Communication about Advance Directives: Are Patients Sharing Information with Physicians? SUZANNE B. YELLEN, LAUREL A. BURTON, and ELLEN ELPERN
Introduction Historically, patients have deferred to physicians' judgments about appropriate medical care, thereby limiting patient participation in treatment decisions. In this model of medical decision making, physicians typically decided upon the treatment plan. Communication with patients focused on securing their cooperation in accepting a treatment decision that essentially had already been made. In recent decades, however, this approach has been challenged and largely replaced with a model that emphasizes patient autonomy and the right of individuals to participate in or even direct decisions related to their medical treatment. In the patient autonomy model of medical decision making, the physician is viewed as an expert consultant who delineates the risks and benefits of all reasonable treatments. After a presumed thorough review of possibilities, the "informed" patient, in collaboration with the physician, selects the treatment most consistent with his or her values and individual treatment goals. The concept of patient autonomy has been applied to the full spectrum of illness, ranging from time-limited treatments to end-of-life decisions. Advance directives (ADs), such as the living will (LW) and durable power of attorney for healthcare (DPAHC), have emerged as a means of promoting patient autonomy, especially in end-of-life decisions. The Patient Self-Determination Act (PSDA) of 1990 has made patient autonomy in end-of-life decisions a key standard. According to this act, institutions receiving federal funds for patient care are required to ask all adult patients whether or not they have some form of AD (e.g., LW or DPAHC). Institutions also must provide written information about ADs, although the specifics of how this information is communicated has, to a large extent, been left in the hands of individual institutions in accordance with state law. The PSDA has tremendous implications for the field of patient-healthcare provider communication. Despite the emergence of ADs with the Quinlan case in 1976 and considerable attention given to life-support issues in both popular and professional publications, communication about ADs between patient and physician has been limited,1"3 although reasons for communication difficulties have not been specified in the literature. One possible explanation for these difficulties is an extension of communication problems noted in the "do-not-resuscitate" (DNR) literature. Specifically, physicians are uncomfortable discussing resuscitation with terminally ill patients4 for fear of taking away hope, unleashing an This research was supported in part by grants from the National Cancer Institute (SRC 92 CA47744-03) and the American Suicide Foundation. Cambridge Quarterly of Healthcare Ethics (1992), 4, 377-387. Printed in the USA.
Copyright © 1992 Cambridge University Press 0963-1801/92 $5.00 + .00
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emotional reaction, or causing a depressive reaction.5 A consequence of this discomfort is that resuscitation discussions are delayed until, on the average, 3-9 days before the patient dies,6 when the patient is not competent to be involved in the discussion. Consequently, resuscitation discussions are more often held with family members (in up to 86% of cases) than with patients directly (22-26% of cases).7 Research in communication about resuscitation and ADs has generally focused on the physician as the initiator and provider of information. Less attention has been paid to the role or desire of patients to initiate end-of-life discussions. As part of a larger investigation of patient opinions and behaviors about ADs, the present study examined patient communication patterns and preferences and sought to determine the extent to which patients direct communication towards physicians about end-of-life treatment decisions and ADs. Patients and potential patients were interviewed to answer the following questions. How many patients discuss treatment preferences about end-of-life decisions directly with physicians? Does communication directed to physicians differ as a function of whether or not patients have a written advance directive? Does communication differ as a function of patient category (e.g., inpatient, outpatient, or nonpatient setting) or health status? Are patients who have advance directives making their physicians aware of them? Do patients feel that the responsibility for initiating discussions about advance directives rests with physicians? What is the preferred method of communication of information about advance directives, in keeping with federal mandate (PSDA)? Hypotheses
