1. Standard of appellate review.

(1) In resolving the petition for writ of review, we must determine whether the evidence, when reviewed in the light of the entire record, supports the Board's decision ( Universal City Studios, Inc. v. Workers' Comp. Appeals Bd. (1979) 99 Cal.App.3d 647, 656 [160 Cal.Rptr. 597]; see National Convenience Stores v. Workers' Comp. Appeals Bd. (1981) 121 Cal.App.3d 420, 424 [175 Cal.Rptr. 378]); and in doing so, we must consider the weight or persuasiveness of all the evidence, not just whether there is substantial evidence in favor of the respondent. ( Skip Fordyce, Inc. v. Workers' Comp. Appeals Bd. (1983) 149 Cal.App.3d 915, 920 [197 Cal.Rptr. 626].)

We are not bound to accept the Board's factual findings where they are illogical, unreasonable, or improbable ( Insurance Co. of North America v. Workers' Comp. Appeals Bd. (1981) 122 Cal.App.3d 905, 911 [176 Cal.Rptr. 365]), where they do not withstand scrutiny when considered in light of the entire record ( Duke v. Workers' Comp. Appeals Bd. (1988) 204 Cal.App.3d 455, 460 [251 Cal.Rptr. 185]), or where on a case-by-case examination we discern an inequitable result when the record is examined for fairness, reasonableness and proportionality in the overall scheme of the workers' compensation law and the purposes sought to be accomplished by that law. ( National Convenience Stores v. Workers' Comp. Appeals Bd., supra, 121 Cal.App.3d at p. 424; Universal City Studios, Inc. v. Workers' Comp. Appeals Bd., supra, 99 Cal.App.3d at pp. 658-659.)

Finally, where the Board's decision is not within the realm of what a reasonable trier of fact could find, the decision is not supported by substantial evidence and must be annulled. ( Skip Fordyce, Inc. v. Workers' Comp. Appeals Bd., supra, 149 Cal.App.3d at p. 921; Insurance Co. of North America v. Workers' Comp. Appeals Bd., supra, 122 Cal.App.3d at p. 911.)

2. Standard of review by Board on reconsideration.

(2) Unlike the court, the Board is empowered on reconsideration to resolve conflicts in the evidence, to make its own credibility determinations, and to reject the findings of the WCJ and enter its own findings on the basis of its review of the record; nevertheless, any award, order or decision of the Board must be supported by substantial evidence in the light of the entire record. ( Lamb v. Workmen's Comp. Appeals Bd. (1974) 11 Cal.3d 274, 280-281 [113 Cal.Rptr. 162, 520 P.2d 978]; Garza v. Workmen's Comp. App. Bd. (1970) 3 Cal.3d 312, 317 [90 Cal.Rptr. 355, 475 P.2d 451]; LeVesque v. Workmen's Comp. App. Bd. (1970) 1 Cal.3d 627 [83 Cal.Rptr. 208, 463 P.2d 432].)

The foregoing standard is not met by simply isolating evidence which supports the Board and ignoring other relevant facts of record which rebut or explain the evidence. ( Lamb v. Workmen's Comp. Appeals Bd., supra, 11 Cal.3d at p. 281; see Garza v. Workmen's Comp. App. Bd., supra, 3 Cal.3d at p. 317; City of Santa Ana v. Workers' Comp. Appeals Bd. (1982) 128 Cal.App.3d 212, 219 [180 Cal.Rptr. 125]; Western Electric Co. v. Workers' Comp. Appeals Bd. (1979) 99 Cal.App.3d 629, 645 [160 Cal.Rptr. 436].)

