Generally, a carrier is liable for benefits under the workers' compensation act if an employee's injury or death arose out of and in the course and scope of his employment. TEX. LAB. CODE ANN. 406.031(a)(2) (Vernon 1996). A benefit claim involving a heart attack, however, is subject to additional compensability requirements. See TEX. LAB. CODE ANN. 408.008 (Vernon 1996). A heart attack is compensable under the Act only if: (1) it occurs at definite time and place and is caused by a specific event occurring in the course and scope of employment; (2) the preponderance of the medical evidence indicates the employee's work rather than the natural progression of a preexisting heart condition or disease was a substantial contributing factor to the heart attack; (3) the heart attack was not triggered solely by mental or emotional stress, unless it was precipitated by a sudden stimulus. Id. A carrier refusing to pay a claim for benefits must provide the claimant and the commission with written notice specifying the grounds for its refusal. See TEX. LAB. CODE ANN. 409.021(a)(2) & 409.022(a) (Vernon 1996). The carrier's notice of refusal, presented on a form known as a "TWCC-21," must provide a "full and complete statement of the grounds for the carrier's refusal to begin payment of benefits." 28 TEX. ADMIN. CODE 124.6(a) & (a)(9) (West 1999). Explanations merely stating a conclusion such as, 'liability is in question,' 'compensability in dispute,' 'no medical evidence received to support disability,' or 'under investigation,' do not satisfy the requirements of this rule. Id. The carrier's defenses to compensability in all subsequent claim proceedings are limited to the grounds listed in its notice of refusal, unless the defense is based on newly discovered evidence that could not have been discovered earlier. TEX. LAB. CODE ANN. 409.022(b) (Vernon 1996). If a carrier does not contest compensability within sixty days after receiving notification of the injury, it waives the right to contest the compensability of the injury. TEX. LAB. CODE ANN. 409.021(c) (Vernon 1996).