Abstract
Background People who have had a prolonged admission to an intensive care unit (ICU) commonly have profound debilitation and weakness. For the delivery of effective exercise training, an accurate assessment of exercise capacity is essential.
Objective The study objectives were to investigate how much ground-based walking is undertaken by inpatients recovering from critical illness within 1 week of discharge from an ICU and to evaluate the feasibility and safety of the Six-Minute Walk Test (6MWT) for this population.
Design This was an observational study.
Methods Within 1 week of discharge from the ICU, functional exercise capacity was measured with the 6MWT. The maximum distance ambulated on the ward in a single session as part of usual clinical management was extracted from the medical notes. The distance achieved during the 6MWT and the maximum distance ambulated on the ward were compared.
Results The participants (N=23) were survivors of a critical illness; their mean age was 57 years (SD=11). The median length of ICU stay was 11 days (interquartile range [IQR]=7). The mean 6-minute walk distance (6MWD) was 179 m (SD=101), and the maximum distance ambulated on the ward was 30 m (IQR=65). There was a moderate association between the distance participants ambulated on the ward and the 6MWD (r=.54). The maximum distance ambulated on the ward, expressed as a percentage of the 6MWD, was 29% (IQR=34%). Five participants (22%) experienced oxygen desaturation (oxygen saturation of <85%) and recovered within 1 minute of resting.
Limitations The maximum distance ambulated on the ward was estimated with premeasured distances.
Conclusions Most participants ambulated at a low percentage of their measured exercise capacity. The 6MWT appears to be a safe and useful test for inpatients recently discharged from the ICU.
Profound debilitation and skeletal muscle weakness are common complications in patients recovering from critical illness.1 Within the intensive care unit (ICU), muscle dysfunction is common, with 25% to 33% of patients who have been mechanically ventilated for more than 4 days demonstrating a reduction in the force-generating capacity of the upper and lower limb muscles.1–3 Survivors of critical illness continue to demonstrate reduced muscle strength, decreased functional exercise capacity, and poor quality of life at 1, 2, and 5 years after discharge from the ICU.4–6 In addition to the individual burden associated with these impairments, the costs for the community are large. Costs are both direct (ie, hospital readmission and inpatient rehabilitation) and indirect (eg, low rates of return to work).7,8 Furthermore, the burden on caregivers, both financially and emotionally, is high.9,10
Exercise training of survivors of critical illness is increasingly being recognized as a key intervention aimed at improving functional recovery and quality of life.11 Exercise started within critical care settings requires the minimization of sedation and involves either passive movement or active exercise, such as strength training and early mobilization or ambulation.12 Exercise training for patients in the ICU has been demonstrated to be safe, to improve exercise capacity and peripheral muscle strength, and to reduce ICU and hospital length of stay.13–15
Specific assessments have been developed to assess the function of patients within the ICU.11 However, as function improves, measures such as the Physical Function ICU Test16 demonstrate a ceiling effect, and the utility of these tests as a basis for exercise prescription has been largely unexplored. For the optimization of physiological adaptations, exercise training should be individually prescribed at a percentage of an individual's peak exercise capacity, measured with an exercise test.17 Studies investigating early exercise-based rehabilitation of inpatients recently discharged from the ICU have not identified an objective measurement that can be used to accurately prescribe exercise. Thus, it is possible that the prescription of exercise training for this population is suboptimal.14,15
The Six-Minute Walk Test (6MWT) is a field-based assessment that has been validated in patients with a range of chronic conditions.18 It has been shown to be safe in survivors of critical illness at the point of hospital discharge14,19,20 and to be a useful measure for assessing recovery in this population.5,6 However, the clinical utility and safety of this test in the period between discharge from the ICU and discharge from the hospital (ie, acute care inpatient stay) for survivors of a critical illness are unknown. It is likely that the prescription of exercise both within the ICU and during the acute care inpatient stay is largely based on individual clinician experience and clinical reasoning rather than objective measures of exercise capacity.
The aim of this study was to investigate, in adults who had been discharged from the ICU to a hospital ward within the preceding 7 days, the following: (1) the maximum distance ambulated during a single ward-based physical therapy treatment session; (2) the relationship between the maximum distance ambulated during a single ward-based physical therapist treatment session and a participant's functional exercise capacity, as determined with the 6MWT; and (3) the safety and feasibility of the 6MWT. We hypothesized that there would be a moderate-to-strong relationship (r≥.5) between the maximum distance ambulated during a single ward-based physical therapist treatment session and a participant's functional exercise capacity.
Method
Design
A prospective observational study that included some data obtained via retrospective chart review was conducted with survivors of critical illness.
