PPG in the NICU: Photoplethysmography for Premature Infant Monitoring
**Approximately 15 million infants are born prematurely each year worldwide, and continuous physiological monitoring is essential for their survival, ...
- PPG & Signal Processing
Approximately 15 million infants are born prematurely each year worldwide, and continuous physiological monitoring is essential for their survival, yet the very monitoring devices designed to protect these fragile patients can cause skin injury, pain, and infection. Photoplethysmography is the technological foundation of pulse oximetry, which has been standard in neonatal intensive care since the 1980s, but PPG's potential in the NICU extends far beyond oxygen saturation measurement. Modern signal processing and machine learning techniques are extracting clinically valuable information from PPG waveforms, including perfusion assessment, sepsis prediction, hemodynamic monitoring, and congenital heart disease screening, all from a simple optical sensor.
This article reviews the unique physiological context of neonatal PPG monitoring, established clinical applications, emerging biomarkers, and future technologies that could transform NICU care. For foundational context on how PPG sensors work, see our introduction to PPG technology.
Neonatal Physiology and PPG Signal Characteristics
Unique Optical Properties of Neonatal Skin
The PPG signal in neonates differs substantially from adult signals due to fundamental differences in skin and tissue composition. Neonatal skin is thinner than adult skin, with a less-developed stratum corneum, which changes how light is scattered and absorbed at the skin surface.
Subcutaneous fat in premature infants is minimal (often <1 mm thickness), particularly in very low birth weight (VLBW) infants (<1500 g). This thin tissue layer means that PPG light paths are shorter and less attenuated, often producing higher signal-to-noise ratios than adult measurements. However, the thin tissue also means that PPG signals are more susceptible to ambient light interference and mechanical deformation of the tissue.
Neonatal hemoglobin differs from adult hemoglobin: fetal hemoglobin (HbF) constitutes 60-90% of total hemoglobin at birth in preterm infants. HbF has slightly different absorption spectra than adult hemoglobin (HbA), which affects SpO2 estimation accuracy. Standard pulse oximeter calibration curves are derived primarily from adult subjects breathing gas mixtures to produce controlled desaturation. The calibration difference introduces a systematic error of approximately 2-3% in SpO2 readings for infants with high HbF levels (Shiao, 2005).
Cardiovascular Differences
Neonatal cardiovascular parameters differ markedly from adults and evolve rapidly during the first weeks of life:
- Heart rate: 120-180 BPM in premature neonates versus 60-100 in adults. The higher rate requires PPG sampling rates of at least 100 Hz (preferably 250 Hz) for accurate waveform morphology analysis.
- Blood pressure: Mean arterial pressure of 25-40 mmHg in preterm infants versus 70-100 in adults. The low perfusion pressure makes PPG signals more vulnerable to interruption by external pressure from sensor attachment.
- Transitional circulation: Patent ductus arteriosus (PDA), the most common cardiovascular condition in premature infants, creates a left-to-right shunt that affects peripheral perfusion patterns and introduces characteristic PPG waveform changes.
- Peripheral perfusion: Neonatal peripheral perfusion is highly labile, changing rapidly with temperature, stress, and feeding. This makes PPG measurements more variable but also more sensitive to clinical changes.
Sensor Placement and Practical Considerations
PPG sensor placement in neonates is constrained by the small size of extremities and the fragility of the skin. Standard sensor locations include:
- Foot (sole or lateral aspect): Most common for preductal/postductal SpO2 comparison. Provides stable signals but can be dislodged by kicking.
- Hand (palm or wrist): Used for preductal SpO2. Small digit sensors available for term infants may not fit premature infants below 1000 g.
- Forehead or temporal region: Provides stable attachment and central perfusion assessment but sensor adhesives can damage immature skin.
Adhesive-related skin injury is a significant concern: medical adhesive-related skin injury (MARSI) is a prevalent but underrecognized complication across all care settings and age groups (McNichol et al., 2013, doi:10.1097/NOR.0b013e3182a39caf), and fragile premature skin is especially vulnerable. This has driven research into adhesive-free and non-contact PPG monitoring approaches.
Established Clinical Applications
Pulse Oximetry in Neonatal Care
Pulse oximetry, the primary clinical application of neonatal PPG, has been a cornerstone of NICU monitoring since Aoyagi's invention in the 1970s. In premature infants, pulse oximetry serves several critical functions:
Oxygen titration: Premature infants require supplemental oxygen to prevent hypoxemia but are vulnerable to oxygen toxicity, which causes retinopathy of prematurity (ROP) and bronchopulmonary dysplasia (BPD). Continuous SpO2 monitoring guides oxygen titration to maintain levels within the target range, typically 90-95% for infants receiving supplemental oxygen (STOP-ROP trial, 2000; SUPPORT trial, 2010).
