Passage
When present in the bloodstream above a threshold partial pressure, a general anesthetic blocks the sensation of pain, induces a state of unconsciousness, and causes muscle paralysis. Many anesthetic agents are administered in their gaseous phase and are absorbed into the bloodstream through the lungs. The rate of diffusion V_gas of an anesthetic across the alveolar membrane with a surface area A and a thickness T is described by the Fick law of diffusion:
Vgas=DAΔPTEquation 1where D is the diffusion coefficient specific for the gas and ΔP is the partial pressure difference across the membrane.An inhaled anesthetic is described by its blood-gas partition coefficient, the ratio of the anesthetic's concentration in the blood to its concentration in the lungs when the partial pressures are equal. The concentration C of an anesthetic gas dissolved in blood is described by the Henry law of solubility:
C=kHPgasEquation 2where k_H is the solubility constant specific for the gas and P_gas is the partial pressure of the gas.Due to the paralytic effects of general anesthetics, mechanical ventilators are usually necessary to assist a patient's breathing during surgery. Positive-pressure ventilators are noninvasive and use external pumps to induce inspiration by forcing air into the lungs through a ventilation mask. Despite the paralytic effects of general anesthesia, patients are often capable of expiration without the active aid of a mechanical ventilator.
-Which of the following characteristics would describe an inhalational anesthetic that is present in a relatively low concentration in the blood when it takes effect?

Question

Passage
When present in the bloodstream above a threshold partial pressure, a general anesthetic blocks the sensation of pain, induces a state of unconsciousness, and causes muscle paralysis. Many anesthetic agents are administered in their gaseous phase and are absorbed into the bloodstream through the lungs. The rate of diffusion V_gas of an anesthetic across the alveolar membrane with a surface area A and a thickness T is described by the Fick law of diffusion:

Vgas=DAΔPTEquation 1where D is the diffusion coefficient specific for the gas and ΔP is the partial pressure difference across the membrane.An inhaled anesthetic is described by its blood-gas partition coefficient, the ratio of the anesthetic's concentration in the blood to its concentration in the lungs when the partial pressures are equal. The concentration C of an anesthetic gas dissolved in blood is described by the Henry law of solubility:

C=kHPgasEquation 2where k_H is the solubility constant specific for the gas and P_gas is the partial pressure of the gas.Due to the paralytic effects of general anesthetics, mechanical ventilators are usually necessary to assist a patient's breathing during surgery. Positive-pressure ventilators are noninvasive and use external pumps to induce inspiration by forcing air into the lungs through a ventilation mask. Despite the paralytic effects of general anesthesia, patients are often capable of expiration without the active aid of a mechanical ventilator.
-Which of the following characteristics would describe an inhalational anesthetic that is present in a relatively low concentration in the blood when it takes effect?

Quizplus · Accepted Answer

11ecba2d_12e0_ce54_a3bf_9154aa9c2f6a_MD0008_00 According to the Henry law of solubility (Equation 2), the concentration C of a gas dissolved in solution is the product of the solubility constant k_H (specific for the gas and solution) and the partial pressure of the gas P_gas in its gas phase: 11ecba2d_12e0_ce55_a3bf_4986dafe85ef_MD0008_00 According to the passage, an anesthetic takes effect when the threshold partial pressure is reached. Rearranging the above equation, the partial pressure of a dissolved gas is inversely proportional to its solubility: 11ecba2d_12e0_ce56_a3bf_d50013c260c8_MD0008_00 In other words, highly soluble anesthetics dissolve easily in the blood, and therefore higher concentrations are required to reach the target threshold partial pressure. In contrast, a poorly soluble gas will require a low concentration in the blood to take effect (Choice D). According to the passage, the blood-gas coefficient is the ratio of the anesthetic's concentration in the blood to its concentration in the lungs at equilibrium. If the concentration in the blood is low, the blood-gas coefficient will also be low. (Choices B and C) An inhalational anesthetic cannot have these combinations of characteristics; the blood-gas partition coefficient is directly proportional to the anesthetic's concentration in the blood, which is also directly proportional to its solubility. Therefore, a poorly soluble gas will have a low concentration in the blood and a low blood-gas partition coefficient; a highly soluble gas will have a high concentration in the blood and a high blood-gas partition coefficient. Educational objective: The concentration C of a gas dissolved in solution is described by the Henry law of solubility: C = k_HP_gas. Therefore, the partial pressure of the dissolved gas is inversely proportional to its solubility. In addition, the blood-gas partition coefficient is directly proportional to the concentration in the blood at equilibrium.

Passage When Present in the Bloodstream Above a Threshold Partial Pressure