An apparatus which interfaces with ventilatory assist devices (ventilators) determining respiratory system resistance, such apparatus associating for change in patient-generated pressure (Pmus) during the perturbation, comprising: a flowmeter, with associated electronic circuitry, that measures the flow rate and volume of gas received by a patient; a pressure sensor that measures pressure near the airway of the patient (Paw); and electronic circuitry which receives the Paw, V and V signals from above mentioned circuitry and which is also connected to the control system of the ventilator, comprising: - circuitry that generates an output that results in a step decrease (negative pulse) in the pressure and/or flow output of the ventilator during selected inflation cycles; - circuitry that measures Paw, V and V at a point (T 0) near the beginning of the pulse (Paw 0, V̇ 0, V 0), at a point (T 1) near the trough of the negative pulse (Paw 1, V̇ 1, V 1) and at a point (T -1); preceding T 0 but after the onset of inspiratory effort (Paw -1, V̇ -1, V 1) and characterized by: circuitry to calculate the value of resistance from the equation: R = [ Paw 0 − Paw 1 + ΔPmus − E V 0 − V 1 / V ̇ 0 − V ̇ − 1 where E is the respiratory system elastance and ΔPmus is the change of Pmus between T 0 and T 1 (Pmus 0 - Pmus 1) as estimated by extrapolation from data obtained at T -1 and T 0 and determined from the equation: ΔPmus = ΔT 1 E V 0 − V − 1 + R V ̇ 0 − V ̇ − 1 − Paw 0 + Paw − 1 / Δt − 1 where ΔT 1 is the time difference between T 1 and T 0 and Δt -1 is the time different between T 0 and T -1. a flowmeter, with associated electronic circuitry, that measures the flow rate and volume of gas received by a patient; a pressure sensor that measures pressure near the airway of the patient (Paw); and electronic circuitry which receives the Paw, V and V signals from above mentioned circuitry and which is also connected to the control system of the ventilator, comprising: - circuitry that generates an output that results in a step decrease (negative pulse) in the pressure and/or flow output of the ventilator during selected inflation cycles; - circuitry that measures Paw, V and V at a point (T 0) near the beginning of the pulse (Paw 0, V̇ 0, V 0), at a point (T 1) near the trough of the negative pulse (Paw 1, V̇ 1, V 1) and at a point (T -1); preceding T 0 but after the onset of inspiratory effort (Paw -1, V̇ -1, V 1) and characterized by: circuitry to calculate the value of resistance from the equation: R = [ Paw 0 − Paw 1 + ΔPmus − E V 0 − V 1 / V ̇ 0 − V ̇ − 1 where E is the respiratory system elastance and ΔPmus is the change of Pmus between T 0 and T 1 (Pmus 0 - Pmus 1) as estimated by extrapolation from data obtained at T -1 and T 0 and determined from the equation: ΔPmus = ΔT 1 E V 0 − V − 1 + R V ̇ 0 − V ̇ − 1 − Paw 0 + Paw − 1 / Δt − 1 where ΔT 1 is the time difference between T 1 and T 0 and Δt -1 is the time different between T 0 and T -1. - circuitry that generates an output that results in a step decrease (negative pulse) in the pressure and/or flow output of the ventilator during selected inflation cycles; - circuitry that measures Paw, V and V at a point (T 0) near the beginning of the pulse (Paw 0, V̇ 0, V 0), at a point (T 1) near the trough of the negative pulse (Paw 1, V̇ 1, V 1) and at a point (T -1); preceding T 0 but after the onset of inspiratory effort (Paw -1, V̇ -1, V 1) and characterized by: circuitry to calculate the value of resistance from the equation: R = [ Paw 0 − Paw 1 + ΔPmus − E V 0 − V 1 / V ̇ 0 − V ̇ − 1 where E is the respiratory system elastance and ΔPmus is the change of Pmus between T 0 and T 1 (Pmus 0 - Pmus 1) as estimated by extrapolation from data obtained at T -1 and T 0 and determined from the equation: ΔPmus = ΔT 1 E V 0 − V − 1 + R V ̇ 0 − V ̇ − 1 − Paw 0 + Paw − 1 / Δt − 1 where ΔT 1 is the time difference between T 1 and T 0 and Δt -1 is the time different between T 0 and T -1. circuitry to calculate the value of resistance from the equation: R = [ Paw 0 − Paw 1 + ΔPmus − E V 0 − V 1 / V ̇ 0 − V ̇ − 1 where E is the respiratory system elastance and ΔPmus is the change of Pmus between T 0 and T 1 (Pmus 0 - Pmus 1) as estimated by extrapolation from data obtained at T -1 and T 0 and determined from the equation: ΔPmus = ΔT 1 E V 0 − V − 1 + R V ̇ 0 − V ̇ − 1 − Paw 0 + Paw − 1 / Δt − 1 where ΔT 1 is the time difference between T 1 and T 0 and Δt -1 is the time different between T 0 and T -1.