Participants, Therapists, and Centers
All patients admitted to the ICU in 2 major Australian teaching hospitals in New South Wales were screened on a daily basis. Participants in this study were recruited as part of a larger study in which outcomes for survivors of critical illness (CIS) were compared with outcomes for survivors of acute lung injury (ALI). Patients were eligible for inclusion if they were between 50 and 70 years of age and had been intubated and ventilated for 4 days or more (CIS group) or if they met the published criteria for ALI, which did not include any age criterion (ALI group).21–23 Criteria for inclusion in the CIS group were chosen to enable comparison of the ALI and CIS groups in the larger program of research. Exclusion criteria comprised immobility before admission; documented neurological disease, including traumatic brain injury; unstable psychiatric disorders; orthopedic injuries with mobility restrictions; inability to understand English; history of recent pulmonary resection; and diagnosis of or treatment for malignant cancer within the preceding 12 months. Informed consent was obtained from all participants.
Outcome Measures
Descriptive variables.
Data recorded during the ICU admission comprised anthropometric and demographic variables, diagnosis, Acute Physiology and Chronic Health Evaluation (APACHE) II score,24 Functional Comorbidity Index,25 total time intubated and ventilated, and length of stay in the ICU. Total hospital length of stay also was documented.
Ground-based walking completed as part of clinical management.
The maximum distance ambulated by a participant during a single physical therapist treatment session both during the ICU admission and within 1 week after ICU discharge to the ward was extracted from the participant's medical notes and recorded. Before the study, walking courses of specified distances were measured within the ICU, wards, and surrounding environments. The treating therapist used these courses to record, in the medical notes, the distances walked by ambulating participants.
6MWT.
Within 1 week of discharge from the ICU to a hospital ward—and on the same day on which participants' records were reviewed—participants who did not have absolute or relative contraindications to exercise testing18,26,27 performed a single 6MWT. One of the authors (J.M.), with the aid of at least one assistant, conducted each test. We chose to have participants perform the 6MWT within 5 to 7 days after discharge from the ICU for the following 2 purposes: (1) to optimize the likelihood that any delirium and physiological instability had resolved and (2) to allow time for the participants to adjust to the new ward environment and the treating physical therapist to become familiar with the participants' impairments. The main outcome of the test was the distance walked (ie, 6-minute walk distance [6MWD]).
The 6MWT was carried out over a 30-m straight course18,26,27 in accordance with the protocol described by the American Thoracic Society.18 At the end of each minute, participants were informed of the time remaining, and standardized encouragement was given. The American Thoracic Society protocol was adapted to include continuous monitoring of heart rate (HR) via telemetry (Polar A1, Polar Electro Oy, Kempele, Finland) and arterial oxygen saturation (Spo2) via pulse oximetry (Rad-5V, Masimo Corp, Irvine, California). Both HR and Spo2 were recorded at rest, at the end of each minute during the 6MWT, and at 2 minutes after test completion. Dyspnea28 was recorded before and after the test and at 2 minutes after test completion. In the event that a participant rested during the test, standardized encouragement to recommence walking was given at 15-second intervals. The number, time, and duration of rests were recorded, and HR, Spo2, and dyspnea were recorded at the beginning of any rests.
The safety and feasibility of the 6MWT were determined on the basis of the number and type of any adverse events observed either during the test or immediately after test completion. We defined an observed adverse event during the 6MWT as an abnormal HR response (eg, a failure of HR to rise or a fall in HR during the test); excessive tachycardia—calculated as an HR of greater than [210 − (0.65 × age)]; signs or symptoms of poor perfusion (eg, sudden pallor, dizziness, fainting); development of an abnormal gait or loss of coordination; signs suggestive of mental confusion; onset of chest pain suggestive of ischemia; or desaturation where Spo2 fell to below 85%. The protocol mandated that the 6MWT was to be terminated immediately following the occurrence of an adverse event or, in the event that Spo2 fell below 85% during the test, a rest was imposed and the participant was encouraged to recommence walking if Spo2 returned to 85%. The participant was monitored for 30 minutes after the completion of the 6MWT by a researcher; thereafter, regular observations were performed by nursing staff in accordance with usual hospital guidelines. Walking aids and supplemental oxygen were used as most recently prescribed in the participants' records.
Data Analysis
The distribution of continuous variables was assessed with frequency histograms and the Shapiro-Wilk test. The maximum distance ambulated during a physical therapist treatment session was expressed as a percentage of the 6MWD. The 6MWD also was expressed as a percentage of a participant's predicted 6MWD.29 Data were expressed as medians and interquartile ranges unless otherwise stated. A Spearman rank correlation coefficient was used to explore the association between the maximum distance ambulated during a single ward-based physical therapist treatment session and a participant's 6MWD.
