Volume 53, Issue 9 p. 841-847
Free Access

The impact of a high-dependency unit on the workload of an intensive care unit

G. Dhond

G. Dhond

Critical Care Complex, Norfolk and Norwich Hospital, Brunswick Road, Norwich NR1 3SR, UK

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S. Ridley

S. Ridley

Critical Care Complex, Norfolk and Norwich Hospital, Brunswick Road, Norwich NR1 3SR, UK

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M. Palmer

M. Palmer

Critical Care Complex, Norfolk and Norwich Hospital, Brunswick Road, Norwich NR1 3SR, UK

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First published: 13 May 2002
Citations: 27
G. Dhond Critical Care Complex, Norfolk and Norwich Hospital, Brunswick Road, Norwich NR1 3SR, UK


While there has been recent support for high-dependency unit development, there are few data reporting the impact of such development on existing critical care facilities. Therefore the aim of this study was to examine the workload and capacity constraints of an adult general intensive care unit before and after the development of an adjacent high-dependency unit. Following the opening of the high-dependency unit, the total number of patients admitted increased by 49%. On the high-dependency unit, more elderly patients were admitted for longer and more frequently following midweek elective surgery. On the intensive care unit, patients' initial severity of illness was lower and their duration of admission decreased; fewer patients were admitted directly from the general wards. The financial benefits of high dependency care may be eroded by the increased use of the critical care services.

The recent long-distance transfer of a neurosurgical patient [1] and the report by Metcalfe and McPherson [2] prompted the Department of Health to issue guidelines concerning admission to and discharge from intensive care and high-dependency units [3]. These guidelines were based on the supposition that many patients presently nursed on intensive care units (ICUs) do not actually require active ICU support but rather close observation and monitoring. Some of these patients were considered ‘inappropriate’ ICU admissions; they may, in fact, have been more suitable for care in an intermediate area, such as a high-dependency unit (HDU). The benefits of intermediate care have been alluded to by indirect evidence from the USA and UK [4–7]. Franklin reported a 39% reduction in cardiac arrest calls from the general ward following the development of an intermediate care area [8]. Gamil and Fanning reported that in the 23 fatalities following over 2000 major surgical procedures, the train of events which ultimately lead to poor outcome could be identified within the first postoperative 24 h [9]. Unfortunately, several national surveys of critical care services suggest that at best only one-third of UK hospitals are supported by HDU facilities [2, 10, 11]. As a result, there is little opportunity to study HDU services. Thus while there may be political support for HDU development, there have been few studies which report the consequences of HDU development on existing critical care facilities.

The aims of this study were to examine the workload (number and demographic details of patients) and capacity constraints (occupancy and number of refusals) of an adult general ICU before and after the development of an adjacent HDU.


Hospital and critical care resources

The Norfolk & Norwich Hospital is a 1100-bed district general hospital serving a population of just under half a million. It is relatively isolated with the closest adjacent district general hospitals being 20 and 45 miles away in opposite directions. Prior to the development of the HDU, the ICU had eight physical bed spaces but staffed so that a maximum of seven ICU patients or six ICU and two HDU patients could be treated. Patients were designated as requiring either ICU or HDU care by the senior nurse prospectively on admission according to the definitions laid down by the Association of Anaesthetists [11].

In January 1996, an adjacent six-bedded HDU was opened. The nursing staff establishment of the ICU was adjusted to staff six ICU beds so making 12 patients the maximum capacity of the HDU/ICU complex. The combined HDU/ICU is run as one large clinical area, being manned by the same medical and nursing staff. The admission and discharge of patients together with their clinical management is the responsibility of the senior ICU medical staff. The HDU/ICU is a general adult unit accepting all types of critically ill patients, except those with acute cardiac conditions who are treated elsewhere in the hospital and patients requiring cardiac or neurosurgery who are referred to the regional centres. The unit has a policy of not transferring critically ill patients because of lack of facilities.


All patients admitted to the ICU initially and later the combined HDU/ICU complex from July 1995 until June 1996 were studied, so representing the 6-month periods before and after the opening of the HDU. The patients' demographic details (age, sex, source of the patient [e.g. general ward, theatre or Accident and Emergency Department], severity of illness, diagnosis and outcome) were recorded prospectively. Their severity of illness was measured using the APACHE II score calculated on the most abnormal variables recorded within the first 24 h and the risk of hospital mortality was estimated using Knaus' coefficients [12]. The presenting diagnosis was classified according to the Intensive Care Society's UK APACHE II study [13]. The patients' durations of admission were calculated by subtraction of the dates and times of discharge from those of admission. Occupancy was calculated on a daily basis using a method which reflected the amount of time patients physically occupied the beds [14].

The patients were divided into four groups:

(a) Group 1: patients classified as needing HDU care but who were nursed on ICU prior to the opening of the HDU.

(b) Group 2: critically ill patients classified as requiring intensive care support nursed on ICU before opening of the HDU.

(c) Group 3: patients admitted directly to the new HDU.

(d) Group 4: critically ill patients admitted to ICU after the opening of the HDU.