Thus, in relying on the opinion of a particular physician in making its determination, the Board may not isolate a fragmentary portion of the physician's report or testimony and disregard other portions that contradict or nullify the portion relied on; the Board must give fair consideration to all of that physician's findings. ( City of Santa Ana v. Workers' Comp. Appeals Bd., supra, 128 Cal.App.3d at p. 219.) (3) As stated in Gay v. Workers' Comp. Appeals Bd. (1979) 96 Cal.App.3d 555, 564 [158 Cal.Rptr. 137], in evaluating the evidentiary value of medical evidence, a physician's report and testimony must be considered as a whole rather than in segregated parts; and, when so considered, the entire report and testimony must demonstrate the physician's opinion is based upon reasonable medical probability. (See Lamb v. Workmen's Comp. Appeals Bd., supra, 11 Cal.3d at p. 281; McAllister v. Workmen's Comp. App. Bd. (1968) 69 Cal.2d 408, 416-417 [71 Cal.Rptr. 697, 445 P.2d 313].) Hence, the Board may not blindly accept a medical opinion that lacks a solid underlying basis and must carefully judge its weight and credibility. ( National Convenience Stores v. Workers' Comp. Appeals Bd., supra, 121 Cal.App.3d at p. 426.)

"'[N]ot all medical opinion constitutes substantial evidence upon which the Board may rest its decision. Medical reports and opinions are not substantial evidence if they are known to be erroneous, or if they are based on . . . incorrect legal theories . . . [or] on surmise, speculation, conjecture or guess.'" ( Insurance Co. of North America v. Workers' Comp. Appeals Bd., supra, 122 Cal.App.3d at p. 917; see Hegglin v. Workmen's Comp. App. Bd. (1971) 4 Cal.3d 162, 169 [93 Cal.Rptr. 15, 480 P.2d 967]; Place v. Workmen's Comp. Appeals Bd. (1970) 3 Cal.3d 372, 378, 90 Cal.Rptr. 424, 475 P.2d 656]; Guerra v. Workers' Comp. Appeals Bd. (1985) 168 Cal.App.3d 195, 199 [214 Cal.Rptr. 58]; Baptist v. Workers' Comp. Appeals Bd. (1982) 137 Cal.App.3d 903, 906 [187 Cal.Rptr. 270].)

(4) When the WCJ's finding of compensable injury is supported by solid, credible evidence, it is to be accorded great weight by the Board and should be rejected only on the basis of contrary evidence of considerable substantiality. ( Lamb v. Workmen's Comp. Appeals Bd., supra, 11 Cal.3d at p. 281; Garza v. Workmen's Comp. App. Bd., supra, 3 Cal.3d at pp. 318-319; Los Angeles Unified School Dist. v. Workers' Comp. Appeals Bd. (1981) 116 Cal.App.3d 393, 400 [171 Cal.Rptr. 841]; Western Electric Co. v. Workers' Comp. Appeals Bd., supra, 99 Cal.App.3d at p. 642.) The WCJ's findings on credibility are entitled to great weight because the WCJ has the opportunity to observe the demeanor of the witnesses and weigh their statements in connection with their manner on the stand. ( Garza v. Workmen's Comp. App. Bd., supra, 3 Cal.3d at pp. 318-319; Western Electric Co. v. Workers' Comp. Appeals Bd., supra, 99 Cal.App.3d at p. 643.)

When the WCJ has relied upon solid, credible evidence to make a wholly supported finding of compensability, the Board may not avoid its obligation to give great weight to that finding by ignoring the evidence on which it is based, since, in doing so, the Board renders a decision which is not supported by substantial evidence in the light of the entire record. ( Lamb v. Workmen's Comp. Appeals Bd., supra, 11 Cal.3d at p. 283.)

(5) Factual findings of the Board are not supported by substantial evidence in light of the entire record where such findings are based upon inferences which cannot be fairly drawn from the evidence, based on evidence lacking probative force, or based on a purely "'fanciful conclusion.'" ( Insurance Co. of North America v. Workers' Comp. Appeals Bd., supra, 122 Cal.App.3d at pp. 910-911.)

