Of missiles and medicine: early warning systems (2006)

The ballistic missile early warning system (BMEWS) became operational in 1959. BMEWS provided long-range warning of a missile attack over the polar region of the northern hemisphere. There were three installations: the United States Air Force facilities of Thule Air Base, Greenland; Clear Air Force Station, Alaska; and the Royal Air Force facility of Fylingdales, in the United Kingdom. Medical early warning systems came a bit later. Formal criteria for alerting a medical emergency team were introduced in 1989 in Liverpool, New South Wales, Australia. As well as cardiorespiratory arrest these criteria included grossly abnormal physiological values or concern regarding the patient [1]. The first published description of a physiologically based early warning score (EWS) for identifying critically ill ward patients was in 1997 [2]. Several physiological parameters were measured. Values within a ‘normal’ range were awarded zero points with increasingly abnormal values assigned points of up to a maximum of three. The points for each physiological parameter were summed to obtain a total score. The EWS is used to follow the patient's physiological well-being and to generate a response to predetermined abnormal values. ‘Track and trigger’ is an apt description of this system. Early warning scores have been constructed using subjective criteria and many different variations are being used throughout the United Kingdom [3]. To date there is little evidence to help us decide which physiological parameters and values are best. This issue contains a report of the use of an EWS during an outbreak of Legionnaires' disease in the summer of 2002 [4]. The authors sought Ethics Committee approval for reviewing the EWS observations from nursing records and observation charts. The Committee's Chairman required that patients, or next of kin, be contacted to ascertain if they did not wish their data to be used (negative consent). Permission was refused by 44 of 498 patients, and their data were excluded from further analysis. If there was no reply it was assumed that there was no objection to the use of information extracted from the hospital records. Research based upon examining old records or databases where data are grouped and anonymised carries little, if any, risk of harm to the patients. Asking each patient for consent is laborious, expensive and time consuming and will make many similar studies very difficult to conduct. Ethics Committee review is an essential safeguard to all research. My experience suggests that many Committees would not require individual consent for this research. However, it is worth noting that 8.8% of those asked actively refused consent. For most research the assumption is made that patients do not wish to participate unless they provide informed consent. Many would argue that assuming that no reply implies consent is an illogical and unacceptable practice. Lack of consent cannot be taken to mean that a patient was willing to participate. Ethical concerns aside, there were some features of note in this study. Recordings of one or more of respiratory rate, pulse rate, systolic blood pressure and temperature at one time were called an observation set. The associated EWS, which also included patient responsiveness and urine output, was noted. Pulse rate, systolic blood pressure and temperature were included in over 90% of the sets, but respiratory rate was less commonly recorded. Responsiveness and urine output were infrequently and poorly recorded. An EWS was assigned to nearly 70% of the sets of observations. Errors in calculation were common and overall only 54.4% of observation sets were associated with a correct EWS. In general, the higher the score, the more likely it was to be misscored and also to be underscored. Patients proven to have Legionnaires' disease were more likely to have incorrectly calculated scores. This study provides an insight into the way EWSs are used in clinical practice. The authors do not make it clear how many observations were from the nursing records and how many from observation charts. Experience suggests that sets of observations are more likely to be recorded on charts, whereas individual results may be found in nursing and medical records. Many charts now incorporate an EWS with instructions on frequency and intensity of monitoring as well as action to be taken based on the EWS. In the discussion, the authors give the opinion that errors were unlikely to have arisen from the rapid introduction of a new system while ward staff were working under pressure. However, EWSs had only just been implemented throughout the hospital at the time of this study. Education and support are essential to ensure that physiological values are completely and accurately recorded and associated scores calculated. The percentage of miscalculated EWSs fell to close to zero over the study period, suggesting that familiarity may increase the accuracy of the scoring. A glance at the calendar for the period also suggests that fluctuations in the percentage of miscalculations shown over the 2-week study period may have been related to increased errors at weekends. Patients in whom Legionnaires' disease was not confirmed were, in general, admitted later in the study period and were less likely to have incorrectly calculated EWSs. I would question the authors' view that it was unlikely that scores were more accurate in this group because staff had become used to the system. Their alternative explanation that experienced staff may ‘manipulate’ the scoring system to support their clinical impression may merit further exploration. It is worth noting that the initial documentation for scoring contained ambiguities in the points allocated to five of the six parameters. It is exceeding laborious, tedious and demanding to ensure completeness and consistency in scoring instructions but it is essential if ward staff are to have confidence in, and enthusiasm for, the system. There is much evidence to show that abnormal physiological values are associated with an adverse outcome. Most hospital patients who have a cardiorespiratory