My brother had a pacemaker implanted after a had flatlining fainting spells. apparently his heart is strong. doctors don't know why this happens.?

Heart block: There is a reflex called "cardioinhibitory vasodepressor syncope" in which people pass out due to very prolonged (>5 second) pauses. A pacemaker is often used but with variable success. For most other indications, such as atrio-ventricular heart block, a pacemaker is the definitive therapy with very good results.

Sure we do: If the pacemaker was implanted specifically because of the fainting spells, the docs are assuming (or have proved) that it was likely due to a low heart rate, or a pause in the heartbeat. Pacemakers fix both.