Although many of the above questions were exploratory in nature, several specific hypotheses were tested. First, we predicted that execution of an AD would be a function of perceived health, such that people who believed their health was "poor" would be more likely to have completed an AD than those who felt their health was either good or fair. Second, it was hypothesized that people with perceived poor health status would be more likely than those in good or fair health to have specifically discussed treatment preferences with their family and/or physicians. Finally, the data were explored to assess the degree to which execution of an AD and communication about treatment preferences were related to patient category (e.g., inpatient, outpatient, or nonpatient setting). Method Subjects Subjects were 131 persons (50 males, 81 females) who volunteered to participate in the study. Participants included hospital inpatients, outpatients, and healthy nonpatients. The inpatient sample included 42 adults utilizing the medical and surgical services at Rush-Presbyterian-St. Luke's Medical Center, a 900-bed tertiary care academic facility serving the Chicago metropolitan area. Fifty outpatients were recruited from persons with appointments at the offices of a cardiologist and an oncology group. Nonpatients were 39 adults who were not in active treatment for a medical problem. All patients were recruited on the basis of their availability to the interviewers on the days of data collection. The refusal rate was 4%. 378
Doctor-Patient Communication Procedure
An interview format was used to administer the questionnaire. Potential study subjects were approached by one of the data collectors who explained the purposes of the study and requirements for participation. People who were unable to speak or understand English were excluded from consideration. Written informed consent was obtained for those who agreed to participate after the purpose of the interview was explained. The study was approved by the Human Investigations Committee of Rush-Presbyterian-St. Luke's Medical Center. Questionnaire
A structured questionnaire developed specifically for this project used a forcedchoice format to assess attitudes, behaviors, and communication patterns related to ADs. (In Illinois, the LW and the DPAHC are the ADs recognized under state law.) The questionnaire consisted of several sections: 1) demographic information; 2) communication about treatment preferences and ADs with physicians; 3) beliefs about ADs; and 4) preferred communication format for receiving information about ADs. Interview questions were pilot tested on a limited number of patients and nonpatients, after which items that caused confusion were identified and revised. Interview procedures were standardized across interviewers, and a standardized definition of an AD was provided to those subjects who did not understand this term. Analyses
A software package8 was used for statistical analyses. Frequency data were tabulated, and percentages were calculated. Planned comparisons between categorical variables (e.g., group of patient, perceived health status, having executed an AD, communication patterns) were analyzed using chi-square analyses in accordance with a priori hypotheses. Percentages for dichotomous responses were converted to proportions and tested with a Test for Significance of Proportion.9 A p-value of .05 was considered to be statistically significant. No adjustments were made for multiple comparisons.10 Results Demographics
The subjects ranged in age from 18 to 88 years. Age was normally distributed, with the mean age of 52.8 years. Mean age for male and female participants did not differ significantly: mean age for males was 53.9 years; mean age for females was 52.1 years. The subjects were predominantly Caucasian, Christian, and well educated, and over half had some form of education beyond high school graduation (Table 1). Chi-square analyses revealed no significant differences among inpatients, outpatients, and nonpatients with respect to communication patterns and preferences; therefore, data were collapsed across patient category and analyzed as a single sample. 379
Suzanne B. Yellen et al. Table 1. Demographics of Subjects in a Study of Patient-Doctor Communication Concerning Advance Directives
Characteristic Sex Male Female Race Caucasian African-American Latino Asian Marital status Single Married Separated/divorced Widowed Religious preference Jewish Catholic Protestant Other None Education Grade School High School Trade/Vocational School College (undergraduate) Postgraduate Recent hospitalization (past 2 years) JNone Once Twice >2 Recent intensive care stay Yes No Health status Good Fair Poor
%of sample 38 62 80 14 2 4 23 56 13 8 7 35 44 10 4 10 39 7 24 19 OO
28 14 19 23 76 61 25 13
Communication of Treatment Preferences
In 88 of the 131 cases (68%), respondents reported that discussions about treatment preferences in the event of illness and patient incompetence had been held (the "communicators"). However, the majority of these discussions were not held with respondents' physicians. Of the 88 respondents who had communicated about treatment preferences with someone, only 13 (14.8%) reported discussing treatment preferences directly with their physician. Considering the entire sample, less than 10% reported speaking with their physicians about treatment pref-
131 interviews 88 discussions about treatment preferences (68%) 13 discussions with doctors (14.77%) 6MD (46.2%)
42 failures to discuss (32%)
72 failures to discuss with doctors (82%) 43 "Pt." (58%)
Figure 1. Patterns of Communication of Treatment Preferences between Potential Patients (Pt.) and Doctors (MD).
