[149]*149ROGERS, Associate Judge:
Appellant Robert Stack was indicted for the second degree murder, D.C.Code § 22-2403 (1981), of Helen Bataineh, who died on June 8, 1983, as a result of a subdural hematoma. According to the government’s theory, Stack caused the fatal hematoma when he struck Bataineh in the face on June 6, 1983. Stack’s defense was that he struck her in self-defense and that the government had failed to establish a sufficient nexus between his action and her death one and one-half days later. The jury found him guilty of the lesser included offense of voluntary manslaughter, D.C. Code § 22-2403 (1981). On appeal, he seeks reversal of his conviction on the grounds that: (1) the in limine limitation of cross-examination of a key government witness violated his Sixth Amendment right to confront witnesses; (2) the jury instructions failed to present fairly the defense theory in violation of his right to a fair trial; (3) the admission of the videotaped deposition of a key government witness, who the government failed to show was unavailable to testify at trial, violated his confrontation rights and Super. Ct. Crim.R. 15; and (4) there was insufficient evidence his blow was the cause of death. We agree that Stack’s first and second contentions require reversal. We also agree that the trial court erred in ruling that a key government witness was unavailable; however, in view of our disposition, we need not decide whether the error was harmless beyond a reasonable doubt. Finally, we hold there was sufficient evidence of causation; accordingly, we reverse and remand for a new trial.
I
On the morning of June 6,1983, while on his way to the hospital for treatment, Robert Stack stopped at James Vaughan’s house where his girlfriend, Rita Morris, lived. Helen Bataineh, the decedent, who sometimes stayed with Morris, was lying on the living room couch. Stack initially remained outside of the house, then went inside, and an altercation developed between him and Bataineh. Bataineh, who had been drinking since the night before, brought two large German Shepherds into the house and went towards Stack with the dogs. Stack was terrified and yelled for Morris’ help. One dog went under a table; Morris took the other dog outside. When Morris returned, she saw Stack take a full swing and slap Bataineh across the left side of the face. The blow knocked Batai-neh to a sitting position on the floor. Stack then kicked her twice in the left rib; Batai-neh looked dazed. Stack stopped hitting Bataineh after Morris told him to “stop hitting her, you’re going to kill her.” He and Morris then left for the hospital. Later that morning, Stack said to Morris, “I guess [Bataineh’s] head hurts now.”1
Around one p.m. Bataineh called a neighbor and the police to report the assault. Officer Lantz testified that Bataineh said she thought some of her ribs were broken. Lantz observed that it was difficult to get information from Bataineh because she lost track of her thoughts easily, although she did not appear to be intoxicated.
An ambulance took Bataineh to Capitol Hill Hospital where she told the emergency room nurse that she had been kicked in the neck and ribs, but did not mention being hit in the face. The examining doctor found no localized tenderness in the neck or bruises or welts in the head or neck, and noted in the medical record “no head trauma.” Chest x-rays revealed five of the eight ribs on Bataineh’s left side were broken. No x-rays or other teste were ordered for her head.
James Vaughan brought Bataineh home from the hospital on June 6 about 7 p.m. That evening he noticed a bruise on the left [150]*150side of Bataineh’s face.2 He and Bataineh remained in the house until the next morning, when Bataineh told Vaughan she felt terrible, but was going to work.
Blair Middleton, for whom Bataineh worked as a live-in nurse’s aid, recounted Bataineh’s activities and deteriorating physical condition during the next two days.3 Joan Park, a cousin of Middleton’s who was staying with him at the time, also described Bataineh’s behavior during this period. Park testified that on June 7 Batai-neh was moving slowly at 8 a.m. and claimed she had been beaten up over the weekend and felt lousy. The next morning, she was still moving slowly, and saying that she did not feel well. She did not do any of the things she normally did in the morning. In addition, her manner of speaking was slow and “sort of drowsy.” Park did not see Bataineh again until after 7:00 when she went upstairs to Bataineh’s bedroom. Bataineh was lying on her stomach, breathing heavily, and Park thought she was sleeping. About an hour later, Park thought Bataineh felt slightly feverish. Less than half an hour later, Park could not hear any breathing and found Bataineh’s body was cold. Middleton told Park to call the police. Bataineh was pronounced dead at 10:57 p.m. that evening.
