Continuous monitoring of vital signs in the general ward
Vital signs were continuously monitored in the general ward using three wearable wireless devices as part of the WARD project:
- Isansys LifeTouch (Isansys Lifecare, Oxfordshire, UK) — a single-lead electrocardiogram (ECG) patch placed over the left anterior thorax, recording heart rate and respiratory rate as an average of the past 60 seconds;
- Nonin WristOx 3150 (Nonin Medical, Minnesota, USA) — a wrist-worn pulse oximeter that measures arterial oxygenation with a calculated average of at least five reliable measurements from the past 60 seconds; and
- Meditech BlueBP-05 (Meditech, Hungary) an inflatable blood pressure cuff device, programmed to measure every 30 minutes during the daytime (07:00 am to 9:59 pm) and every 60 minutes during the night-time (10:00 pm to 06:59 am).
Data were transmitted through Bluetooth to a bedside gateway and from the gateway to a hospital server via a secure hospital internet connection. Research staff attended patients daily to ensure proper function of the equipment as well as patient comfort. Clinical staff and patients were blinded to the collected vital signs data.
Serious adverse events
SAEs during the first 96 hours in the general ward were assessed according to a predefined manual with internationally recognised standards for diagnostic classification, and were rated as SAEs if considered life-threatening, resulting in death, required prolongation of existing hospitalisation, or resulted in significant disability or incapacity. Complications (SAEs) can be diagnosed by on-call medical doctor or assessed by the investigator by review of the patient’s record according to the criteria defined below:
- pneumonia — demonstration of an infiltrate by the radiographic imaging plus either fever > 38°C, dyspnoea, cough, new or increased expectorate, or pleuritic chest pain;
- sepsis — suspected infection with organ dysfunction; a total Sequential Organ Failure Assessment (SOFA) score ≥ 2 points;
- septic shock — sepsis and persistent hypotension requiring vasopressors to maintain mean arterial pressure ≥ 65 mmHg and lactate ≥ 2 mmol/L (despite adequate volume resuscitation).
The primary exposure was the maximum PACU score at any time point during the PACU stay.
Secondary exposures included the maximum PACU score at prespecified 6-hour intervals starting on admission and until discharge. Maximum PACU subscores during the PACU stay and at discharge were calculated for circulatory vital parameters (combined scores from pulse and systolic blood pressure) or respiratory vital parameters (combined scores from respiratory rate and oxygen saturation).
The primary outcome was the cumulative duration of any vital signs abnormalities (heart rate, blood pressure, respiratory rate, and arterial oxygen saturation) during the first 96 hours in the general ward or until discharge, whichever came first. Thresholds for circulatory (heart rate > 130 min-1 or < 30 min-1 and systolic blood pressure < 90 mmHg or > 220 mmHg) and respiratory (SpO2 < 85% and respiratory rate > 24 min-1) vital signs abnormalities were set.
Secondary outcomes were the cumulative duration of circulatory vital signs abnormalities (heart rate, systolic blood pressure), cumulative duration of respiration abnormalities (SpO2 and respiratory rate), and the frequency of any SAE during the first 96 hours in the general ward or until discharge, whichever came first.
Specific pre-analytical algorithms removed noise and artefacts before analysing continuous data. Heart rate and respiratory rate were recorded with a one-minute sampling frequency derived from automatic detection of the QRS complex and R peaks in the single-lead ECG, digitised at 1000 samples per second. Every minute, 10-second segments of the ECG underwent a computerised filtration process, where signal quality was ensured based on QRS complex analysis. The heart rate, respiratory rate, and SpO2 included both raw values and a calculated average per minute. An SpO2 change > 4% per second was considered an artefact and thus removed from the final analysis. The duration of abnormal blood pressure measurements was calculated as the time between the abnormal value and the previous normal value, plus the time until the first subsequent normal value divided by two, thus assuming the abnormal value started and ended midway between the normal and abnormal value.
Due to the lack of similar studies on duration of vital signs abnormalities, we did not perform a formal power calculation in this exploratory study. Based on the existing literature, we expected a 20–30% complication rate.
Data are presented using descriptive statistics either as medians with interquartile ranges (IQRs) or means with 95% confidence intervals (95% CIs), dependent on distribution. The duration of vital signs abnormalities was measured as minutes during the entire monitoring time (96 hours). SAEs were presented with percentages for categorical data. The primary analysis was the association between the maximum PACU score and the cumulative duration of vital signs abnormalities (any). Secondary analyses were the associations between maximum and discharge circulatory and respiratory PACU subscores and duration of circulatory and respiratory vital signs abnormalities respectively. Associations were tested by Spearman rank correlation, with a significance level of P = 0.05. The predictive value of the PACU scores for SAEs in the general surgical ward were calculated as the area under the receiver operating characteristic (AUROC) curve with 95% CIs. All data analyses were conducted using the R Foundation Statistical Program (R v.3.6.2) and R Studio (v.1.2.5001).
One-hundred patients were prospectively included. Eight patients were excluded due to being transferred directly from the PACU to the ICU (n
= 4), change of surgical procedure (n
= 3), or withdrawn consent (n
= 1), leaving 92 patients for analysis. The mean age was 69.6 years (95% CI, 68.5–70.3); 30 patients (33%) were female, 36 (39%) underwent an oesophageal resection, and the median ASA score was 2. During the first 96 hours in the general ward, there was one ICU admission, mortality was zero, and the median hospital length of stay was 9.6 days (IQR, 7.6–15.9 days). All patients received standardised general anaesthetic (six patients received volatile anaesthesia) followed by a multimodal postoperative analgesia plan. Demographic characteristics are detailed in Table 1
. The median PACU stay was 19 hours (IQR, 19–21 hours), and patients were successively discharged to the general surgical ward, with 92 patients admitted for 12–18 hours, 62 patients admitted for 18–24 hours, and only seven patients remaining in the ICU after 24 hours. The median maximum PACU score was 7 and occurred predominantly during the first 6 hours after PACU arrival (Figure 1
(median score, 2) and systolic blood pressure (median score, 1) were the most frequent vital parameter abnormalities in the PACU.
Vital signs abnormalities
The median duration of continuous monitoring was 91 hours (IQR, 71–95 hours) per patient. We recorded a median 84 hours (IQR, 64–94 hours) of respiratory and heart rate, 55 hours (IQR, 30–76 hours) of SpO2
, and 83 (IQR, 32–116) blood pressure measurements per patient for analysis. The primary outcome of duration of cumulative vital signs abnormalities (any) was a median duration of 114 minutes (IQR, 23–324 minutes; range, 0–1755 minutes) and occurred in 88 patients (96%) during the time of continuous monitoring (Table 2
No significant correlation between the maximum PACU score and duration of (any) vital signs abnormalities during the first 96 hours in the general ward was found (R
= 0.13; P
= 0.22) (Figure 2
). The maximum circulatory PACU score and circulatory discharge score were significantly correlated to the duration of circulatory vital signs abnormalities in the general ward (R
= 0.32 [P
= 0.0021] and R
= 0.26 [P
= 0.014] respectively). The maximum respiratory PACU score and the respiratory discharge score were not significantly correlated to the duration of respiratory abnormalities in the ward (R
= -0.02 [P
= 0.85] and R
= -0.056 [P
= 0.6] respectively).