arrest have documented physiological derangement in the preceding hours [5–7]. Similarly, a high percentage of ward patients who die have abnormal values documented in the hours before death [8]. Other studies have shown that this holds true for patients admitted from the wards to intensive care [9, 10]. The more physiological abnormalities a ward patient has, the higher their hospital mortality [11]. It therefore makes sense to combine a set of physiological measurements into a scoring system. The parameters used are those that are easy to measure on a ward. However, it is possible that other measurements may be better. We know that increasing values of a score are associated with increased mortality as well as decisions to admit a patient to a critical care area or to limit treatment [12, 13]. However, we have little information about which parameters to include and the appropriate values for the scores. Some systems use a total score for a trigger and others also include a threshold in a single parameter. It is also possible that EWSs work well for some groups of patients and poorly for others. Patients within a hospital are already a selected group with a higher mortality and morbidity than the population outside. Early warning scores are a way of selecting a subgroup of patients who are particularly at risk. They are screening tools and are not designed to predict outcome. A good score is one which will identify most of the patients who need additional support and few who do not, while only missing small numbers of those who might also benefit. The EWS is also an objective means of describing sick patients, e.g. ‘this patient has an EWS of four’. Instructions may state that staff must be summoned at certain values of the EWS and define the response to increasing values. An EWS may therefore empower ward nurses to summon senior staff and mandate a response from doctors. It may be particularly useful as more patients are cared for in single rooms and initiatives, such as the ‘hospital at night’ programme, limit the number of doctors on wards. To acquire an EWS, measurements have to be made. Good chart design will assist in obtaining a set of accurate and complete observations. These days, pulse rate, blood pressure, temperature and oxygen saturation are usually machine-measured. Measuring respiratory rate is not that easy. A reliable means of counting it would be useful. Level of consciousness or urinary output may be the earliest or only signal of impending crisis and are also important. Even in sick patients, routine measurements on the wards are often infrequent. A large randomised Australian study investigating the effect of introducing Medical Emergency Teams (METs) reported that a complete record of blood pressure, heart rate and respiratory rate was only available in 19% of patients in the 15 min before a cardiac arrest, unplanned intensive care admission or unexpected death [14]. Four Finnish hospitals investigated how often physiological measurements were made in the 24 h preceding a cardiac arrest on the wards [15]. Of the 56 patients who arrested, seven had no recorded observations in the 8 h preceding the event. Although respiratory distress was one of the commonest warning signs before the cardiac arrest, only one patient had any recorded respiratory rate measurements. Introduction of early warning scoring coupled with an educational programme can be effective in increasing the frequency with which respiratory rate is recorded [16]. In Melbourne, Australia, activation of the MET occurs more commonly around the time of routine nursing observations and handover [17]. These findings imply that more frequent or automated observations might detect physiological abnormalities earlier. In England, some 3% of acute hospital beds are designated as critical care (Department of Health hospital activity statistics). Outside these areas and the operating theatres, few patients are continuously monitored. Cheap and robust monitors incorporating EWSs are one way in which the reliability and intensity of ward monitoring can be improved while ensuring the accuracy of the EWS calculated from the raw physiological data. Systems are being developed for automating early warning scoring. In the future computerised analysis of multiple simultaneous physiological inputs collected from a monitor may prove to be the best way of identifying at-risk patients at an early stage (see http://www.oxford-biosignals.com/). Identifying patients is not enough as, even when set criteria are achieved, the response may be delayed or inappropriate [15]. The ballistic missile warning system illustrates issues associated with early warning systems. It is expensive and complex to obtain, analyse and interpret early warning data. Much has to be invested in systems to ensure that they work as intended and do not give false alarms or miss real threats. The United States may have had 30 min warning of a missile attack. The United Kingdom only had three. Being forewarned is useful only if you have time to get to a shelter or respond with retaliation or antimissile missiles. If the attack is overwhelming there may be little practical that can be done to limit the death and destruction. The best solution is to remove all threat of an attack so that the early warning system is unnecessary. Applying these lessons to health care, early warning systems require the collection and processing of accurate, reliable and timely data. The score based upon this data must be capable of identifying appropriate patients. Patients must be in hospital long enough for this to happen and for a response to be initiated. Early warning of deterioration is only of value if outcomes can be changed. For patients who trigger an EWS, outcome can only be improved if there is an effective response involving clinicians with appropriate training, skills, experience and resources. There is some evidence that early intervention is beneficial and intuitively it makes sense [18]. Finally, early warning systems may not be necessary if all hospital patients who are, or may become, critically ill have access to an appropriate critical care or monitored location.