The government called two expert witnesses, Dr. Michael Bray, who performed the autopsy, and Dr. Vernon Armbrust-macher, who reviewed the medical records, autopsy report, and a tape recording of Bataineh’s voice. The doctors testified that Bataineh’s death was caused by a sub-dural hematoma4 on the right side of her brain: the hematoma had swollen and compressed the brain, eventually causing a hemorrhage within the brain stem which resulted in death. In their opinion, the trauma which had caused the hematoma could have occurred between one and four days before Bataineh’s death. Dr. Bray testified, to a reasonable medical certainty, that the hematoma was consistent with a punch or a kick to the neck on the afternoon of June 6. He estimated that the trauma which caused the hematoma could have occurred as late as 10 p.m. on June 7. Dr. Armbrustmacher testified, in response to hypothetical questions which recounted Bataineh’s activities from June 6 until her death, that, to a reasonable medical certainty, the slap to Bataineh’s face on June 6 was “very consistent” with the type of event required to cause the fatal hemato-ma. Dr. Armbrustmacher thought that the evidence Bataineh was drowsy on June 8 was an ominous sign of increasing intracra-nial pressure. Both doctors opined that the bruise on the lower left side of Bataineh’s jaw, close to her chin, occurred at approximately the same time as the trauma which caused the hematoma. They agreed, however, that a fall or any event causing a sudden, sharp rotation of the head could result in a fatal hematoma. Because of Bataineh’s history of heavy drinking, they expressed caution about ascribing any particular injury as the cause of the hemato-ma, although neither doctor found any evidence of the type of bruises or scrapes which are usually seen when an intoxicated person falls down the stairs.
The defense expert, Dr. Richard Linden-berg, testified that the fatal hematoma was less than two and a half days old and could have begun to form as little as four to six hours before Bataineh’s death. In his opinion, the autopsy report was more consistent with evidence that Bataineh had fallen on June 8, and the fall had caused the hemato-ma, than with evidence that Stack’s slap on June 6 had caused the hematoma. He based his opinion on the absence of edema5 (an indication of age of the injury) in the [151]*151brain, the absence of a notation in the hospital records of head or facial bruises on June 6, and the fact that Bataineh was able to go to work on June 7 and perform the tasks associated with grocery shopping and meal preparation. On cross-examination, Dr. Lindenberg admitted that the bruise on Bataineh’s chin might have been covered by make-up, and that Stack had not told him Vaughan had claimed to have seen the bruise on the night of June 6 or his (Stack’s) slap of Bataineh’s face had knocked her off her feet.6
The defense also called James Calloway, a tenant in Vaughan’s house. He testified that when he came home around ten o’clock at night, Vaughan had told him that Batai-neh had been beaten by Rita Morris’ boyfriend, Frank. Calloway did not notice any injuries on Bataineh’s body.7 He saw Bata-ineh sitting on the floor, and testified that she may have been drinking, and had complained her back hurt. Stack did not testify-
II
Stack contends first that he was denied his Sixth Amendment right to confront witnesses when defense counsel was not allowed to cross-examine Vaughan about his assaults of Bataineh prior to June 5, 1983. He argues the trial judge thereby imper-missibly limited cross-examination which would have shown Vaughan’s bias and motive to fabricate.8
A criminal defendant has a Sixth Amendment right to confront and cross-examine government witnesses. Davis v. Alaska, 415 U.S. 308, 315, 94 S.Ct. 1105, 1109, 39 L.Ed.2d 347 (1974); Lawrence v. United States, 482 A.2d 374, 376 (D.C. 1984). If the trial court limits that right, and if an error of constitutional magnitude is found, the appellate court must determine whether reversal is required, see, e.g., Davis v. Alaska, supra, 415 U.S. at 318, 94 S.Ct. at 1111; Springer v. United States, 388 A.2d 846, 857 (D.C.1978), or whether the error was harmless beyond a reasonable doubt under Chapman v. California, 386 U.S. 18, 24, 87 S.Ct. 824, 828, 17 L.Ed.2d 705 (1967). See Delaware v. Van Arsdall, — U.S.-, 106 S.Ct. 1431, 89 L.Ed.2d 674 (1986); see also Lawrence, supra, 482 A.2d at 378. Once a trial judge has allowed enough cross-examination on an appropriate issue to satisfy the Sixth Amendment, limitation of further cross-examination will be reviewed for abuse of discretion. See Goldman v. United States, 473 A.2d 852, 856 (D.C.1984); Springer, supra, 388 A.2d at 854. Implicit in this standard is an evaluation of the importance of the subject matter and the witness whom the defendant seeks to cross-examine, measured against the degree of cross-examination permitted. Where the proffered testimony of a witness establishes an element of the offense or affects the core of the defense, the court is more likely to find constitutional error if cross-examination is severely limited on that issue. See Lawrence, supra, 482 A.2d at 377; Goldman, supra, 473 A.2d at 857-58; Springer, supra, 388 A.2d at 855-56.
