on a “good day” all the 24 patients maintained it. Postural orthostatic tachycardia (POT) (increase in heart rate ≥30 beats/min) or severe POT (heart rate ≥120 beats/min) was observed on the “bad days” in 10 patients (43 %) who did not suffer from the severe tachycardia on the “good days”, suggesting the exaggerated sympathetic nervous activation.”
“Not only may patients be affected by more than one of these pathophysiologies but also the phenotype of POTS has similarities to a number of other disorders, e.g., chronic fatigue syndrome, Ehlers-Danlos syndrome, vasovagal syncope, and inappropriate sinus tachycardia.”
A reader was just prescribed this and asked for an opinion… what PubMed find is:
- “Inappropriate sinus tachycardia (IST) often causes palpitations, dyspnea, and exercise intolerance,…Ivabradine is effective and safe in short- and medium-term treatment of IST. 
- “In this cohort, ivabradine significantly improved symptoms associated with inappropriate sinus tachycardia and completely eliminated them in approximately half of the patients.”
- “Postural orthostatic tachycardia syndrome (POTS) is associated with tachycardia on orthostasis…. At the time of data analysis, 11 (55%) patients continued to use ivabradine, the median duration of treatment was 25 weeks (range 7–113), median daily dose 5 mg (range 2.5–15) taken in one or two divided doses…All those who continued to take ivabradine (55% of POTS patients) reported fewer episodes of palpitations and tachycardia. Eight (44% of those who tolerated ivabradine) reported a reduction in fatigue…. Ivabradine appears to control POTS-related symptoms with an efficacy similar to conventional treatment.
Role of Bacteria?
Above we see that in CFS, tachycardia increases with the severity of CFS (good day/bad day) this hints that bacteria may play a role in this condition. This aspect does not appear to have been studied yet. Back in 2013, I posted my own observations of a 30% drop in heart rate after taking prescript-assist. It is also associated with throat infections.
- “Staphylococcus aureus (S. aureus) causes a variety of infections… [symptoms include] tachycardia … the significant predictors of mortality(death) were prior hospitalization and antibiotic intake,” 
- Tachycardia in a newborn with enterovirus infection .
There is a potential to reduce or eliminate tachycardia by a shift of bacteria. The use of ivabradine is another route. It also appear to have desired side-effects:
Ivabradine inhibits the production of proinflammatory cytokines and inducible nitric oxide synthase in acute coxsackievirus B3-induced myocarditis .
- Coxsackievirus is associated with many cases of CFS.
Unfortunately, it has been studied only on it’s impact in a few areas.