Role of the Funding Source
This study was partially funded by the Intensive Care Foundation, Australia.
Results
Data collection occurred between April 2011 and September 2012. The flow of participants through the study is shown in Figure 1. Twenty-three patients recovering from critical illness participated in the study. The characteristics of the participants are shown in Table 1.
Participant flow diagram. *Neurological disease, n=25; malignant cancer, n=14; orthopedic injury, n=19; death, n=2; other, n=3. †Orthopedic injury, n=5; neurological disease, n=3; malignant cancer, n=3; death, n=2; other, n=5. ‡Cognitively unable to consent, n=4; consent denied, n=3; expedited discharge from hospital, n=2; other, n=2. §Consent denied, n=3; death, n=1. ‖Expedited discharge from hospital, n=7; death, n=2; other, n=1. #Expedited discharge from hospital, n=2; death, n=1. ALI=acute lung injury, CIS=survivor of critical illness, ICU=intensive care unit.
Participant Characteristics (N=23)a
Table 2 shows, for each participant, data for age, diagnosis, APACHE II score, and walking aids and supplemental oxygen used during the 6MWT as well as the 6MWD and the maximum distances ambulated in the ICU and on the ward. The mean 6MWD was 179 m (SD=101, range=49–372); this value was equivalent to 27% (SD=15%) of the participants' predicted 6MWD.25 The maximum distance ambulated on the ward was 30 m (interquartile range=65, range=0–200). The maximum distance ambulated on the ward, expressed as a percentage of the 6MWD, was 29% (interquartile range=34%).
Data Recorded for Each Participanta
Data for blood pressure, HR, Spo2, and dyspnea scores recorded during the 6MWT are shown in Table 3. Blood pressure and HR responses were within acceptable limits for all participants during and after the 6MWT. Oxygen desaturation occurred to various degrees in the entire sample but was mostly within acceptable limits. Thirteen participants required between 1 and 3 rests during the 6MWT (Tab. 4), all because of fatigue. An adverse event occurred in 5 participants (22%) and, in all cases, was attributable to the occurrence of oxygen desaturation (Spo2 <85%). In these 5 participants, Spo2 recovered within 1 minute of resting, and 3 participants recommenced walking. The remaining 2 participants did not recommence walking because of fatigue.
Observations Recorded During the Six-Minute Walk Testa
Data Recorded for 13 Participants (57%) Who Rested During the Six-Minute Walk Testa
The mean time between discharge from the ICU and completion of the 6MWT was 6 days (SD=1). The difference between the performance of the 6MWT and documentation of the maximum distance ambulated on the ward was 1 day (SD=1). There was a moderate association between the 6MWD and the maximum distance ambulated on the ward (r=.54, P=.008) (Fig. 2).
Scatterplot with line of best fit showing the relationship between the 6-minute walk distance and the maximum distance ambulated on the ward.
Discussion
To our knowledge, this is the first study to report on the safety and feasibility of the 6MWT for inpatients recently discharged from the ICU. The distance participants ambulated on the ward during a physical therapist treatment session, relative to their functional exercise capacity, also was explored. Our data suggest that the 6MWT is a feasible test for this patient population. An adverse event was observed in 22% of the participants (n=5) and, in all cases, was attributable to oxygen desaturation (Spo2 of <85%), which recovered within 1 minute of resting. Furthermore, because the greatest 6MWD achieved in our sample was 350 m (48% of the predicted 6MWD), it appears unlikely that the test will be subject to a ceiling effect. A moderate association between the 6MWD and the maximum distance participants ambulated on the ward was demonstrated. However, during a physical therapist treatment session, participants ambulated, on average, less than 30% of the distance they walked during the 6MWT; this finding suggests that usual clinical practice was characterized by underprescription of ground-based walking exercise.
Research with survivors of critical illness, to date, has focused primarily on strategies for the implementation of exercise training within the ICU.13–16,30,31 Decreased sedation and early mobilization of patients within the ICU have been demonstrated to improve outcomes, such as functional exercise capacity, quadriceps muscle force, and self-perceived functional status.14,15,32 Exercise-based rehabilitation after hospital discharge also has been shown to improve functional exercise capacity and decrease anxiety and depression.19,33 Despite these findings, exercise-based strategies implemented on the hospital ward after discharge from the ICU to improve patient outcomes have been largely unexplored. For some other inpatient populations, such as patients hospitalized with an acute exacerbation of chronic obstructive pulmonary disease, the dose of walking-based exercise has been prescribed on the basis of the 6MWD (ie, at least 75% of this distance) or average walking speed achieved during the 3-minute walk test (ie, either 40% or 70% of this walking speed).34,35 However, this approach has not been described for other inpatient populations, including survivors of an ICU admission. It is likely that an individually tailored and structured ground walking program based on objective measures of functional exercise capacity will optimize the effectiveness of any inpatient rehabilitation program.