The government argues that this case is indistinguishable from Beale v. United States, 465 A.2d 796 (D.C.1983), cert. de[152]*152nied, 465 U.S. 1030, 104 S.Ct. 1293, 79 L.Ed.2d 694 (1984), because defense counsel failed to link Vaughan’s prior assaults of Bataineh to the pending charges.9 The government also relies on Brown v. United States, 409 A.2d 1093 (D.C.1979), in support of the trial judge’s restriction on the defense to avoid inquiry about assaults which were too remote in time to have caused the hematoma.10 Stack, relying on Davis v. Alaska, supra, 415 U.S. at 316, 94 S.Ct. at 1110, contends that questions relating to the partiality of a witness are always relevant, and that the defense proffer provided a sufficient indication of good faith. Further, he maintains the trial court misapprehended the issue by basing its ruling on Beale and Brown, which did not involve cross-examination of a “crucial” government witness on bias or motive to fabricate.
Vaughan was one of four witnesses who testified that they spent time with Bataineh between the time of her release from the hospital and her death. The defense claimed Stack did not cause the injury which led to Bataineh’s death and that some other person or an accidental fall did. Vaughan’s testimony, therefore, was important.11 Whether it was crucial, however, going to the heart of the defense, depends on whether defense counsel’s proffer included sufficient indicia of the reliability of the evidence that Vaughan hit Bataineh and caused the fatal injury.
At the conclusion of Vaughan’s direct examination, the prosecutor announced that the government would object to any questions about fights between Vaughan and Bataineh that occurred prior to June 5, 1983, and to evidence that the police had come to Vaughan’s house on any prior occasions. The prosecutor argued the questions were impermissible under Beale, supra, 465 A.2d 796, and Hall v. United States, 454 A.2d 314 (D.C.1982),12 because there was no factual predicate indicating that the altercations had occurred during the relevant period and thus they were too remote in time; more than Vaughan’s mere presence had to be shown. Alternatively, the government argued that, even if questions about the prior beatings were relevant, the questions should not be allowed because they would result in jury speculation about what might have happened on Monday night, June 6. The prosecutor also questioned the relevancy to prior altercations of defense questions about the bruise near Bataineh’s chin.
Defense counsel responded13 that Vaughan had told her about the prior beat[153]*153ing himself, that the instant case was distinguishable from Beale and Hall because Vaughan was the only person, other than Bataineh’s employer (Middleton) and Stack, who was with Bataineh during the relevant period, and that medical evidence was expected to establish that the bruise near Bataineh’s chin was “very significant” with regard to the cause of death. Defense counsel also proffered that Rita Morris would testify Vaughan had admitted beating Bataineh in the past and had beaten her after she left the hospital. Morris also would testify that Vaughan became exasperated with Bataineh when she had been drinking.
The trial judge correctly ruled that defense counsel could inquire about the bruise near Bataineh’s chin since the evidence was relevant to the ultimate issue in the case and the proffer was sufficiently reliable. Vaughan was the first person to notice the bruise on the night of June 6. When defense counsel sought to cross-examine Vaughan, no evidence had been introduced to explain the presence of the bruise along the lower left side of Batai-neh’s jaw, near her chin.14 Nor did the evidence suggest that the bruise had appeared during the six hours on June 6 when she was at Capitol Hill Hospital.