Statistical methods

If the distribution was (approximately) normal (e.g. age), a two-sample t-test was used to compare the groups. For data with a skewed distribution (e.g. APACHE score, duration of admission and risk of hospital mortality), the Wilcoxon rank sum test was used. Differences in distributions of sex, source, diagnostic categories and outcomes were examined using Chi-squared analysis. One-way analysis of variance was used to determine whether mean daily occupancy was significantly different for any day of the week for either the high-dependency or intensive care beds. A p value of less than 0.05 was considered significant and the 95% confidence intervals (95% CI) are shown in parentheses where appropriate.


Patient demographics

The total number of patients treated in the critical care complex increased by 49% following the opening of the HDU, from 430 admitted to the ICU alone to 641 admitted to the HDU/ICU complex in its first 6 months (Table 1).

Table 1. Demographic details of patients admitted to ICU alone and HDU/ICU complex before and after opening the HDU (95% CI in parentheses).

Comparing the patients requiring only HDU care (Groups 1 and 3), Group 3 were significantly older (61 vs. 53 years, p < 0.05). However, there were no significant differences in their sex distribution, severity of illness at admission and outcome. With respect to the patients requiring ICU care (Groups 2 and 4), there was a significant decrease in the severity of illness at admission of the patients in Group 4 (APACHE score 13 vs. 15, p < 0.05) (Fig. 1) but no significant differences in their risk of hospital mortality (Fig. 2).

Details are in the caption following the image

Distribution of APACHE scores in the four groups (numbers given in table below).

Details are in the caption following the image

Distribution of risk of hospital mortality in the four groups (numbers given in table below).

The distribution of the diagnostic categories, admission types, duration of admission and sources of patient referral are shown in Tables 2 and 3. There was no significant variation over time in the numbers of patients admitted within the main diagnostic categories to either HDU or ICU. There were significant differences in the distribution of type of admissions for the HDU patients. In the new HDU, a greater proportion of patients (Group 3) were admitted electively rather than as emergencies, reversing the trend in the previous 6 months (p < 0.05). There was no significant difference in the ratio of elective to emergency admissions to the ICU (Groups 2 and 4) over the study period. Over half the patients throughout the study period were admitted from the operating theatres (all groups). With respect to the source of admission, there were significantly fewer patients admitted to the ICU (Group 4) from the general wards following the opening of the HDU (13% vs. 22%, p < 0.05). There was no significant variation in the sources of patients to the new HDU (Group 1) compared to the first 6 months (Group 3), although ‘step-down’ transfers from the adjacent ICU accounted for only 14% of the HDU admissions.

Table 2. The distribution of diagnostic categories among the HDU and ICU groups (percentage in parentheses).
Table 3. Type and duration of admission and distribution of patient sources (numbers & percentage or 95% CI in parentheses).


Figure 3 shows that the weekly occupancy of the ICU and HDU/ICU complex is highly variable. There are, however, matching peaks and troughs in occupancy of HDU and ICU in August, September, December and April, reflecting the contraction of HDU facilities as the demand for ICU increased. There were significant changes in the occupancy of both intensive care and high-dependency beds when the HDU was opened (Table 4). The occupancy of the ICU decreased (77–71%, p < 0.05) after the HDU opened while the occupancy of the high-dependency beds increased (35–61%, p < 0.001).

Details are in the caption following the image

Variations in occupancy in ICU beds (dotted line) and HDU beds (solid line) before and after the new HDU.

Table 4. The mean daily bed occupancy (%) of the ICU and ICU/HDU complex.

Table 5 shows the mean daily occupancy of the ICU and HDU/ICU complex for each weekday. There was no difference in ICU occupancy with day of the week either before or after the HDU opened. However, there was a statistically significant difference in the occupancy of high-dependency beds after the HDU opened (p < 0.001), with a higher mean daily occupancy on Thursday and Friday and a lower occupancy at the weekend.

Table 5. Occupancy of the HDU with day of the week.

The duration of admission of the high-dependency patients increased significantly with the introduction of the new HDU (p = 0.04), despite no observed increase in their severity of illness (Table 3). At the same time the duration of admission of the patients admitted to the ICU shortened significantly (p = 0.03), but this may have been associated with a significant reduction in their severity of illness at admission. There was a marked decrease in the rate of cancellation of major elective operations following the opening of the new HDU from 28 in the 6 months before to 18 in the 6 months after the opening of the HDU; no patient was cancelled more than once in the 6 months following the opening of the HDU.


The impact of high-dependency care on critical care services is difficult to assess in a rigorous scientific manner because of ethical and logistic considerations. A randomised control trial, allocating patients to receive high-dependency or continued general ward care, may be difficult to conduct when patients are randomly allocated to remain on a hard-pressed general ward as opposed to being admitted to the HDU which has spare capacity. Randomly allocating patients to be discharged directly to either the general ward or HDU from the ICU is probably more feasible. However, the results of this study suggest that only a small percentage of HDU admissions originated from the ICU as ‘step-down’ care. Once an HDU is established, only observational studies are really possible because of clinical pressure to admit patients if capacity permits. Other well recognised confounding factors, such as the rate of referral fluctuating when HDU/ICU are known to be full, and admission criteria varying as the critical care services become busier, makes measurement of the matching of supply and demand difficult. Under such circumstances following analysis of workload, only inferences may be drawn and therefore in this study we decided to look for only clear changes and trends.

