Agents used for anaesthesia are generally metabolized by liver and eliminated by kidney. Which of the following anaesthetic agent is NOT metabolized by body?

Nitrous oxide (N2O; laughing gas) is the only inorganic anesthetic gas in clinical use. There is no change with blood pressure, hea rate, systemic vascular resistance, and cardiac output. Respiratory tidal volume decreases but respiratory rate is increased. By increasing CBF and cerebral blood volume, nitrous oxide produces a mild elevation of intracranial pressure. Nitrous oxide also increases cerebral oxygen consumption (CMRO2). Nitrous oxide appears to decrease renal blood flow by increasing renal vascular resistance. This leads to a drop in glomerular filtration rate and urinary output. During emergence, almost all nitrous oxide is eliminated by exhalation. Ref: Morgan, Jr. G.E., Mikhail M.S., Murray M.J. (2006). Chapter 7. Inhalation Anesthetics. In G.E. Morgan, Jr., M.S. Mikhail, M.J. Murray (Eds), Clinical Anesthesiology, 4e.