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 devices for pets, equine performance monitoring systems, and precision livestock farming applications extend PPG's reach from the veterinary clinic to farms, stables, and pet homes. Animal physiology presents unique challenges — from coat interference and pigmentation effects to species-specific heart rate ranges — that require tailored solutions.

Veterinary PPG: Core Applications

Pulse Oximetry in Anesthetized Animals

Pulse oximetry via PPG is a standard of care in veterinary anesthesia for dogs, cats, horses, and large animals. The Veterinary Anesthesia and Analgesia Society guidelines recommend continuous SpO2 monitoring for all anesthetized patients. PPG provides both SpO2 and heart rate from a single non-invasive sensor.

Sensor placement by species:

Dogs and cats:

• Tongue (highest signal quality when accessible during anesthesia)
• Toe webbing (convenient, minimal pigmentation issues in most breeds)
• Ear pinna (inner surface, hairless area)
• Vulva or prepuce (recumbent patients)
• Lip fold

Horses:

• Lip, tongue, or gum during general anesthesia
• Nasal septum for sedated standing procedures
• External ear canal during field procedures

Exotic species (birds, reptiles):

• Birds: metatarsal web, toenail base, cloaca
• Reptiles: cloaca, esophagus, or tail base (melanistic skin significantly impairs signal)
• Small mammals (rabbits, ferrets): ear pinna is standard

Calibration differences across species: Commercial pulse oximeters are calibrated using human hemoglobin dissociation curves. Animal hemoglobin oxygen-binding properties differ:

• Cats have trichromatic hemoglobin variants that shift the PPG-SpO2 relationship
• Ruminants (cattle, sheep) have fetal-type hemoglobin in some age groups
• Birds and reptiles have nucleated red blood cells that scatter light differently

These differences cause systematic SpO2 bias in standard human-calibrated veterinary devices — typically 1-4% underestimation in cats and variable errors in exotic species. Species-specific calibration algorithms are available in some veterinary-specific pulse oximeters.

Equine Cardiovascular Monitoring

Resting Heart Rate and HRV in Horses

Horses are exceptional cardiovascular athletes. Resting heart rate (28-44 bpm) is lower than humans, and peak exercise heart rate (210-240 bpm) is substantially higher. The massive equine vagal tone at rest — reflected in high HRV — dramatically reduces with sympathetic activation at the onset of exercise.

Equine HRV reference ranges:

• SDNN (resting): 60-120 ms (much higher than human 30-80 ms)
• RMSSD (resting): 50-100 ms
• LF/HF ratio (resting): 0.3-0.8 (low, reflecting parasympathetic dominance)

PPG sensor placement for resting equine HRV uses clip sensors on the ear pinna or nasal mucosa. Adhesive sensors on the chest wall (cardiac apex area, hairclipped) provide good quality during stable standing.

Equine Performance Monitoring

Thoroughbred racing and endurance equestrian sports use heart rate monitoring for training load assessment, recovery monitoring, and competition pacing:

Training zones in horses:

• Zone 1 (aerobic base): HR < 150 bpm
• Zone 2 (aerobic training): 150-170 bpm
• Zone 3 (lactate threshold): 170-190 bpm
• Zone 4 (VO2max): 190-210 bpm
• Zone 5 (sprint/race effort): > 210 bpm

PPG-based wearable systems (Polar Equine, Bontrager Equine HR systems) provide real-time heart rate during gallops, enabling trainers to quantify physiological load. Post-exercise HRV recovery rate — time for HR to return from >180 bpm to <80 bpm — tracks fitness adaptation across training cycles.

Exercise-Induced Pulmonary Hemorrhage (EIPH)

EIPH (bleeding from pulmonary capillaries during strenuous exercise) affects 60-80% of racehorses. PPG monitoring during and after high-intensity exercise cannot directly detect pulmonary hemorrhage, but SpO2 reduction during peak exercise (SpO2 < 92% in horses vs. normal > 97%) suggests significant ventilation-perfusion mismatch that may indicate pulmonary pathology including EIPH.

