Plasma lidocaine concentrations in conscious horses after cervicothoracic (stellate) ganglion block with 1% lidocaine HCl solution.

This study examines the levels of lidocaine, a local anesthetic, in the blood of horses after it was used to block signals in the cervicothoracic ganglion, a cluster of nerve cells. The research was conducted on 12 horses using different dosages for unilateral and bilateral blockades, measured at various intervals over a two-hour period.

Objective and Methodology

• The objective of the study was to identify arterial and venous plasma concentrations of lidocaine after its injection into the cervicothoracic ganglion (CTG), a cluster of nerve cells located in neck and upper chest region.
• 12 adult horses, nonmedicated, were taken into the study, their average body weight being 422 +/- 59 kg.
• Two different dosages of 1% lidocaine HCl solution were used; 2.9 +/- 0.5 mg/kg body weight for inducing unilateral CTG blockade in 8 horses and 4.8 +/- 0.8 mg for bilateral CTG blockade in 4 horses.

Procedure and Measurements

• Blood samples were collected before the injection, and at regular intervals after injection – 5, 15, 30, 45, 60, 75, 90, 105, and 120 minutes.
• Plasma lidocaine concentrations were determined using gas chromatography, an analytical method that separates and analyzes compounds.
• Symptoms of cervicothoracic sympathetic blockade, which confirms the successful delivering of lidocaine into the CTG, were marked by Horner’s syndrome and excessive sweating over the face, neck, and thoracic limbs of the horse.

Results

• The study found that the mean maximal venous concentrations of lidocaine reached 0.86 +/- 0.33 micrograms/ml in about 26 minutes after the unilateral CTG blockade and 1.14 +/- 0.25 micrograms/ml in roughly 31 minutes after bilateral CTG blockade.
• The mean venous and arterial concentrations of lidocaine were not significantly different after 45 and 120 minutes of injection.
• The venous concentrations of lidocaine were consistently higher than the arterial concentrations, as evidenced in two horses wherein right CTG and brachial plexus were temporarily anesthetized after the repeated administration of lidocaine into the right CTG.