The trial judge erred, however, in ruling that defense counsel could question Vaughan only about his assaults of Batai-neh after June 5. The judge viewed evidence of any other assaults as irrelevant unless linked to “the relevant time period,” a period which he did not define but appeared to base on the prosecutor’s claim that evidence of assaults before the weekend of June 5 would be too remote in time. The evidence of Vaughan’s relationship with Bataineh and the fact that he had assaulted her in the past was relevant to the ultimate issue in the case, and might have caused a reasonable jury to conclude there was a reasonable possibility that Vaughan had hit Bataineh on the evening of June 6 either out of exasperation with her drinking or because of other behavior which had provoked him in the past. The government’s theory of causation was based on circumstantial proof of guilt. Vaughan admitted being alone with Batai-neh after her release from the hospital. He alone saw a bruise near her chin which had not been observed before she left the hospital. The defense proffered medical evidence from which the jury could conclude that Stack’s slap of Bataineh’s face had not caused her death. The probative value of the prior assaults testimony went beyond a general suggestion that someone other than Stack could have caused the fatal blow.
Therefore, even assuming the Beale-Brown rule applies, the proffered evidence was admissible; it was not extrinsic to the ultimate issue in the case, it had clear indicia of reliability, and nothing in the record suggests it was likely to confuse or mislead or result in speculation by the jury. To require a greater proffer for the admissibility of such exculpatory evidence would distort Beale and Brown. Brown, on which Beale relied, analyzed the admissibility of exculpatory evidence in terms of the proposition that “the accused in a criminal prosecution has a fundamental right to call witnesses in his own defense,” limited only by the requirement that there be “sufficient indicia that the evidence is reliable.” 409 A.2d at 1097 (citations omitted). The court required no more than that evidence that someone else committed the crime of which the defendant is charged “clearly [154]*154link that other person to the commission of the crime,” and that its probative value be weighed against its prejudicial impact, including its propensity to mislead or confuse the jury. Id. Stack’s proffer established the necessary linkage through Vaughan’s presence with Bataineh on June 6 and a “new,” unexplained injury at that time.
Alternatively, as Stack contends, the evidence of Vaughan’s prior assaults of Bataineh was clearly admissible to show bias and motive to fabricate. See Collins v. United States, 491 A.2d 480, 487 (D.C. 1985) (“well-reasoned” suspicion standard) (citing United States v. Pugh, 141 U.S. D.C. 68, 71, 436 F.2d 222, 225 (1970) (so long as cross-examination is not “an improbable flight of fancy” nor “utterly implausible” it is permissible)). The evidence of the parties’ phlegmatic relationship was relevant to explain how the “new” injury might have occurred; the nature of that relationship was, according to the defense proffer, admitted by Vaughan. Questions about Vaughan’s prior assaults would not have been a “flight of fancy,” and the record does not suggest that the prior incidents were so remote in time as to be irrelevant to the parties’ relationship on June 6, 1983. See Lawrence, supra, 482 A.2d 374; cf. Jones v. United States, 477 A.2d 231, 243 (D.C.1984) (evidence of prior threats many years ago relevant to show motive); United States v. Bobbitt, 146 U.S. App.D.C. 224, 228, 450 F.2d 685, 689 (1971) (evidence of prior threat many years ago was admissible to show that bad blood between the defendant and the victim had continued).
Accordingly, since evidence about the relationship between Vaughan and Ba-taineh was relevant, in view of the circumstantial nature of the government’s evidence of causation, to the jury’s assessment of Vaughan’s credibility, the trial court erred in refusing to let defense counsel inquire about Vaughan’s assaults of Bataineh prior to June 5. Because such cross-examination went to the heart of the defense theory, the error was of constitutional magnitude. Goldman, supra, 473 A.2d at 857 (applying Chapman standard). Given the equivocal nature of the medical testimony, we hold that keeping “from the jury relevant and important facts bearing on the trustworthiness of crucial testimony” was not harmless error beyond a reasonable doubt.