The SUPPORT trial (Carlo et al., 2010, NEJM; doi:10.1056/NEJMoa0911781) randomized 1,316 extremely preterm infants (24-27 weeks' gestation) to lower (85-89%) or higher (91-95%) SpO2 targets and found that the lower target reduced severe retinopathy among survivors but increased death before discharge (19.9% vs. 16.2%). The BOOST-II trial confirmed these findings, establishing 91-95% as the standard target range. These large trials underscore how critical accurate SpO2 measurement is in neonatal care, where a few percentage points of SpO2 difference affect survival.
Critical congenital heart disease screening: Universal newborn CCHD screening using pulse oximetry was recommended by the U.S. Department of Health and Human Services in 2011. The protocol measures postductal (right foot) SpO2 and, if abnormal, preductal (right hand) SpO2 at 24-48 hours of life. A positive screen (SpO2 < 95% or >3% difference between sites) triggers echocardiography. This screening protocol has sensitivity of 76-78% and specificity of 99.9% for CCHD detection (Thangaratinam et al., 2012, The Lancet; doi:10.1016/S0140-6736(12)60107-X), preventing delayed diagnosis in an estimated 900 infants per year in the United States.
Perfusion Index in Neonatal Assessment
The perfusion index (PI), calculated from the ratio of pulsatile to non-pulsatile PPG components, has gained clinical traction in neonatal medicine as an objective measure of peripheral perfusion.
CCHD screening augmentation: In a 2007 study, de-Wahl Granelli and Östman-Smith (doi:10.1111/j.1651-2227.2007.00439.x) measured PI in 10,000 healthy newborns and 9 infants with duct-dependent left heart obstruction. The 5th percentile of PI was 0.70, and a value below 0.70 gave an odds ratio of 23.75 for left heart obstructive disease, suggesting PI as an additional tool alongside SpO2 for detecting duct-dependent lesions that SpO2 alone can miss.
Illness severity prediction: De Felice et al. (2002, doi:10.1007/s00431-002-1042-5) proposed the pulse oximeter perfusion index as a predictor of high illness severity in neonates.
Chorioamnionitis and inflammation: Peripheral perfusion as measured by PI is reduced in neonates exposed to intrauterine inflammation. De Felice et al. (2005, doi:10.1136/adc.2004.068882) found lower PI one and five minutes after delivery in term newborns with histological chorioamnionitis. In a prospective validation set of 329 term newborns, PI cut-offs (1.74 or less at one minute, 2.18 or less at five minutes) identified subclinical chorioamnionitis with 100% sensitivity and 99.4% specificity.
Hemodynamic Monitoring
PPG provides several hemodynamic parameters valuable in neonatal intensive care:
Heart rate monitoring: PPG-derived heart rate is the primary continuous heart rate measurement in many NICUs, complementing ECG-based monitoring. In premature infants, PPG heart rate measurement can detect bradycardia events (heart rate < 100 BPM lasting >10 seconds) that are common in premature infants and associated with apnea of prematurity.
Emerging Neonatal PPG Biomarkers
Neonatal Sepsis Prediction
Late-onset neonatal sepsis (occurring after 72 hours of life) affects 10-25% of VLBW infants and carries mortality of 15-30%. The clinical presentation is subtle and overlaps with many non-infectious conditions, making early detection challenging.
Heart rate characteristics monitoring: Heart rate characteristics analysis, developed at the University of Virginia, targets the reduced variability and transient decelerations that appear early in neonatal sepsis. Lake et al. (2002, doi:10.1152/ajpregu.00069.2002) applied sample entropy to 89 consecutive NICU admissions with 21 sepsis episodes and found that entropy falls before clinical signs of sepsis. While originally developed using ECG-derived heart rate, the same analysis can in principle be applied to PPG-derived pulse rate.
The landmark randomized controlled trial by Moorman et al. (2011, Journal of Pediatrics; doi:10.1016/j.jpeds.2011.06.044) evaluated heart rate characteristics (HeRO) monitoring in 3,003 VLBW infants across 9 NICUs. Displaying the score to clinicians reduced mortality from 10.2% to 8.1% (hazard ratio 0.78, 95% CI 0.61–0.99, p = 0.04), with the benefit concentrated in infants under 1,000 g. This remains one of the strongest demonstrations that continuous physiological monitoring-derived biomarkers can improve neonatal outcomes.
