I have commented before on functional impairment associated with LBBB. See here.
J Aalen and colleagues (1) see here demonstrated that cardiac output in asymptomatic patients with isolated LBBB is very sensitive to afterload as in elevated arterial pressure as would also occur with strenuous exercise. (Full text is available on line,)
Dr Aalan studied the effect of increasing afterload on left ventricular ejection fraction (LVEF) in 11 asymptomatic patients with isolated ( or lone) LBBB. Afterload was increased not by exercise by increasing blood pressure (increase of 38 +/-12 mm Hg) by pneumatic extremity constrictors and handgrip exercise .
The controls subjects decreased their LVEF from 60 to 54 while the LBBB patients decreased their LVEF from 56 to 42. The increased heart rate and after load accentuated the effect of the left ventricular dyssynchronous contraction which consists of early septal contraction with bulging of the lateral left ventricular wall and delayed lateral wall contraction and bulging of the septum.
The severity of cardiac functional impairment is not reflected by the resting LVEF. Most studies have indicated at most a mild decrease. On a personal note, when I developed a LBBB my decrease in running speed was by about 20% -close to the 25% decrease in LVEF note in Aalan's paper. (decrease from a fairly comfortable 12 minute mile run to a difficult 15 minute mile with unusual calf discomfort)
I received 2 comments on the blog post mentioned in the first paragraph from readers indicating their exercise history after LBBB onset was similar to mine.
Lone LBBB is often considered asymptomatic and often is at rest. A person not doing strenuous exercise would likely not notice any problem and probably for that reason many texts describe LBBB as asymptomatic. For example, Mayo Clinic Patient website -" in most people BBB does not cause symptoms " and Up to Date "LBBB can also be seen in asymptomatic patients with structurally normal hearts."(Both accessed on 2/6/19) The key here is "with structurally normal hearts". Patients with already reduced EFs may experience significant worsening of exercise ability with onset of LBBB.
1) Aalen J et al Afterload hypersensitivity in patients with left bundle branch block . Jan, 2018 JACC Cardiovas imaging.