Ill
Stack next contends that the trial judge’s instructions did not give a fair presentation of the defense theory to the jury. Because he admitted the truth of a good part of the government’s case, he argues that his defense required the jury to be advised of the significance of his independent cause theory. See Laughlin v. United States, 128 U.S.App.D.C. 27, 34, 385 F.2d 287, 294 (1967), cert. denied, 390 U.S. 1003, 88 S.Ct. 1245, 20 L.Ed.2d 103 (1968) (citing Levine v. United States, 104 U.S. App. D.C. 281, 261 F.2d 747 (1958)).
A “defendant is entitled to an instruction on his theory of the case when properly requested by counsel and when the theory is supported by any evidence.” Montgomery v. United States, 384 A.2d 655, 660 (D.C.1978); see Hale v. United States, 361 A.2d 212, 216 n. 9 (D.C.1976). Although the instruction need not be given in the exact language requested, Fludd v. United States, 336 A.2d 539, 541 n. 3 (D.C. 1975); Leftwich v. United States, 251 A.2d 646, 649 (D.C.1969), the trial court commits reversible error when it refuses to present adequately a defendant’s theory of the defense. See Levine, supra, 104 U.S.App. D.C. at 282-83, 261 F.2d at 748-49 (reversible error to deny defense request for an instruction “where special facts present ev-identiary theory which if believed defeats the factual theory of the prosecution_”). The trial court is not required to “rehearse the evidence, especially where the effect would be ... to give special emphasis to the defendant’s testimony.” Montgomery, supra, 384 A.2d at 660 (quoting Laughlin, supra, 128 U.S.App.D.C. at 34, 385 F.2d at 294). Nor may the court give instructions [155]*155that favor the government’s version of the evidence. See Levine, supra, 104 U.S.App. D.C. at 283, 261 F.2d at 749. For this court the key question is whether the instructions to the jury were an adequate statement of the law, see Leftwich, supra, 251 A.2d at 649; Spade v. United States, 277 A.2d 654, 656 (D.C.1971), and our task is to review the instructions as a whole to determine whether, they fairly and fully presented the defense theory. See Montgomery, supra, 384 A.2d at 661.15
Defense counsel requested the trial judge to instruct the jury that:
The theory of the defense in this case is that Robert Stack did not cause the death of Helen Bataineh. The fight between Helen Bataineh and Robert Stack, which was started by Helen Bataineh, resulted in injuries to her ribs but did not cause the subdural hematoma from which she died. The subdural hematoma resulted from independent causes which happened after her release from the hospital.
The defendant has no burden to show or prove by what means the decedent died. The burden of proving how the decedent died always remains with the government.
If you have a reasonable doubt whether the fight between Helen Bataineh and Robert Stack resulted in injuries from which she died, then you must find him Not Guilty.
The trial judge’s complete instruction on the defense theory was: “Ladies and gentlemen, the theory of the defendant is that he did not inflict wounds from which the deceased died. He also asserts the defense of self-defense.” The judge elaborated on the self-defense theory, but made no mention of the theory of independent cause. Elsewhere the judge instructed the jury on proximate cause, stating that the defendant was responsible for the foreseeable and likely consequences of his acts.16 In instructing on the elements of second degree murder and voluntary manslaughter, the judge told the jury that to convict it must find the defendant “inflicted an injury or injuries upon the deceased and that the deceased have died as a result of such injuries.” The judge gave the standard instructions on reasonable doubt and the government’s burden of proof.
We hold that the instructions inadequately expressed the defense theory of independent cause. In effect, as Stacks points out, the instruction told the jury “no more than that the defendant denied killing the decedent — a general denial.” The phrase “independent cause,” or its equivalent, and the legal principle involved were not mentioned in the instructions. In addition, the judge’s expansion of the standard instruction on causation in murder and manslaughter had the effect of emphasizing the government’s theory of causation. The single “denial” sentence instructing on Stack’s theory was also minimized in significance by the contrast between it and the lengthy (two pages in the transcript) instructions on self-defense. Indeed, the instructions as a whole undercut the focus and force of any suggestion in the instructions about independent cause as well as defense counsel’s closing argument that [156]*156the government had failed to negate the independent cause theory. See Laughlin, supra, 128 U.S.App.D.C. at 34, 385 F.2d at 294 (“there was a risk that the jury might find the defendant guilty although believing his testimony, because of failure to appreciate the significance of defendant’s evidentiary theory.”).
