Systolic Blood Pressure in Anesthetized Bitches Measured with Photoplethysmography
Authors: Vaccarin Charline , Gasparotto Jean Carlos , Vargas Daniel , Libardoni Roberta , Gutierres Velasquez Omar , Teixeira da Silva Cristiana , Jesser Segat Hecson , Lopes Carlize , Curvello de Mendonça Müller Daniel , Vasconcellos Soares André 
Published: December 2024

Background:  Photoplethysmography is widely used in human medicine, with few studies on its use in veterinary medicine. Its sensor detects fluctuations in blood volume at the site, providing direct readings of cardiac pulse and peripheral oxygen saturation, as well as estimating cardiac output, respiratory rate and blood pressure. This study aimed to evaluate the use of photoplethysmography and compare it to vascular Doppler ultrasound as an indirect method of measuring systolic blood pressure in bitches undergoing elective ovariohysterectomy, using the invasive assessment of systolic blood pressure as a reference.
Materials, Methods and Results: After clinical and laboratory evaluation, 34 healthy bitches were selected to undergo elective ovariohysterectomy. After food and water fasting, patients received pethidine hydrochloride intramuscularly as pre-anesthetic medication, followed by anesthetic induction with fentanyl citrate and propofol intravenously. General anesthesia was maintained by inhalation with isoflurane diluted in 100% oxygen. Intraoperative analgesia consisted of continuous infusion of fentanyl citrate intravenously. The animals were randomly divided into 2 groups, the thoracic limb group (TLG) and the pelvic limb group (PLG). In each patient, non-invasive blood pressure measurement was obtained simultaneously with Doppler (DOP) and photoplethysmography (PPG). The sensors of both devices were placed on the end of the same limb. The PPG sensor was positioned in the interdigital region. In patients belonging to the TLG, the Doppler sensor was placed in the ventral region of the thoracic limb, under the ulnar artery. In PLG patients, the Doppler sensor was placed in the dorsal region of the pelvic limb, over the dorsal artery of the foot. The sphygmomanometer was positioned close to the sensors. For SBP measurement, the cuff was inflated until the Doppler sound signal and the plethysmographic wave were lost. The cuff was then deflated until the Doppler pulse sound resumed and the photoplethysmography showed at least 2 continuous waves on a regular basis. The corresponding pressure value observed on the manometer consisted of the systolic blood pressure (SBP). The same 2 evaluators performed all SBP measurements: 1 responsible for the DOP method and the other for the PPG method; both were blind to the other's findings, thus minimizing potential bias in the results. All animals underwent cannulation of the auricular artery for invasive measurement of systolic blood pressure, using a multiparameter monitor. All blood pressure measurements were performed at 5-min intervals, as well as obtaining additional parameters (heart and respiratory rate, esophageal temperature, partial tissue oxygen saturation, carbon dioxide concentration) and electrocardiographic monitoring. All parameters were documented for further statistical analysis. A strong correlation (r² = 0.95) was obtained between the DOP and PPG methods regardless of the limb on which the sensors were placed. There was a low correlation between the invasive method of measuring systolic blood pressure and the other methods. There was better agreement between the DOP and PPG methods (r2 = -0.0061; P = 0.85) when systolic blood pressure was measured in the TLG.
Discussion: In the PLG, the values ​​obtained with the DOP and PPG methods were significantly higher than those obtained with the invasive method, while the values​​obtained in the TLG differed slightly. It was found that the best measurement site by non-invasive methods was the thoracic limb. It was concluded that the non-invasive methods showed a low correlation with the invasive method; however, both methods had similar characteristics and photoplethysmography can be used to replace the vascular Doppler method.
Keywords: vascular Doppler, photoplethysmographic wave, plethysmography, pulse oximetry, veterinary medicine.