erences (Fig. 1). Most often, these discussions were held with family members and/or close friends. Data were further examined to determine the percentage of subject agreement with a statement that physicians should initiate discussions with patients about ADs and to explore whether there were differences between respondents who had discussed treatment preferences with their physicians vs. those who had not. Neither the communicators nor noncommunicators seemed to believe that the burden of initiating AD discussions rested with the physician. Only 6 of the 13 communicators (46%) and 27 of the noncommunicators (37.0%) believed that physicians should initiate these discussions. Communication Patterns among Patients with or without an Advance Directive
Thirty-one (23.7%) of the 131 patients in the study had completed an advance directive at the time of interview. Of this sample of "completers," however, only 15 (48%) had informed their physician of the existence of their AD. Further, a significant proportion of completers (19.4%; z = —3.44, p < .01) failed to discuss the terms of the AD or treatment preferences with their physicians, and a small minority (9.7%) of patients with an AD failed to notify physicians, family members, or friends of the fact that they had an AD. Having an AD does not necessarily mean that this information will be transmitted to attending physicians. The pattern of data for the group of "noncompleters" was similar to that of the completers. Ninety-nine patients (76.3%) had not completed an AD at the time of the interview. Of this group of respondents, 60 (61%) had discussed treatment preferences with someone; however, only 8 (8%; z = —8.4, p < .01) had discussed preferences directly with a physician. Data were also examined to determine if perceived health status influenced the likelihood of executing a LW, discussing treatment preferences with someone, or discussing them specifically with a physician. We hypothesized that people who perceived their health status as "poor" (as opposed to "good" or "fair") would be 381
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more likely to have an AD and to have discussed treatment preferences with their loved ones as well as their physicians. A chi-square analysis indicated only an effect for health status on execution of an AD; x2 (di = 2) = 8.62, P = .013. People who believed their health was poor were more likely to have completed an AD than were people whose health was believed to be good or fair. Perceived health had no effect on either discussion of treatment preferences (P = .78, ns) or the likelihood of discussing treatment preferences with a physician (P = .82, ns). Comfort in Discussion of Advance Directives with Various Hospital Personnel
Most patients anticipated either some or a great deal of comfort in discussing ADs with their physicians, and overall comfort level with physicians was markedly higher than that with other healthcare or hospital personnel (Table 2). Of the 131 participants, 91 (69.5%) anticipated that they would be very comfortable discussing ADs with physicians and 32 (24.5%) felt that they would be somewhat comfortable. Patient Beliefs about Advance Directives
Study participants indicated agreement or disagreement with statements designed to evoke attitudes about ADs. Several attitudinal statements were presumed related to known communication difficulties that physicians have in discussions of foregoing life-sustaining treatments (e.g., discussion of CPR in terminally ill patients). The percentage of subjects that agreed with a statement "Patients want their physician to bring up the issue of ADs first" was only 43%, and 44% believed that discussion of ADs was depressing and related to thoughts about dying. However, some (21%) agreed that ADs were primarily necessary for those who were old or sick. With the exception of responses to the statement that ADs were for those who were old or sick (21%, z = —7.25, P < .01), the probability agreement with these statements did not differ significantly from chance. Communication of Advance Directives
Each subject's first choice of various methods in which information about ADs could be communicated was recorded, and percentages were calculated for each class of response. Choices were written information, videotaped information, face-
Table 2. Comfort Levels of Potential Patients Discussing Advance Directives Treatment team member
Not at all comfortable
Attending physician Psychologist Nurse Chaplain Admit clerk Social worker Lawyer
91 (69.47%) 48 (36.64%) 59 (45.04%) 72 (54.96%) 15 (11.45%) 48 (36.64%) 36 (27.48%)
32 (24.43%) 45 (34.35%) 46 (35.11%) 22 (16.79%) 40 (30.53%) 49 (37.40%) 34 (25.95%)
8 (6.11%) 38 (29.01%) 26 (19.85%) 37 (28.24%) 75 (57.25%) 34 (25.95%) 61 (46.56%)
Table 3. Preferences for Communication about Advance Directives Manner of delivery
% of sample
Written information Video presentation Face-to-face discussion Combination Do not want information No preference
31.8 0.7 39.3 17.8 2.9 6.1
to-face discussion, or a combination of the above (Table 3). The most preferred method of communication involved face-to-face discussion (39%). The provision of written information was selected by 32% of the sample, and a combination of written information with face-to-face discussion was selected by 18% of subjects. Over half of our sample preferred that some form of face-to-face communication be a part of the process of delivery of information about ADs. Discussion Our data have implications for both patient-doctor communication and the Patient Self-Determination Act (1990). Physicians have difficulty initiating discussions about various end-of-life issues, often delaying these discussions until the probability of dialogue with the patient becomes a moot point. 