Stack clearly met his burden to show that he was entitled to an instruction which expressly set forth his theory of an independent cause. Fersner v. United States, 482 A.2d 387, 393 (D.C.1984) (evidence viewed most favorably to defendant). Defense counsel did not insist on the precise language requested. Assuming, as the government suggests, that the first paragraph of the requested instruction would have unduly emphasized the defense version of the evidence, the judge easily could have made appropriate modifications. That defense counsel argued the independent cause theory to the jury in closing does not cure the error since the jury must be instructed on the legal principles which are to guide its deliberations, and the court has the obligation to state those principles in the instructions.
IV
Stack further claims that allowing a key government witness to testify by color video-tape deposition although the witness was willing and available to testify at trial violated his constitutional right to confrontation and Super.CtCrim.R. 15(e).17
Due process requires that the prosecution make a reasonable, good faith effort to secure a witness’ presence. Ohio v. Roberts, 448 U.S. 56, 74, 100 S.Ct. 2531, 2543, 65 L.Ed.2d 597 (1980); Warren v. United States, 436 A.2d 821, 826-27 (D.C. 1981). The government bears a substantial burden to show that a witness is unavailable to testify at trial. As described in Ohio v. Roberts, supra, although “the law does not require the doing of a futile act ...[,] if there is a possibility, albeit remote, that affirmative measures might produce the declarant, the obligation of good faith may demand their effectuation.” 448 U.S. at 74, 100 S.Ct. at 2543 (emphasis in original).18 The trial court must determine whether a witness is unavailable, and if so, whether and under what circumstances the witness’ deposition should be admitted at trial. See Warren, supra, 436 A.2d at 825. The issue is whether there was sufficient evidence to support the trial judge’s ruling that the government met its burden to show Middleton was unavailable to testify at trial, and that his deposition was admissible into evidence. Id. at 829. We review the trial court’s determinations for abuse of discretion. Id. at 831.
This court has made clear that it has not sanctioned “a new category of medical unavailability in all cases where witnesses are likely to suffer adverse emotional or psy[157]*157chological effects as a result of testifying against their assailants.” Id. In Warren, supra, the defendant was charged with kidnapping and various sexual assaults and armed robbery, and the complainant’s availability to testify was disputed. Two psychiatrists, one of whom was appointed by the trial judge, testified about the complainant’s psychological unavailability. Id. at 828-29. This court affirmed the trial judge’s finding that the complainant was unavailable because of “extreme circumstances” consisting of a high likelihood of temporary psychological injury, perhaps even psychosis, and a possibility of permanent psychological injury.
Middleton, according to the prosecutor, was an “essential” witness in the government’s effort to establish the absence of an independent cause of Bataineh’s death. In support of the motion to take Middleton’s deposition under Super.Ct.Crim.R. 15(a),19 the government contended that he was unavailable to testify at trial because of his poor medical condition and the likely consequences to his health which the emotional stress of testifying would cause. The motion referred to statements by Middleton’s cardiologist, and offered a letter from Middleton’s personal physician. The unsworn letter described Middleton, who was 71 years old at the time of his deposition, as suffering from hypertension, a narrowing of the main heart valve, congestive heart failure and diabetes. The letter, which was written six months before the trial, expressed concern that testifying at trial could have “potentially very serious” ill effects on Middleton “if he is placed under any great emotional stress or tension.” A second unsworn letter from the same doctor, written three months before the trial stated that a courtroom appearance would place Middleton and his life in jeopardy. At his deposition four and one-half months before trial, Middleton testified that he suffered from high blood pressure, diabetes, shortness of breath and was too fat; he had a pacemaker, and had to avoid excitement. During a telephone conversation with the trial judge shortly before trial, however, Middleton told the judge that he was willing to appear; the judge commented lightly after the telephone conversation that he thought his “charm and personality” had caused Middleton to say he was willing to testify at trial. Stack disputed the severity of Middleton’s illness, the partiality of his doctor and the absence of sworn testimony, as well as whether, assuming these medical facts, they were sufficient to establish that Middleton was unavailable to testify as a witness at trial.
