A system integrated with a respiratory treatment device and configured to measure airway impedance, the system comprising: a conduit configured to provide a flow path for gas toward and away from an airway of a subject using the respiratory treatment device during inhalation and exhalation, respectively, the conduit including a first branch configured to communicate inhaled gas toward the airway of the subject and a second branch configured to communicate exhaled gas away from the airway of the subject to the ambient environment; a first valve disposed within the second branch of the conduit to affect gas flow through the flow path formed by the conduit such that gas exhaled by the subject is communicated via the second branch; a second valve disposed within the first branch of the conduit such that inhaled gas is communicated by the first branch and exhaled gas is prevented from being communicated by the first branch, the second valve comprising a one-way valve; one or more sensors disposed within the second branch of the conduit, or within the first valve, and configured to provide a signal conveying information associated with one or more characteristics of gas in the flow path formed by the conduit while the gas flow through the flow path is affected or unaffected by the first valve; and one or more processors configured to execute one or more computer program modules, the one or more computer program modules comprising: an airway impedance monitoring module configured to determine an impedance metric of the airway of the subject based on the signal provided by the one or more sensors during the one or more exhalations for which gas flow through the flow path is affected or unaffected by the first valve, wherein the first valve comprises an interrupter valve configured to affect gas flow through the flow path formed by the conduit by temporarily interrupting the gas flow through the flow path formed by the conduit during exhalation until a detected pressure of the exhaled gas becomes stable. a conduit configured to provide a flow path for gas toward and away from an airway of a subject using the respiratory treatment device during inhalation and exhalation, respectively, the conduit including a first branch configured to communicate inhaled gas toward the airway of the subject and a second branch configured to communicate exhaled gas away from the airway of the subject to the ambient environment; a first valve disposed within the second branch of the conduit to affect gas flow through the flow path formed by the conduit such that gas exhaled by the subject is communicated via the second branch; a second valve disposed within the first branch of the conduit such that inhaled gas is communicated by the first branch and exhaled gas is prevented from being communicated by the first branch, the second valve comprising a one-way valve; one or more sensors disposed within the second branch of the conduit, or within the first valve, and configured to provide a signal conveying information associated with one or more characteristics of gas in the flow path formed by the conduit while the gas flow through the flow path is affected or unaffected by the first valve; and one or more processors configured to execute one or more computer program modules, the one or more computer program modules comprising: an airway impedance monitoring module configured to determine an impedance metric of the airway of the subject based on the signal provided by the one or more sensors during the one or more exhalations for which gas flow through the flow path is affected or unaffected by the first valve, wherein the first valve comprises an interrupter valve configured to affect gas flow through the flow path formed by the conduit by temporarily interrupting the gas flow through the flow path formed by the conduit during exhalation until a detected pressure of the exhaled gas becomes stable. an airway impedance monitoring module configured to determine an impedance metric of the airway of the subject based on the signal provided by the one or more sensors during the one or more exhalations for which gas flow through the flow path is affected or unaffected by the first valve, wherein the first valve comprises an interrupter valve configured to affect gas flow through the flow path formed by the conduit by temporarily interrupting the gas flow through the flow path formed by the conduit during exhalation until a detected pressure of the exhaled gas becomes stable.