Non-contact physiological monitoring of post-operative patients in the intensive care unit

Abstract

Prolonged non-contact camera-based monitoring in critically ill patients presents unique challenges, but may facilitate safe recovery. A study was designed to evaluate the feasibility of introducing a non-contact video camera monitoring system into an acute clinical setting. We assessed the accuracy and robustness of the video camera-derived estimates of the vital signs against the electronically-recorded reference values in both day and night environments. We demonstrated non-contact monitoring of heart rate and respiratory rate for extended periods of time in 15 post-operative patients. Across day and night, heart rate was estimated for up to 53.2% (103.0 h) of the total valid camera data with a mean absolute error (MAE) of 2.5 beats/min in comparison to two reference sensors. We obtained respiratory rate estimates for 63.1% (119.8 h) of the total valid camera data with a MAE of 2.4 breaths/min against the reference value computed from the chest impedance pneumogram. Non-contact estimates detected relevant changes in the vital-sign values between routine clinical observations. Pivotal respiratory events in a post-operative patient could be identified from the analysis of video-derived respiratory information. Continuous vital-sign monitoring supported by non-contact video camera estimates could be used to track early signs of physiological deterioration during post-operative care.

Fig. 1. Agreement between the reference heart rate values (computed from the ECG and PPG) and the camera estimates for the valid video camera data, comprising a total estimated time of approximately 103.0 h.

a The differences between the camera and reference monitors are normally distributed. b The Bland-Altman plot presents a minimal sensor bias. c The scatter plot shows high correlation between the two devices, with a Pearson correlation coefficient of 0.98. d The distribution of the mean values shows that most of the heart rate estimates are within the expected physiological range for adults.

Fig. 2. Agreement between the reference respiratory rate values (computed from the chest impedance pneumogram) and the camera estimates for the valid video camera data, comprising a total estimated time of approximately 119.8 h.

a The differences between the two estimates. b The Bland-Altman plot presents negligible sensor bias. c The plot shows a positive correlation between the two devices, with a Pearson correlation coefficient of 0.82. d The distribution of the mean values shows that most of the respiratory rate estimates are within the expected physiological range for adults. brpm = breaths/min.

Fig. 3. Reference and video camera-derived traces for two 30-min sample periods.

Heart rate and respiratory rate values for the first period are shown in panels (a) and (b), respectively. Heart rate and respiratory rate values for the second period are shown in panels (c) and (d), respectively. Reference values were provided by the patient monitoring equipment (black), and manual nursing estimates (green). Fluctuations in heart rate associated with patient movement can be observed in (a) from t = 21 min to t = 25 min. The second 30-min sample (c, d) documents a period of deterioration from t = 20 min. The camera system produced frequent observations of both vital signs in during the episode of clinical deterioration. Only one manual respiratory rate was recorded during this period, as the impedance system was disconnected.

Fig. 4. Respiratory monitoring of a patient over a 14-h period.

The video camera-derived estimates (in red) are compared against the reference values provided by the bedside monitoring equipment (in black). Periods during which the privacy blind was drawn, and thus video monitoring was interrupted due an obstructed camera view of the patient, are shown in the bottom panel. Sample frames from the video camera recording during invasive respiratory support and after extubation are shown as inserts at t = 22:15 and t = 09:15.

Fig. 5. Visualisation of the local respiratory effort of a patient later diagnosed as having a ruptured diaphragm, as confirmed by chest CT, and a right-sided pneumothorax.

(a) Respiratory rate estimated from the camera. Heat maps show the magnitude of the respiratory effort from regions on the chest during time stages (b) S1, (c) S2 and (d) S3. Each super-pixel in the heat maps was coloured according to the amplitude of the respiratory signal (in pixel units) computed from the 30 × 30 ROI centred on the super-pixel. The non-stationary respiratory rate was measured with a sustained increase in respiratory rate from t = 18 min to t = 27 min. A progression towards subdiaphragmatic left-lateral respiratory movements is observed as increased signal intensity over the lower left-side in the respiratory map for stage S3.

Fig. 6. Monitoring apparatus in the Churchill Hospital ICU in Oxford, United Kingdom.

a Patient monitor at the bedside and the non-contact vital-sign monitoring equipment at the end a patient’s bed. b Set-up of the trolley containing the non-contact vital-sign monitoring equipment. The components are shown in their respective positions—(A) optical camera; (B) infrared LED source; (C) thermal camera; (D) interface equipment; (E) workstation (in cabinet); (F) privacy blind (folded up).