11 Traditionally, the responsibility for communication failures has rested on the shoulders of the physician. Our data show communication difficulties but suggest that they may be bidirectional. Patients may not see themselves as active participants in the medical decision-making process and therefore may fail to either discuss end-of-life treatment options with physicians or to notify physicians of the existence and content of their ADs. Our study indicated that 1) patients are communicating treatment preferences and wishes to limit treatment but are not discussing them with their physicians; 2) patients who have considered treatment limits and have formalized them in an AD do not necessarily share this information with their physicians; and 3) having an AD or communication about treatment preferences is not a function of patient or nonpatient settings. Our data also suggest that communication failures usually do not stem from either anticipated discomfort in discussing ADs with physicians or from the belief that it is the physician's responsibility to initiate the discussion. The relatively few empirical studies focusing on the extent to which physicians and patients directly discuss life-sustaining treatment12'13 and DNR orders14-15 report competing findings. Lo et al.,16 who examined patient attitudes toward discussing life-sustaining treatments in a chronic illness patient population (e.g., cancer, angina, chronic obstructive pulmonary disease (COPD), chronic renal failure), found that 70% of patients discussed treatment preferences with either family or friends, but only 6% of patients had discussed life-sustaining treatments directly with physicians. The limited number of doctor-patient discussions of resuscitation preferences (3-16% of respondents17"19) has also been found in other studies. Further, Lo et al. found that 68% of respondents wanted to have discus383
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sions about life-sustaining treatments with their physicians, but only 53% of this group preferred that their physician bring up the matter of life-sustaining treatment first, a finding that is supported in the present study. In contrast, Stolman et al.20 found that 51% of their sample recalled discussing resuscitation directly with a physician. The failure to obtain consistent results among these studies may be explained by methodological differences in the way the subject population was defined. Stolman and her colleagues derived their sample from a pool of terminally ill patients, 73% of whom had already been designated as DNR following discussion with their doctors. The present study, as others, derived its sample from populations in which death was not imminent and resuscitation status had not necessarily been addressed. [Frankl et al.21 found that neither communication nor desire for communication about life support was related to terminal or nonterminal illness status. However, terminal illness was defined by specific diagnosis (e.g., cancer, AIDS, end-stage liver disease) rather than by the expectation that death would occur within 6 months.] The relatively high percentage of doctor-patient communication about resuscitation found by Stolman et al. might thus be limited to illness contexts in which death is imminent, and future research should examine communication patterns across various levels of illness severity. In nonterminally ill patient populations, however, it is obvious that despite increased public awareness about ADs within the past 5 years (in our sample, 75% had heard of either LW or DPAHC and 71% had a relatively accurate understanding of their purpose), there has not been a concomitant increase in the likelihood of patients and physicians talking about ADs. A potential limitation of the generalizability of these communication-specific findings involves the demographics of our sample, which was predominantly white and well-educated. A well-educated sample may result in an over- rather than underestimation of the probability that physicians and patients are talking to one another. However, level of education and verbal ability may be irrelevant when it comes to discussing topics related to death and end-of-life decisions. Irrespective of the direction of the bias, these findings warrant replication using a sample more representative of the Chicago metropolitan area. Our data did not address specific reasons for communication failures. One possible reason is that time restrictions of office visits with physicians do not permit exploration of future treatment (or lack of treatment) possibilities. Another possibility is that patients may share information about ADs with their primary physician but not with a medical specialist. A more plausible explanation, however, involves role ambiguity on the part of both patient and physician that occurs as the focus of treatment changes from curative to palliative care. Stewart and Roter22 offered a model of doctor-patient relationships that varies as a function of both physician control and patient control, such that relationships can be described as paternalistic (high physician control-low patient control), mutual (high physician and patient control), consumerist (low physician control-high patient control), or default (low physician and patient control). Assuming their model is dynamic, the actual expression of the doctor-patient relationship changes as a function of illness stage and needs of the patient.23 Thus, a shift in physicianpatient control may accompany the transition from curative to palliative treatment, and previously well-defined roles may become ambiguous. Role ambiguity and associated discomfort caused by this shift have important implications for communication, particularly in sensitive end-of-life issues. Because roles are ambiguous, each member in the doctor-patient dyad may look to the other for clues as 384