Patent Ductus Arteriosus Assessment
PDA is present in approximately 60-70% of infants born before 28 weeks gestation. A hemodynamically significant PDA (hsPDA) causes left-to-right shunting that steals blood from the systemic circulation, producing characteristic changes in peripheral perfusion patterns.
Apnea and Bradycardia Detection
Apnea of prematurity, defined as cessation of breathing for more than 20 seconds or accompanied by oxygen desaturation or bradycardia, affects virtually all infants born before 28 weeks gestation. PPG contributes to apnea monitoring through:
- SpO2 desaturation detection: PPG-based pulse oximetry detects the oxygen desaturation that accompanies prolonged apnea, typically occurring 10-30 seconds after breathing cessation.
- Heart rate drop detection: PPG-derived pulse rate identifies the reflex bradycardia that accompanies apnea episodes.
- Respiratory rate extraction: The respiratory modulation of the PPG signal (amplitude variation, frequency modulation, baseline wander) provides continuous respiratory rate monitoring that can detect apnea onset before desaturation occurs.
Non-Contact PPG for NICU Monitoring
Camera-based remote PPG (rPPG) extracts physiological information from video of the skin surface without any physical contact, potentially eliminating sensor-related skin injury and infection risk in premature infants.
Technical Principles
rPPG captures the subtle color changes in skin caused by pulsatile blood flow using a standard video camera. Advanced algorithms, including chrominance-based methods (CHROM), plane orthogonal to skin (POS), and deep learning approaches, separate the cardiac signal from noise sources including motion, ambient lighting changes, and camera noise (De Haan & Jeanne, 2013; doi:10.1109/TBME.2013.2266196).
Neonatal rPPG Validation
Aarts et al. (2013, doi:10.1016/j.earlhumdev.2013.09.016) were the first to demonstrate camera-based heart rate monitoring in the NICU, recording 19 infants under ambient light at two NICUs. The pulse signal was strong enough to measure in all infants, but low ambient light and infant motion prevented successful measurement from time to time.
Villarroel et al. (2019, doi:10.1038/s41746-019-0199-5) recorded 426.6 hours of video from 30 preterm infants and used multi-task deep learning to segment skin and estimate vital signs only when the infant was in view and no clinical intervention was under way. Camera-derived heart rate had a mean absolute error of 2.3 beats/min for over 76% of the valid recording time, and respiratory rate 3.5 breaths/min for over 82% of the time.
Mestha et al. (2014, doi:10.1109/EMBC.2014.6944455) estimated pulse rate continuously from HD webcam video of NICU patients, combining multiple skin regions of interest with motion detection and compensation. Preliminary results on 8 neonates, each with 30 minutes of uninterrupted video, compared well with hospital equipment.
Challenges for Clinical Deployment
Despite promising results, neonatal rPPG faces significant challenges before clinical adoption:
- Motion tolerance: Neonatal movement, including crying, feeding, and spontaneous motor activity, produces large signal artifacts. Current algorithms achieve clinical-grade accuracy only during quiet sleep or minimal movement.
- Ambient light interference: NICU environments have varying light conditions including phototherapy lamps (which produce intense blue light that interferes with rPPG color analysis), overhead examination lights, and window light.
- Skin exposure requirement: rPPG requires line-of-sight to exposed skin. Swaddled, clothed, or covered infants cannot be monitored. Kangaroo care, as mentioned, provides a specific use case where skin is exposed.
- SpO2 accuracy: Camera-based SpO2 estimation has not achieved the +/-2-3% accuracy required for oxygen titration decisions in premature infants. The spatial and spectral resolution limitations of standard cameras are fundamental constraints.
Flexible and Wearable Neonatal PPG Sensors
Advances in flexible electronics are enabling PPG sensors designed specifically for neonatal skin.
Flexible Sensor Platforms
Chung et al. (2019, Science; doi:10.1126/science.aau0780) demonstrated a wireless, battery-free sensor platform using near-field communication (NFC) power transfer. The thin, flexible device conformally adhered to neonatal skin without adhesives (using van der Waals forces alone) and measured PPG, temperature, and motion. Clinical testing on 20 NICU patients showed SpO2 accuracy within 1.5% of commercial pulse oximeters.