The quantum and quality of evidence of Middleton’s unavailability to testify at trial does not measure up to that in Warren. The evidence consisted only of unsworn hearsay, Middleton’s view of his health, and the trial judge’s evaluation of Middleton’s health based on observations of Middleton at his deposition, and the judge’s telephone conversation with Middleton. Nor does the evidence of unavailability rise to the level of evidence that was present in either Ohio v. Roberts, supra, or Harrison v. United States, 435 A.2d 734 (1981) (en banc),20 where the witness’ una[158]*158vailability involved facts about which a layperson would be knowledgeable. Here physical unavailability was not an issue; Middleton lived nearby, he was not physically impaired but still active, going to work every week, he had appeared without adverse consequence for the deposition, and he was willing to testify at trial.21
The trial judge initially had stated correctly that he would need to conduct a voir dire hearing to determine, on the basis of competent evidence, Middleton’s unavailability for medical reasons. See Warren, 436 A.2d at 828-30.22 Although Harrison, supra, implicitly suggests that expert testimony is not required, 435 A.2d at 738, 740, Warren, supra, which was decided after Harrison, recognized that expert testimony was required to determine psychological unavailability because the trial judge’s personal observations were “pointless in view of his lack of psychiatric expertise.” 436 A.2d at 830; Warren v. United States, 515 A.2d 208, 209-10 (D.C.1986) (per curiam) (“testimony of independent court-appointed psychiatrist will usually be the proper foundation for a finding of psychological unavailability of a witness”).23 This requirement is consistent with the rule in a number of other jurisdictions.24
Accordingly, we hold that where a crucial government witness’ unavailability is contested, and the witness’ unavailability is based on the effect which a trial appearance would have on particular medical conditions as well as the witness’ general health, expert evidence is required to support a finding that the witness is unavailable to testify at trial. Many older people undoubtedly suffer from the type of medical problems described by Middleton, and probably would prefer not to testify at trial. Although deference to the trial judge is appropriate where a witness’ appearance and reaction to questioning in the atmosphere of a trial must be evaluated, the trial judge’s reliance on hearsay and his (and Middleton’s) impressionistic, personal opinions about the effect of a trial appearance on Middleton’s health are an insufficient basis on which to deny Stack the opportunity to exercise a fundamental constitutional right. Cf. Mattox v. United States, 156 U.S. 237, 242-43, 15 S.Ct. 337, 339-40, 39 L.Ed. 409 (1895); Springer, supra, 388 A.2d at 854. Because we reverse and remand this case on other grounds, we need not consider whether Stack’s inability [159]*159to confront Middleton at trial was harmless beyond a reasonable doubt.25
V
Finally, Stack contends that the evidence of causation was insufficient to support his conviction because the government failed to show a sufficient nexus between Stack’s slap of Bataineh on June 6, and her death on June 8. The government concedes on appeal that its medical evidence, standing alone, was insufficient to convict,26 but maintains that it met its burden through the medical testimony coupled with the lay testimony about Bataineh’s deteriorating physical condition and the absence of any evidence another event led to her death.
Upon review of the denial of a motion for judgment of acquittal, we view the evidence most favorably to the government, drawing all reasonable inferences in its favor. Patterson v. United States, 479 A.2d 335, 337-38 (D.C.1984); Boyd v. United States, 473 A.2d 828, 832 (D.C.1984). Neither this court nor the trial court may [160]*160“usurp the jury’s prerogative of determining credibility, weighing the evidence, and drawing reasonable inferences of fact.” Boyd, supra, 473 A.2d at 832; see Glasser v. United States, 315 U.S. 60, 80, 62 S.Ct. 457, 469, 86 L.Ed. 680 (1941); In re A.H.B., 491 A.2d 490, 496 (D.C.1985). Reversal is required “only where there is no evidence upon which a reasonable mind could infer guilt.” Patterson, supra, 479 A.2d at 338; see Franey v. United States, 382 A.2d 1019, 1022 n. 6 (D.C.1978); Crawford v. United States, 126 U.S.App.D.C. 156, 158, 375 F.2d 332, 334 (1967); Curley v. United States, 81 U.S.App.D.C. 389, 392-93, 160 F.2d 229, 232-33, cert. denied, 331 U.S. 837, 67 S.Ct. 1511, 91 L.Ed.2d 1850 (1947). Because Stack introduced evidence after his motion for acquittal was denied at the close of the government’s case, only the motion at the close of all the evidence is before us, see Hawthorne v. United States, 476 A.2d 164, 168 n. 10 (D.C.1984); consequently, we determine the propriety of the trial court’s denial in light of all the evidence. Id.
