Non-contact heart rate monitoring utilizing camera photoplethysmography in the neonatal intensive care unit — A pilot study
Introduction
The heart rate (HR) is a critical parameter to assess the well-being of infants in the neonatal intensive care unit (NICU). Episodes of bradycardia occur frequently in premature infants due to a variety of causes, including immature respiratory development or sepsis [1], [2]. Current HR monitoring techniques include electrocardiography (ECG) and pulse oximetry based on photoplethysmography (PPG) [3]. These techniques are reliable and inexpensive. However, there are several disadvantages because both are contact devices using adhesive sensors. Placement and removal of the sensors and the attachment to wires cause discomfort, stress, pain and sometimes even epidermal stripping. The latter occurs particularly in infants < 27 weeks of gestational age, because in these infants the bond between the sensor and dermis is stronger than the epidermal–dermal junction [4], [5], [6]. The patches and wires appear on X-rays and complicate reading the film. Furthermore, the obtrusiveness of the wires impairs parent–child bonding, especially during kangaroo mother care. Anand and Scalzo suggested that pain, stress and maternal separation of NICU patients have a negative impact on cognitive development [7]. In addition, a study by Chen et al indicates that repetitive application and removal of patches, adversely affect the infant's well-being and developmental outcome [8]. In view of the above, a contactless HR monitoring technique would be highly desirable.
Recently, camera based PPG was demonstrated [9], [10], [11], [12], [13], [14], [15], [16], [17]. This novel technique is entirely contactless but uses the same principle as contact PPG, better known as pulse-oximetry. Blood absorbs light more than surrounding tissue so variations in blood volume affect light transmission and reflectance. While contact PPG predominantly uses transmissive mode, camera PPG is typically performed in reflective mode. The cardiovascular pulse waves cause changes in the volume of arterioles which result in minute pulsatile skin color changes. These color changes may be compared to blushing (more blood, redder skin), except that they are less intense. While invisible to the eye, these ‘micro-blushes’ can be detected with a camera.
The aim of this pilot study is to investigate the feasibility of camera based PPG for contactless HR monitoring in newborn infants in the NICU with ambient light. We did not use dedicated illumination as it might hinder the infant and would increase the experimental footprint in the NICU.
Section snippets
Study design
Infants were studied in the NICU in both the Children's Hospital of Orange County (CHOC), California, USA and in the Máxima Medical Center (MMC), The Netherlands. Institutional Review Board approval (CHOC#090768 and UCI#2009-7046, clinicaltrials.gov: NCT00989859, Philips Research-MMC #2010-075) and informed parental consent were obtained prior to measurements. Since the objective was to explore potential challenging conditions of the technique no exclusion criteria were defined and any infant
Results
A total of 19 infants with gestational ages ranging from 25 to 42 weeks, postnatal ages ranging from 3 days to 4 weeks, and weights from 470 to 3810 g were studied. Two were ventilated, one of which with HFOV. One infant had sepsis. No inotropics had been administered to the infants at the time of the measurement. Ambient light levels in the NICU varied from 20 to 180 lx. For comparison, typical office work floor light intensities range from 400 to 750 lx. During phototherapy to treat neonatal
Discussion
The most fundamental condition for camera based PPG to be feasible in the NICU is that the cardiovascular induced PPG signal has to be detectable in the infants at an arbitrary anatomical location. This means that non-cardiovascular related changes in skin optical reflectivity (such as caused by redistribution of venous blood, e.g. during sepsis or by repositioning a limb causing changes in hydrostatic pressure) have to be smaller than those induced by the cardio-vascular pulse wave. We could
Conflict of interest statement
The authors have no conflict of interest to declare.
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Current affiliation: Department of Pediatrics, Rijnstate Hospital, Wagnerlaan 55, 6800 TA Arnhem, The Netherlands.