A system for stimulating a diaphragm to provide a diaphragm assist level, comprising: an electrode positionable within a blood vessel of a patient; an electronic storage device storing instructions for controlling the system; and a processor configured to execute the instructions to: (a) obtain an attribute of a breath cycle of the patient; (b) based on a stimulation parameter, the attribute of the breath cycle, and a desired value of a breath parameter characteristic of the diaphragm assist level, transmit a stimulation signal to the electrode to enable the electrode to recruit a phrenic nerve of the patient during a breath cycle that includes respiratory assistance from a ventilator; (c) determine an actual value of the breath parameter from a previous breath; (d) compare the actual value of the breath parameter to the desired value of the breath parameter; (e) either maintain the stimulation parameter if the actual value of the breath parameter is within a selected range of the desired value of the breath parameter; or modify the stimulation parameter if the actual value of the breath parameter is not within the selected range of the desired value of the breath parameter; and (f) repeat steps (a)-(e). an electrode positionable within a blood vessel of a patient; an electronic storage device storing instructions for controlling the system; and a processor configured to execute the instructions to: (a) obtain an attribute of a breath cycle of the patient; (b) based on a stimulation parameter, the attribute of the breath cycle, and a desired value of a breath parameter characteristic of the diaphragm assist level, transmit a stimulation signal to the electrode to enable the electrode to recruit a phrenic nerve of the patient during a breath cycle that includes respiratory assistance from a ventilator; (c) determine an actual value of the breath parameter from a previous breath; (d) compare the actual value of the breath parameter to the desired value of the breath parameter; (e) either maintain the stimulation parameter if the actual value of the breath parameter is within a selected range of the desired value of the breath parameter; or modify the stimulation parameter if the actual value of the breath parameter is not within the selected range of the desired value of the breath parameter; and (f) repeat steps (a)-(e). (a) obtain an attribute of a breath cycle of the patient; (b) based on a stimulation parameter, the attribute of the breath cycle, and a desired value of a breath parameter characteristic of the diaphragm assist level, transmit a stimulation signal to the electrode to enable the electrode to recruit a phrenic nerve of the patient during a breath cycle that includes respiratory assistance from a ventilator; (c) determine an actual value of the breath parameter from a previous breath; (d) compare the actual value of the breath parameter to the desired value of the breath parameter; (e) either maintain the stimulation parameter if the actual value of the breath parameter is within a selected range of the desired value of the breath parameter; or modify the stimulation parameter if the actual value of the breath parameter is not within the selected range of the desired value of the breath parameter; and (f) repeat steps (a)-(e).