to how to proceed in communication about ADs, CPR, and other decisions to actively limit aggressive treatment, with the result that no communication occurs. Better understanding of the shift in doctor-patient roles when treatment changes form curative to palliative care could reduce discomfort associated with role shifts and foster better communication in the doctor-patient relationship. Attempts cited in the literature have focused on clarifying both the role of the physician and the manner of delivery of information in palliative care.24 Attention is also being devoted to changing the patient role. Research is beginning to address the question of doctor-patient communication from a patient perspective by determining patient communication style as either active or passive and then teaching the patient to be more direct with his or her healthcare provider in the medical interview.25 This method, along with other forms of "assertiveness training" could give patients increased role flexibility and encourage more participation in the communication process. However, all patients may not desire a high degree of communication, and physicians should direct initial inquiries into determining the extent to which patients want to be involved in the medical decision-making process. For example, our results indicate that only a small percentage (3%) of people do not want to receive information about ADs, whereas others26 have found higher proportions (30-41%) of patients who do not want to talk to their doctors about life-support preferences. The PSDA specifies that procedures must be established to provide information to patients about ADs, but the extent and manner in which this information is disseminated may differ widely among institutions. Although the spirit of the act encourages timely discussion of ADs, literal adherence to the law will more likely involve routine questions asked by admitting clerks (e.g., "Do you have a living will? Would you like to receive information about it?") than establishment of a process to increase doctor-patient discussion. Our results suggest that simply providing written information to patients about ADs and expecting them to actively pursue discussion of this topic with physicians will be inadequate. Over 50% of our sample preferred either face-to-face communication about ADs or discussion along with written information. The lack of an opportunity for physicianor medical center-initiated communication may likely result in failure on the part of patients to review materials about ADs received in the admitting department. Research in New York State at Memorial-Sloan Kettering Cancer Center has indicated that legislation can have virtually no impact on patients being admitted with an appropriate resuscitation status. 27 Without active communication about ADs between patients and their physicians, it is likely that the impact of the PSDA on the number of ADs executed will be minimal. Remediation of communication difficulties should be directed to both physicians and patients, particularly because physicians cannot reliably predict resuscitation preferences of patients.28'29 Physicians may not ask patients about whether or not they have completed ADs, and patients who have completed them may not inform their physicians,30 as indicated in the present study. If physicians are aware that patients do not necessarily communicate about their ADs, then physicians may become more vigilant in making requests for this type of information a standard in medical practice. Assessing physician beliefs about the impact of AD discussions with patients might help identify and challenge inappropriate assumptions regarding the anticipated reactions of patients. Findings of Emmanuel et al.31 suggest a prevalent belief among both patients and physicians that planning for ADs is primarily for 385
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older and sicker people. This belief was cited as a barrier to planning for 24% of their sample; however, the manner in which these data were generated did not allow for determination of whether or not 76% actually disagreed with the belief. Further, their results indicated that the younger, healthier people in their sample were actually more interested in discussing and planning ADs than were the older, sicker people. In the present study, this specific belief was assessed systematically and revealed a low percentage of agreement. This finding, which was statistically significant, provides additional support to the findings of Emmanuel et al. and suggests that physicians who believe that they should not initiate discussions about ADs with younger and healthier patients may be operating under a false assumption. Physicians may be reluctant to discuss life-sustaining treatments due to misperceptions about the consequences of initiating these discussions.32 However, the assumed vulnerability of patients to discussions of ADs has not received empirical support either in past research33 or in the present study. Although almost half of our sample (44%) believed that discussion of ADs could be depressing and related to thoughts of dying, only a very small percentage (3%) stated that they would not want to receive this information, suggesting that although these discussions may be distressing, patients are still willing to engage in dialogue about ADs with physicians and other hospital personnel. The anticipation of patient distress on the part of the physician should not be used as an excuse for failing to discuss this important topic. Our study and others34 indicate that beliefs about the impact of AD discussions are indeed important; however, further systematic investigation is needed to precisely determine those beliefs of patients and physicians that interfere with initiation of discussions. Efforts to improve communication should be directed towards patients as well as physicians. Preliminary findings of experiments conducted by Degner35 suggest that increasing patient participation results in increased hope, "fighting spirit," and improved quality of life. Most importantly, there was no evidence of harm by increasing patient involvement in the decision-making process. As the PSDA is implemented, there is a moral necessity to train and empower patients and physicians alike to engage each other in on-going, prospective discussions about treatment and care preferences. Neither physicians nor patients need fe^r such communications. Indeed, for both groups, this mutual enhancement of patient autonomy carries the potential for mutual benefit to the doctorpatient dyad.