Continuous SpO2 logging with synchronized GPS and heart rate data during training gallops provides a comprehensive exercise physiology dataset for identifying horses with clinical or subclinical EIPH.

Precision Livestock Farming

Cattle Monitoring

Dairy and beef cattle monitoring using wearable sensors (neck collars, ear tags, leg bands) incorporates accelerometry, temperature, and PPG for health management:

Estrus detection: Estrus (ovulation) produces physiological changes detectable via PPG — elevated heart rate, increased peripheral perfusion, and HRV changes reflecting the hormonal surge. PPG-based estrus detection in cattle achieves 85-90% sensitivity, comparable to activity-based detection but with the additional information from autonomic state.

Bovine respiratory disease (BRD): BRD is the most economically significant disease in beef cattle, costing >$800 million annually in the US. Early detection is critical for treatment efficacy. Elevated resting heart rate (>80 bpm in adult cattle; normal: 50-80 bpm) and reduced HRV are early BRD indicators. Wearable ear-tag PPG that alerts to heart rate elevation triggers early examination before respiratory distress is clinically obvious.

Stress during handling and transport: Cattle stress responses during gathering, loading, transport, and unprocessing produce acute autonomic activation measurable via PPG. Objective stress quantification enables welfare assessment and facility design improvement that is currently based on subjective behavioral scoring.

Swine Monitoring

Pigs are physiologically similar to humans, making them valuable biomedical research models. In commercial pork production:

• Gestation monitoring: Sow heart rate and HRV during late gestation predicts farrowing timing and may identify sows at risk for dystocia
• Post-weaning stress syndrome: Piglet HRV monitoring in the first week post-weaning tracks adaptation to the stress of maternal separation and diet change
• Transport stress: Pre-slaughter transport stress is associated with meat quality defects (PSE — pale, soft, exudative pork). PPG heart rate monitoring during transport provides welfare and product quality data simultaneously

Poultry

Poultry pulse oximetry and heart rate measurement require contact sensors that tolerate wing and body movement. Heart rate in broiler chickens at rest is 250-350 bpm — too fast for standard clinical PPG algorithms. Specialized algorithms with wider bandpass filters (up to 8 Hz fundamental) accommodate poultry cardiac rates.

Thermal imaging combined with facial PPG provides remote (non-contact) heart rate estimation in poultry flocks during health surveys, avoiding the handling stress that confounds measurements.

Pet Wearable Health Monitoring

Consumer Pet Wearables

The pet wearable market has grown substantially. Devices include GPS trackers with activity monitors, but true physiological PPG monitoring remains technically challenging for several reasons:

Coat interference: Dog and cat fur significantly attenuates light transmission. Standard surface-contact PPG requires sensor placement on hairless areas (paw pads, ear pinnae, mucosal surfaces) that are impractical for continuous wear. Longer-wavelength NIR light (>900 nm) penetrates further through fur than visible red light, partially addressing this limitation.

Motion artifact: Pets move more erratically than humans wearing wrist sensors. PPG artifact rejection algorithms trained on human movement patterns perform poorly on dog or cat activity profiles.

Species-specific heart rate ranges:

• Dogs: 60-160 bpm depending on size (small dogs up to 180 bpm)
• Cats: 140-220 bpm at rest
• Rabbits: 180-250 bpm

Consumer-grade wearable PPG algorithms assume heart rate in the 40-200 bpm range — inadequate for cats.

Commercial products with PPG: The Whistle Smart Collar (dogs) incorporates an optical PPG sensor on the inner collar surface for resting HR measurement. Fi Smart Collar includes activity monitoring without PPG. True continuous physiological grade PPG in pet wearables remains aspirational for most species as of 2026.

Veterinary Clinical PPG Applications

In clinical settings, PPG complements other monitoring:

Sedated procedure monitoring: Many veterinary procedures (dental cleaning, radiographs, minor surgeries) use sedation. Continuous SpO2 and HR monitoring during sedation is a patient safety standard. Clip sensors on ear pinna or toe are standard.

Chronic disease monitoring: Dogs with congestive heart failure, cardiac arrhythmias, or pulmonary hypertension benefit from periodic PPG assessment. Home monitoring wristwatch-equivalent products for dogs are in development.