3. Board majority erred in amending findings.

(6) Applying the cited authorities on the Board's power to reject the WCJ's findings and substitute its own findings, we first note the Board majority patently disregarded the mandate that it review the evidence in the light of the entire record. ( Lamb v. Workmen's Comp. Appeals Bd., supra, 11 Cal.3d at pp. 280-281; Garza v. Workmen's Comp. App. Bd., supra, 3 Cal.3d at p. 317; LeVesque v. Workmen's Comp. App. Bd., supra, 1 Cal.3d at p. 637.) Instead, the Board majority erroneously relied on isolated fragmentary portions of Dr. Gillis's deposition and later report while disregarding substantial evidence that tended to contradict the isolated fragments relied on, such as Dr. Gillis's original report, almost all of Dr. Gromis's extensive reports and testimony expressing his opinion as a reasonable probability, and the medical literature showing the causal relationship between acute heart attacks and strokes occurring soon thereafter. ( City of Santa Ana v. Workers' Comp. Appeals Bd., supra, 128 Cal.App.3d at p. 219; see Lamb v. Workmen's Comp. Appeals Bd., supra, 11 Cal.3d at p. 281; Garza v. Workmen's Comp. App. Bd., supra, 3 Cal.3d at p. 317; Western Electric Co. v. Workers' Comp. Appeals Bd., supra, 99 Cal.App.3d at p. 645.)

Moreover, Dr. Gillis's changed opinion in his deposition and final report was to a large extent based on surmise, speculation, conjecture, and guess, and thus did not constitute substantial evidence upon which the Board majority could rest its decision. ( Insurance Co. of North America v. Workers' Comp. Appeals Bd., supra, 122 Cal.App.3d at p. 917;see Hegglin v. Workmen's Comp. App. Bd., supra, 4 Cal.3d at p. 169; Place v. Workmen's Comp. App. Bd., supra, 3 Cal.3d at p. 378; Guerra v. Workers' Comp. Appeals Bd., supra, 168 Cal.App.3d at p. 199; Baptist v. Workers' Comp. Appeals Bd., supra, 137 Cal.App.3d at p. 906). The Board majority could not blindly accept the changed opinion of Dr. Gillis, which lacked a solid, underlying basis; it should have weighed Dr. Gillis's changed opinion and his credibility against the contrary medical evidence relied upon by the WCJ and the credibility of Dr. Gromis, who testified at the trial and was found credible by the WCJ. ( National Convenience Stores v. Workers' Comp. Appeals Bd., supra, 121 Cal.App.3d at p. 426; see Garza v. Workmen's Comp. App. Bd., supra, 3 Cal.3d at p. 319.) The evidence relied upon by the Board simply did not constitute contrary evidence of considerable substantiality to warrant the Board's rejection of the WCJ's findings which were based on the solid, credible evidence evinced in Dr. Gromis's reports, Dr. Gillis's original report, and the medical literature. ( Lamb v. Workmen's Comp. Appeals Bd., supra, 11 Cal.3d at p. 283; see Garza v. Workmen's Comp. App. Bd., supra, 3 Cal.3d at p. 319.)

We are not bound to accept the Board's factual findings where, as here, they are unreasonable ( Insurance Co. of North America v. Workers' Comp. Appeals Bd., supra, 122 Cal.App.3d at p. 911), are not within the realm of what a reasonable trier of fact could find (ibid.; Skip Fordyce, Inc. v. Workers' Comp. Appeals Bd., supra, 149 Cal.App.3d at p. 921), do not withstand scrutiny when viewed in the light of the entire record ( Duke v. Workers' Comp. Appeals Bd., supra, 204 Cal.App.3d at p. 460), and lead to an inequitable result when the entire record is examined for fairness, reasonableness, and proportionality in the overall scheme of the workers' compensation law and the purposes sought to be accomplished by that law. ( National Convenience Stores v. Workers' Comp. Appeals Bd., supra, 121 Cal.App.3d at p. 424; Universal City Studios, Inc. v. Workers' Comp. Appeals Bd., supra, 99 Cal.App.3d at pp. 658-695.)

For the foregoing reasons, we conclude the Board erred in amending the WCJ's findings.

The opinion and decision of respondent Board after reconsideration is annulled; and the matter is remanded for further proceedings consistent with the views expressed herein.