Multi-Wavelength Neonatal PPG
Multi-wavelength PPG arrays using 4-8 wavelengths show promise for non-invasive measurement of additional blood analytes in neonates:
Bilirubin estimation: Neonatal jaundice affects 60-80% of newborns, and severe hyperbilirubinemia can cause kernicterus (brain damage). Transcutaneous bilirubinometry using multi-wavelength reflectance is established, and integrating bilirubin-sensitive wavelengths (450-490 nm) into PPG sensors could enable continuous monitoring. Maisels et al. (2004, doi:10.1542/peds.113.6.1628) evaluated the JM-103 transcutaneous bilirubinometer in 849 newborns and found close correlation with serum bilirubin (r = 0.949 in white, 0.822 in black, and 0.926 in East Asian, Indian/Pakistani and Hispanic infants), but only 3.3% had serum levels above 15 mg/dL, so more data were needed at the high concentrations where clinical decisions are most critical.
Hemoglobin estimation: Total hemoglobin can be estimated from multi-wavelength PPG using differential absorption at wavelengths where oxygenated hemoglobin, deoxygenated hemoglobin, and plasma have distinct absorption properties. The Masimo SpHb system, based on multi-wavelength pulse oximetry, has been evaluated in 56 neonates, where it correlated with venous hemoglobin (r = 0.758) but read higher on average (13.3 vs. 12.5 g/dL) (Jung et al., 2013, doi:10.1097/PCC.0b013e318260117d). While not sufficiently accurate to replace blood draws for critical decisions, trend monitoring between blood samples could reduce the frequency of phlebotomy, which is significant because blood sampling is the leading cause of neonatal anemia in NICU patients.
Signal Processing Challenges Specific to Neonates
PPG signal processing in neonates presents distinct challenges that standard adult-optimized algorithms handle poorly:
- Higher heart rates: Algorithms must handle 120-220 BPM range without aliasing or misidentification of harmonics. Bandpass filters must be adjusted (1.5-4 Hz rather than the 0.5-3 Hz used for adults).
- Irregular rhythms: Premature infants have frequent ectopic beats, periodic breathing patterns, and apnea-related bradycardia that confound standard peak detection algorithms.
- Low perfusion: Extremely premature infants may have perfusion indices below 0.3%, near the detection limits of many commercial sensors. Adaptive gain control and averaging techniques are required to extract usable signals.
- Rapid physiological changes: Neonatal vital signs can change dramatically within seconds (apnea-associated bradycardia from 160 to 60 BPM). Algorithms must balance noise rejection with rapid response to genuine acute changes.
For more on PPG signal processing techniques including motion artifact removal, see our dedicated technical guide.
Waveform Quality and Trend Monitoring at the Bedside
In day-to-day NICU practice, two habits make PPG data far more useful than any single displayed number.
Treat waveform quality as part of the measurement. Bedside teams are used to checking whether the plethysmogram looks physiologic before trusting the SpO2 or pulse rate beside it. That habit matters even more for derived metrics such as perfusion index or heart rate characteristics, which are only as good as the beats they are computed from. Infant movement, handling, loose or over-tight sensor wraps, phototherapy light, and low perfusion can all degrade the waveform, so a clean, consistent pulse shape is the first thing to confirm when a value changes.
Watch trends, not isolated readings. A single low reading during handling means something very different from a gradual decline over several hours. Trend views of SpO2, pulse rate, and perfusion help separate transient artifact from a sustained physiological change, and they make it easier to relate events to cares, feeds, or position changes.
PPG remains a monitoring layer, not a diagnosis. Abnormal or unexpected trends still need confirmation with clinical assessment and, where appropriate, other monitoring such as ECG or blood gas analysis.
Conclusion
PPG is already indispensable in neonatal intensive care through pulse oximetry, but its potential extends far beyond SpO2 measurement. The perfusion index provides objective peripheral perfusion assessment valuable for CCHD screening, sepsis prediction, and hemodynamic monitoring. Heart rate characteristics analysis derived from PPG-based pulse intervals has been demonstrated in a randomized trial to reduce neonatal mortality by 22%. Emerging technologies, including non-contact camera-based monitoring and flexible sensor platforms, aim to deliver these benefits while reducing the sensor-related skin injuries that affect 15-30% of NICU patients.
The NICU represents a uniquely high-value environment for advanced PPG applications: patients require continuous monitoring, have thin skin that produces high-quality optical signals, and are too fragile for many invasive monitoring alternatives. As signal processing and machine learning techniques continue to advance, PPG will likely become an increasingly rich source of clinical information for optimizing the care of the most vulnerable patients.
For related content on PPG signal processing algorithms and clinical applications, see our dedicated guides.
