An elderly client with a history of stable chronic obstructive pulmonary disease, alcohol abuse, and cirrhosis has a serum theophylline level of 25.8 mcg/mL (143 umol/L). Which clinical manifestation associated with theophylline toxicity should worry the nurse most?

Theophylline has narrow therapeutic index and plasma concentrations >20 mcg/mL (111 umol/L) are associated with theophylline drug toxicity. Toxicity can be acute or chronic. Conditions associated with chronic toxicity include advanced age (>60), drug interactions (eg, alcohol, macrolide and quinolone antibiotics), and liver disease. Acute toxicity is associated with intentional or accidental overdose. Symptoms of toxicity usually manifest as central nervous system stimulation (eg, headache, insomnia, seizures), gastrointestinal disturbances (eg, nausea, vomiting), and cardiac toxicity (eg, arrhythmia).