Post-surgical recovery: ICU-equivalent monitoring in veterinary hospitals uses PPG alongside ECG, blood pressure, and temperature for continuous critical patient monitoring.

Regulatory and Validation Considerations

Veterinary medical devices in the US are regulated as Class I or Class II devices by the FDA or fall outside FDA jurisdiction as commercial animal products. Unlike human medical devices, mandatory clinical validation requirements are less stringent for veterinary PPG devices.

This regulatory difference means many veterinary PPG devices lack systematic accuracy validation. Published veterinary pulse oximetry accuracy studies show large variability between devices and species combinations. When selecting veterinary PPG equipment, prioritizing devices with published peer-reviewed accuracy data in the target species is important.

FAQ

Why is cat PPG harder than dog PPG? Cats have much higher resting heart rates (140-220 bpm) than dogs, requiring wider-bandwidth PPG algorithms. Cats also resist sensor application more consistently than most dogs. The combination of motion artifact from resistance behaviors and high heart rate makes continuous PPG challenging in conscious cats. Sedated cats, where tongue or mucosa sensors can be applied, provide excellent signal quality.

Can you use a human pulse oximeter on a dog? Human pulse oximeters will display SpO2 and heart rate values for dogs, but the accuracy is not validated. Calibration curves derived from human hemoglobin absorption properties introduce systematic bias for canine hemoglobin. Heart rate range limitations (some human devices alarm below 40 bpm, which is normal for large-breed dogs) may also cause false alarms. Veterinary-specific devices with validated calibration are preferred for clinical use.

What is the normal SpO2 for a horse? Healthy awake horses have SpO2 of 97-100% at rest. During peak athletic effort, SpO2 can decrease to 85-92% in elite Thoroughbreds due to diffusion limitation at extreme cardiac outputs. SpO2 below 95% in an awake resting horse is abnormal and indicates respiratory compromise or equipment artifact.

How do you measure heart rate in a fish? PPG in fish (used in aquaculture research) uses transmitted light through caudal fin or gill arches. Fish heart rates are slow (40-120 bpm in many species) and temperature-dependent. Immersion sensor systems with the fish in a measurement chamber are the standard approach for research use. Continuous wearable PPG for fish remains experimental.

Can PPG detect pain in animals? Animals cannot self-report pain intensity. Autonomic pain responses — elevated heart rate, reduced HRV, increased peripheral vasoconstriction reducing PI — correlate with behavioral pain indicators in veterinary pain scales. PPG-derived pain indices have been validated in research settings for dogs and horses, providing objective pain assessment that supplements behavioral scoring.

Are there species differences in the PPG waveform shape? Yes. Ruminants (cattle, sheep, goats) have distinctly biphasic PPG waveforms with prominent dicrotic notch due to their unique cardiovascular anatomy. Horses show prominent respiratory sinus arrhythmia detectable in PPG. Birds and reptiles have markedly different waveform morphology due to different cardiac anatomy and physiology, making direct application of mammalian analysis algorithms problematic.

References

1. Mathews, K., Kronen, P.W., Lascelles, D., Nolan, A., Robertson, S., Steagall, P.V.M., & Yamashita, K. (2014). Guidelines for recognition, assessment and treatment of pain. Journal of Small Animal Practice, 55(6), E10-E68. doi:10.1111/jsap.12200

2. Evans, D.L., & Rose, R.J. (1988). Cardiovascular and respiratory responses to exercise in Thoroughbred horses. Journal of Experimental Biology, 134(1), 397-408. doi:10.1242/jeb.134.1.397

3. Muir, W.W., & Hubbell, J.A.E. (2009). Equine Anesthesia: Monitoring and Emergency Therapy (2nd ed.). Saunders Elsevier. doi:10.1016/B978-1-4160-5207-4.00001-2

4. Maatje, K., Loeffler, S.H., & Engel, B. (1997). Predicting optimal time of insemination in cows that show visual signs of estrus by estimating onset of estrus with pedometers. Journal of Dairy Science, 80(6), 1098-1105. doi:10.3168/jds.S0022-0302(97)76037-8