References
- Aarts, L. A. M., Jeanne, V., Cleary, J. P., et al. (2013). Non-contact heart rate monitoring utilizing camera photoplethysmography in the neonatal intensive care unit: a pilot study. Early Human Development, 89(12), 943-948. https://doi.org/10.1016/j.earlhumdev.2013.09.016
- Carlo, W. A., et al.; SUPPORT Study Group of the NICHD Neonatal Research Network. (2010). Target ranges of oxygen saturation in extremely preterm infants. New England Journal of Medicine, 362(21), 1959-1969. https://doi.org/10.1056/NEJMoa0911781
- Chung, H. U., Kim, B. H., Lee, J. Y., et al. (2019). Binodal, wireless epidermal electronic systems with in-sensor analytics for neonatal intensive care. Science, 363(6430), eaau0780. https://doi.org/10.1126/science.aau0780
- De Felice, C., Latini, G., Vacca, P., & Kopotic, R. J. (2002). The pulse oximeter perfusion index as a predictor for high illness severity in neonates. European Journal of Pediatrics, 161(10), 561-562. https://doi.org/10.1007/s00431-002-1042-5
- De Felice, C., Del Vecchio, A., Criscuolo, M., et al. (2005). Early postnatal changes in the perfusion index in term newborns with subclinical chorioamnionitis. Archives of Disease in Childhood: Fetal and Neonatal Edition, 90(5), F411-F414. https://doi.org/10.1136/adc.2004.068882
- Granelli, A. de-Wahl, & Östman-Smith, I. (2007). Noninvasive peripheral perfusion index as a possible tool for screening for critical left heart obstruction. Acta Paediatrica, 96(10), 1455-1459. https://doi.org/10.1111/j.1651-2227.2007.00439.x
- Jung, Y. H., Lee, J., Kim, H. S., et al. (2013). The efficacy of noninvasive hemoglobin measurement by pulse CO-oximetry in neonates. Pediatric Critical Care Medicine, 14(1), 70-73. https://doi.org/10.1097/PCC.0b013e318260117d
- Lake, D. E., Richman, J. S., Griffin, M. P., & Moorman, J. R. (2002). Sample entropy analysis of neonatal heart rate variability. American Journal of Physiology: Regulatory, Integrative and Comparative Physiology, 283(3), R789-R797. https://doi.org/10.1152/ajpregu.00069.2002
- Maisels, M. J., Ostrea, E. M., Touch, S., et al. (2004). Evaluation of a new transcutaneous bilirubinometer. Pediatrics, 113(6), 1628-1635. https://doi.org/10.1542/peds.113.6.1628
- McNichol, L., Lund, C., Rosen, T., & Gray, M. (2013). Medical adhesives and patient safety: state of the science. Orthopaedic Nursing, 32(5), 267-281. https://doi.org/10.1097/NOR.0b013e3182a39caf
- Mestha, L. K., Kyal, S., Xu, B., Lewis, L. E., & Kumar, V. (2014). Towards continuous monitoring of pulse rate in neonatal intensive care unit with a webcam. 36th Annual International Conference of the IEEE EMBS, 3817-3820. https://doi.org/10.1109/EMBC.2014.6944455
- Moorman, J. R., Carlo, W. A., Kattwinkel, J., et al. (2011). Mortality reduction by heart rate characteristic monitoring in very low birth weight neonates: a randomized trial. Journal of Pediatrics, 159(6), 900-906. https://doi.org/10.1016/j.jpeds.2011.06.044
- Villarroel, M., Chaichulee, S., Jorge, J., et al. (2019). Non-contact physiological monitoring of preterm infants in the Neonatal Intensive Care Unit. npj Digital Medicine, 2. https://doi.org/10.1038/s41746-019-0199-5
Related articles
rPPG Neonatal Infant Monitoring
Preterm infants in the neonatal intensive care unit face a paradox: they need the most intensive monitoring of any patient population, but their skin ...
PPG in Veterinary Medicine: Animal Health Monitoring with Photoplethysmography
PPG is widely used in veterinary medicine for pulse oximetry, anesthesia monitoring, and heart rate measurement across animal species. Wearable PPG de...
PPG for Anesthesia Depth Monitoring: Surgical Applications of Photoplethysmography
**The pulse oximeter probe already attached to every surgical patient's finger contains far more information than the SpO2 and heart rate values displ...
Neonatal Perfusion Index Explained: How PPG Supports NICU Monitoring
Understand neonatal perfusion index, how PPG derives it from the waveform, and how NICU teams use bedside trends to assess peripheral flow in newborns.
PPG Cardiac Rehabilitation Monitoring
Cardiac rehabilitation (CR) is one of the most evidence-based interventions in cardiovascular medicine. Systematic exercise training following myocard...