Although the government is not required to negate every possible inference of innocence, Chaconas v. United States, 326 A.2d 792, 798 (D.C.1974), the preclusion of defense counsel’s attempt to cross-examine Vaughan on his prior assaults of Batai-neh deprived the jury of the opportunity to weigh Vaughan’s credibility in the light of relevant, admissible evidence, and preserved inviolate the favorable inference regarding the cause of death on which the government must rely to meet its burden of proof. As discussed in Part II, supra, Vaughan’s testimony was a crucial part of the government’s effort to prove that Bata-ineh’s death was not independently caused by an event occurring after Stack had slapped her.27 Vaughan’s testimony therefore helped establish proximate cause, a crucial element necessary for conviction. Since the trial judge’s limitation on Stack’s cross-examination of Vaughan violated Stack’s right of confrontation, we must decide if Vaughan’s testimony should be disregarded in determining whether the government met its burden of proof.
Our research has not found a case explicitly addressing the right to confront adverse witnesses in the context of evidentia-ry sufficiency. However, it has been held repeatedly that the appropriate remedy where the defense has not been permitted to cross-examine a key government witness is to remand for a new trial. See Delaware v. Van Arsdall, supra, 106 S.Ct. 1431; Davis v. Alaska, supra, 415 U.S. 308, 94 S.Ct. 1105; Lawrence, supra, 482 A.2d 374; Goldman, supra, 473 A.2d 852; Springer, supra, 388 A.2d 846. This court has declined to hold the evidence insufficient and a new trial therefore barred even when it has acknowledged that without the witness’ testimony the government’s evidence would have been insufficient to convict. Lawrence, supra, 482 A.2d at 377; Goldman, supra, 473 A.2d at 858; Springer, supra, 388 A.2d at 857.
This approach makes sense in the framework of our judicial system. The right to test an adverse witness’ reliability and veracity through cross-examination is vital to maintaining the “integrity of the fact-finding process.” Chambers v. Mississippi, 410 U.S. 284, 295, 93 S.Ct. 1038, 1046, 35 L.Ed.2d 297 (1973) (quoting Berger v. California, 393 U.S. 314, 315, 89 S.Ct. 540, 541, 21 L.Ed.2d 508 (1969)); Howard v. United States, 473 A.2d 835, 838 (D.C.1984); see Delaware v. Van Arsdall, supra, 106 S.Ct. [161]*1611431). If the first trial is fundamentally flawed as a result of a violation of a defendant’s right of confrontation, and the error is not harmless beyond a reasonable doubt, Delaware v. Van Arsdall, supra, a remand is required to insure that the defendant receives a fair trial. Given the totality of the evidence presented by the government, we can determine that Vaughan was a crucial witness, but we do not know whether further cross-examination would have been effective, nor whether the government would have been able to rehabilitate cross-examination undermining Vaughan’s credibility. The government obtained a favorable ruling limiting cross-examination at the trial and was entitled to rely on it by assuming Vaughan’s testimony would not need to be buttressed.
The evidence of Stack’s guilt, with Vaughan’s testimony, was sufficient to send the case to the jury. The jury could reasonably believe the government’s witnesses, and conclude that an accident or injury had not occurred after Stack struck Bataineh and before her death. In view of the evidence of the force of the blow that Stack delivered and the medical evidence that Bataineh’s injuries could have caused the fatal subdural hematoma, a reasonable jury could find beyond a reasonable doubt that Stack’s actions were the proximate cause of Bataineh’s death.
Accordingly, the judgment is reversed, and the case remanded for a new trial.
Reversed and remanded.