Participants who ambulated farther during the 6MWT also ambulated farther on the ward with their treating physical therapist. However, the distance ambulated with the treating therapist was a small proportion of the distance achieved during the 6MWT. This finding indicates that participants had the capacity to ambulate significantly farther than they did during treatment with the physical therapist and that ground-based walking exercise was underprescribed. Thus, its effectiveness may have been compromised.
For patients who have been transferred to a hospital ward after discharge from an ICU, the implementation by treating physical therapists of short walks relative to the patients' exercise capacity may be the result of a perception that these patients—who are of higher acuity or have multiple comorbidities, or both—may not be able to exercise safely. However, in the present study, the only adverse events that occurred during the 6MWT were attributable to oxygen desaturation. No participants required additional oxygen, and all events were ameliorated with rest. The amount of exercise undertaken also may be limited by other factors, such as a reluctance of patients to mobilize, a lack of awareness of a patient's actual exercise capacity, or both. The timing of the intervention during the day (ie, in relation to meals and medications) also may compromise the distance ambulated by patients. It seems unlikely that time constraints were an important factor contributing to the implementation of short walks relative to an individual's exercise capacity noted in the present study, as only 6 minutes would have been needed for the participants to ambulate a distance equivalent to their 6MWD during a physical therapist session.
For survivors of a critical illness, it is likely that supervised ambulation is only one component of a comprehensive physical therapy exercise program. Earlier work showed that survivors of a critical illness have impairments in muscle strength and the ability to perform functional tasks and are at risk of falling during the acute care recovery phase on the ward.14,36 To address these impairments, physical therapists likely will implement upper and lower limb resistance exercises as well as balance and functional retraining.14,37 Although not assessed in the present study, engaging in exercises other than ambulation may have reduced the amount of ground-based walking undertaken by our study participants.
For a population with long-lasting impairments that contribute to substantial ongoing costs for the community,4,6,8 optimizing exercise prescription to improve functional outcomes is important. Effective and safe exercise prescription involves a thorough evaluation of a patient, including assessments of the patient's medical history, objective measures of functional exercise capacity, and the cardiorespiratory response to exercise.11,16,31,34 Nevertheless, there is a paucity of data to guide clinicians about which assessments are appropriate to use during the acute care inpatient stay after discharge from the ICU. The 6MWT was well tolerated by the participants in the present study. Furthermore, this test yielded useful data and was a safe and feasible means of assessing functional exercise capacity. Such data could be used as a basis for prescribing exercise training during the acute care inpatient stay after critical illness.
Limitations
The main limitation of the present study was the reliance on documentation by the treating physical therapist of the maximum distance ambulated during a single treatment session on the ward and in the ICU for each participant. The actual distance ambulated was not objectively measured by the researcher. Nevertheless, given the magnitude of the discordance between the distance documented and the 6MWD, it is unlikely that measurement error—which is likely to have been random—accounts for our findings. Furthermore, because the time taken to ambulate the distance documented by the physical therapist on the ward and in the ICU was not recorded, it was also not possible to determine the intensity at which a participant ambulated.
In conclusion, the present study demonstrated that many inpatients recently discharged from the ICU ambulate at a small proportion of the distance walked in the 6MWT. The use of objective measures such as the 6MWT to aid in more accurate exercise prescription could lead to improved outcomes. Further research about the optimal exercise prescription for this population is needed to improve long-term outcomes. Specifically, trials should investigate whether an individually tailored exercise program based on objective measures of exercise capacity, such as the 6MWT, and implemented after discharge from the ICU can improve long-term functional outcomes.
Footnotes
Mrs Waters, Dr Hill, Dr Jenkins, Dr Johnston, and Ms Mackney provided concept/idea/research design. All authors provided writing and data analysis. Mrs Waters and Ms Mackney provided data collection. Ms Mackney provided project management and fund procurement. The authors thank Mr Daniel Robins and the physical therapists at the participating hospitals for their assistance with this study.
Ethical approval for the study was obtained from the human research ethics committees of Hunter New England Local Health District, The University of Newcastle, and Curtin University.
This research was presented at the Australia Physiotherapy Association Conference; October 17–20, 2013; Melbourne, Victoria, Australia.
This study was partially funded by the Intensive Care Foundation, Australia.
- Received June 29, 2014.
- Accepted March 23, 2015.
- © 2015 American Physical Therapy Association
References
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