Notes 1. Emmanuel L, Barry M, Stoeckle J, et al. Advance directives for medical care —a case for greater use. New England Journal of Medicine 1991;324:889-895. 2. Gamble E, McDonald P, Lichstein P. Knowledge, attitudes, and behavior of elderly persons with living wills. Archives of Internal Medicine 1991; 151:277-80. 3. LaPuma J, Orentlicher D, Moss R. Advance directives on admission: clinical implications and analysis of the Patient Self-Determination Act of 1990. Journal of the American Medical Association 1991;266:402-5. 4. Stolman C, Gregory J, Dunn D, et al. Evaluation of patient, physician, nurse, and family attitudes toward do-not-resuscitate orders. Archives of Internal Medicine 1990; 150:653-8. 5. Holland J. Clinical course of cancer. In: Holland J, Rowland J, eds. Handbook ofPsychooncology. New York: Oxford University Press, 1989:75-100.
Doctor-Patient Communication 6. Bedell S, Pelle D, Maher P, et al. Do-not-resuscitate orders for critically ill patients in the hospital: how are they used and what is their impact? Journal of the American Medical Association 1986;256:233-7. 7. See note 6. Bedell et al 1986:233-7. 8. Wilkinson L. SYSTAT: The System for Statistics. Evanston, Illinois: SYSTAT, Inc., 1987. 9. Bruning J, Kintz B. Computational Handbook of Statistics. Glenview, Illinois: Scott, Foresman and Co., 1977. 10. Rothman KJ. No adjustments are needed for multiple comparisons. Epidemiology 1990;l:43-6. 11. See note 6. Bedell et al. 1986:233-7. 12. Lo B, McLeod G, Saika G. Patient attitudes to discussing life-sustaining treatment. Archives of Internal Medicine 1986; 146:1613-5. 13. Frankl D, Oye R, Bellamy P. Attitudes of hospitalized patients toward life support: a survey of 200 medical inpatients. American Journal of Medicine 1989;86:645-8. 14. Seckler A, Meier D, Mulvihill M, et al. Substituted judgment: how accurate are proxy predictions? Annals of Internal Medicine 1991; 115:92-8. 15. Schmerling R, Bedell S, Lilienfeld A, et al. Discussing cardiopulmonary resuscitation: a study of elderly outpatients. Journal of General Internal Medicine 1988;3:317-21. 16. See note 12. Lo et al. 1986:1613-5. 17. See note 13. Frankl et al. 1989:645-8. 18. See note 14. Seckler et al. 1991:92-98. 19. See note 15. Schmerling et al. 1988:317-21. 20. See note 4. Stolman et al. 1990:653-8. 21. See note 13. Frankl et al. 1989:645-8. 22. Stewart M, Roter D. Communication with Medical Patients (Introduction). Newbury Park, California: Sage Publications, 1989. 23. See note 22. Stewart, Roter. 1989. 24. Kinzel T. Relief of emotional symptoms in elderly patients with terminal cancer. Geriatrics 1988;43:61-68. 25. Degner L. Fostering patient participation in treatment decision-making. Paper presented at the 1991 International Consensus Conference on Doctor-Patient Communication, Toronto, Canada, 1991 Nov. 26. See note 13. Frankl et al. 1989:645-8. 27. O'Hare D. DNR orders in New York State. Paper presented at Controversies in the Care of the Dying Patient Conference, Lake Buena Vista, Florida, 1991, Feb. 28. See note 14. Seckler et al. 1991:92-8. 29. Uhlmann R, Pearlman R, Cain K. Physicians' and spouses' predictions of elderly patients resuscitation preferences. Journal of Gerontology: Medical Sciences 1988;43:M115-121. 30. See note 3. LaPuma et al. 1991:402-5. 31. See note 1. Emmanuel et al. 1991:889-895. 32. See note 3. LaPuma et al. 1991:402-5. 33. See note 1. Emmanuel et al. 1991:889-895. 34. See note 1. Emmanuel et al. 1991:889-895. 35. See note 25. Degner. 1991.