Expanding the critical care services with a new HDU resulted in a significantly increased workload within 6 months, with the total number of patients passing through the HDU/ICU complex increasing by 49%. Most of this increase was accounted for by patients admitted to the new HDU; the number of patients referred for postoperative high-dependency care increased almost 250%. Prior to opening the HDU, patients were admitted predominantly to the ICU for high-dependency care after emergency surgery but with the new HDU, a greater elective workload could be undertaken. This was reflected by the busiest period for the HDU being midweek when elective surgery is maximal. Previously most of these postoperative patients would have been returned to the general ward. Studies by Gamil [9] and Leeson-Payne [15] suggested that many patients on the general wards would benefit from closer observation and monitoring. Although there is not a direct link between the outcome in terms of mortality in the present study and their HDU admission, it is difficult not to surmise that being cared for on a unit with a nursing establishment of three whole-time equivalents per bed is better than staying on a general ward with less than one whole time equivalent per bed.

Other indices also suggest an improvement in quality of care. Once the HDU was opened, the patients remained on HDU for longer despite there being no increase in their severity of illness. This was probably due to increased capacity leading to less pressure on beds but did allow more time to ensure physiological stability. Most high dependency patients were successfully stabilised on HDU, for only 13 patients admitted to the HDU subsequently deteriorated and ‘stepped-up’ to the ICU. These may represent premature and possibly inappropriate HDU admissions; however, this level of upgrading is in line with other reports. Crosby [16] reported that less than 5% of patients needed upgrading to ICU from an HDU adjacent to but separate from the ICU, a similar situation to Norwich. The patients admitted to the new HDU (Group 3) were significantly older, though not more severely ill, than those who received high-dependency care prior to the opening of the HDU (Group 1). Elderly patients are particularly prone to physiological derangement following surgery. The increased capacity and flexibility afforded, principally by the HDU, for postoperative care contributed to the marked decrease in the cancellation rates for major elective surgery. Reports of activity following the development of an HDU in Sheffield [17] showed a similar reduction in all parameters (postponed operations, premature discharges, transfers out, refusals) that implied capacity was previously constrained.

The development of the HDU/ICU complex also had an impact on the patient profile of the ICU. Although there were no significant differences in age distribution, type of admission or distribution of diagnostic categories, the ICU patients (Group 4), despite their risk of hospital mortality remaining the same, were significantly less physiologically deranged on admission following the opening of an HDU. There are numerous possible explanations but two of the most important are that with the additional capacity, the patients are referred earlier to the ICU prior to acute decompensation. Also in the presence of an HDU, patients are now stabilised prior to return to the general ward and this prevents organ failure and critical illness developing. This is supported by the significant reduction in the number of patients being referred to ICU from the general wards following the opening of the HDU. If the ICU admits the most severely ill patients in the hospital, then a reduction in their number and severity of illness would imply that overall the acuity of illness in the hospital as a whole declined following the opening of the HDU. This is an important finding and will require confirmation in other hospitals.

The shorter stay of the ICU patients following opening of the HDU may reflect their lower severity of illness on admission. The use of the HDU as a ‘step-down’ facility may have contributed but only 14% of ICU patients were discharged to the HDU. The opening of an HDU produced a large increase in the workload for the Critical Care Complex; a previously unmet demand for observation and monitoring was realised following opening of the HDU. Increasing the capacity of the Critical Care Complex by 71% (7–12 beds) did not produce a corresponding reduction in occupancy. It is important to appreciate that increased demand for high-dependency care from elsewhere in the hospital is likely to be expressed and the HDU may not reduce the ICU workload. Edwards & Stockwell [18] reported that ICU bed usage remained the same (851 vs. 978 patients days) following HDU opening. Although HDU costs may be much higher than the general ward because of sophistication of care requiring increased nurse/patient ratio and the high cost of consumables, pharmacy and capital assets, the cost is, at most, only 50% of that of ICU [19]. However, the total cost of critical care services is unlikely to be reduced by the same proportion because of increased workload and so HDU development may not ameliorate funding difficulties. Even so, any cost implications are likely to be offset by improvements in the quality of care inferred by the findings of the present study.

In conclusion, as more elderly patients undergo major elective surgery, so the demand for high-dependency care will rise. The opening of the HDU at the Norfolk & Norwich Hospital has enabled us to offer this level of optimal care to a greater number of such patients and for a longer time. Although no difference in mortality was observed in this study, more patients benefited from care on the HDU within the first 24 h of surgery; this may have improved postoperative care and prevented complications. In terms of impact on the ICU, we observed a reduction in severity of illness at admission and a concomitant shortening of stay in the ICU population. Although care on HDU is cheaper than ICU management, we have not observed a reduction in demand for ICU care with the introduction of the HDU. Indeed the HDU appears to have generated new demand for critical care services and is unlikely to relieve the pressure on ICU beds per se or to reduce the overall cost of critical care.