Opinion by JUDGE SMITH.

Beaudoin Construction Company and Transamerica Insurance Company, petitioners, seek to reverse an order of the Industrial Commission granting death benefits to the widow of Gerald E. McDowell. The Commission found that McDowell died from a "heart attack" that was proximately caused by an unusual exertion arising out of, and within, the course of his employment. We affirm.

On March 20, 1978, decedent obtained work through the union hall as a laborer for Beaudoin Construction Company. That day, decedent performed various tasks which included breaking up blacktop pavement three to four inches thick, loading the chunks of pavement each of which weighed from ten to twenty-five pounds into a wheelbarrow, and pushing the wheelbarrow which weighed seventy to eighty pounds, fifty to seventy-five feet up an incline.

During the morning hours, decedent was observed by his supervisor as appearing to be short of breath while working the jack-hammer. The supervisor noted that decedent appeared to be somewhat out of shape with respect to the demands of the job. At lunch, both men took a one-half hour lunch break. After lunch, the decedent returned to work loading the chunks of pavement into the wheelbarrow, while the supervisor left for ten minutes. Upon his return the supervisor found decedent unconscious on the ground. The decedent was taken to a hospital where the emergency physician pronounced him dead as a result of a cardiac arrest.

Prior to his employment with Beaudoin Construction Company, decedent had been unemployed for a period of three months during which time decedent's only physical labor consisted of performing household chores. Medical testimony adduced at the Workmen's Compensation hearing showed that decedent had undergone significant deconditioning as a result of the three month layoff.

Prior to this death, decedent had been employed as a construction laborer for nine years. During this time, decedent performed a number of construction related tasks with varying physical demands. This pattern of employment was somewhat modified in November of 1976 when decedent learned that he had an advanced condition of coronary artery disease. After learning of his condition, decedent sought, but did not always obtain, construction tasks that were less physically demanding.

Testimony at the Workmen's Compensation hearing indicated that although the type of labor performed by the decedent on the date of his death was not unusual in the construction industry, it was unusual for the decedent considering his age, physical condition, and recent work history.

I.

Injury or death resulting from a heart attack is only compensable when caused by an unusual exertion arising out of and in the course of employment. Section 8-41-108(2.5), C.R.S. 1973 (1976 Cum. Supp.). The unusual exertion referred to in § 8-41-108(2.5) does not require that the work causing the attack be different in nature from the employee's usual work. City County of Denver v. Industrial Commission, 195 Colo. 431, 579 P.2d 80 (1978). That case stands for the proposition that the unusual exertion doctrine is to be applied according to the employee's work history rather than the work patterns of his profession in general.

Petitioners contend that in order to determine the meaning of unusual exertion, the Commission must compare the employee's normal work activities and physical demands thereof with the physical demands of the job on the day that the injury occurred. Petitioners submit that in applying the unusual exertion doctrine to the employee's work history, the Commission may not consider periods of unemployment and non-activity. Petitioners thus contend that since decedent was by profession a construction laborer, and since the work claimant performed on the date of his death did not materially differ from normal activities for a construction laborer, that, therefore, the claimant's work activities resulting in his death did not require unusual exertion. We disagree.

The dispositive question is whether the Commission may consider periods of unemployment as part of an employee's work history.

In adopting the rule in Denver v. Industrial Commission, supra, that the unusual exertion doctrine is to be applied according to the employee's work history, the Supreme Court rejected any standard based upon work patterns for the employee's profession in general. This was done, we conclude, in order to assure that each employee is considered as an individual. What is unusual exertion for a fifty-seven-year-old laborer with preexisting heart disease may differ significantly from what would be unusual exertion for a twenty-two-year-old laborer who is in excellent physical condition.

If the unusual exertion doctrine is to apply on an individual basis, then a baseline level of fitness of the individual must be determined before a trier of fact can decide what constitutes unusual exertion for that particular individual. This determination must necessarily include consideration of periods of unemployment and consequent deconditioning. For these reasons, we hold that the Commission properly considered decedent's recent unemployment as part of his work history.

II.

Petitioners also contend that the Commission erred in allowing claimant's physician to testify as an expert in the field of cardiology because the witness was not `board certified' in cardiology. We disagree.

Evidence disclosed that the witness was a licensed physician in the State of Colorado, that he was in the process of completing a cardiology fellowship, and that he had extensive experience treating patients for heart related ailments.

Rulings on qualifications of an expert witness and the admission of opinion evidence by an expert are within the sound discretion of the hearing officer. White v. Director of Division of Labor, 30 Colo. App. 393, 493 P.2d 676 (1972). We find no error in the conclusion of the hearing officer that claimant's physician was qualified to testify as an expert in the field of cardiology.

III.

Finally, petitioners contend that the Commission erred in basing its decision upon medical testimony that was not couched in terms of reasonable medical probability. For this reason, petitioners contend that the Commission's ruling is unsupported by the evidence. We disagree.

Evidence in a Workmen's Compensation proceeding must establish a reasonable causal connection between the accident and injury, but need not establish it with reasonable "medical" certainty. Ringsby Truck Lines, Inc. v. Industrial Commission, 30 Colo. App. 224, 491 P.2d 106 (1971).

Here, upon substantial evidence, the Commission concluded that there was a reasonable probability that the decedent's heart attack was caused by the labor performed on the date of his death. Claimant's medical expert testified that cardiac arrest occurring subsequent to the stress of hard physical labor established the causal connection between the accident and the aggravation of the preexisting coronary condition.

When the evidence as a whole, together with the reasonable inferences drawn therefrom, is sufficient to substantiate the Commission's conclusion that there is a causal connection between the accident and the injury, it makes no difference that the precise medical causes of the injury remain in doubt. See Industrial Commission v. Royal Indemnity Co., 124 Colo. 210, 236 P.2d 293 (1951). The Commission's findings, being supported by the record, and its conclusions, being properly based upon those findings, will not be disturbed by this Court. Martin v. Industrial Commission, 43 Colo. App. 521, 608 P.2d 366 (1980).

We have considered the petitioners' other assertions of error and find them to be without merit.

Order affirmed.

JUDGE BERMAN concurs.

JUDGE VAN CISE dissents.