The alveolar venous anesthetic gradient is determined by the amount of tissue uptake of an inhalation agent anesthetic . This depends on the tissue solubility, tissue blood flow and arterial-tissue partial pressure difference.
The brain has a high perfusion and equilibrates rapidly with the inhalation agent anesthetic , muscle has 1/20th perfusion of brain, so equilibration takes 20 times as long.
Fat-blood coefficients are higher, ranging from 2.3 for N20 to 61 for methoxyflurane; however, because of low perfusion in fat, the inhalation agent anesthetic concentration rises slowly.
The vessel-rich group (VRG) comprising the brain, heart, splanchnic bed, kidney and endocrine glands form 10% of the body weight (BW) but receive 75% of the cardiac output, 90% equilibration occurs in 4-8 minutes.
After this, the muscle and skin group (MG) with a lower perfusion equilibrates in 2-4 hours. Next, the fat group serves as an effective depot but equilibration ranges from 2—30 hours. Lastly, the vessel poor group consisting of ligaments, tendons, bone and cartilage have little perfusio, so poor in uptake.
Uptake in various tissues of inhalation agent anesthetic
8 minutes (VRG) – Vessel rich group
2—4 hours (MG) – Muscle group
2—30 hours (FG) – Fat group
Poor uptake (VPG) – Vessel poor group
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