1. An implantable neural stimulator for delivering chronic neural stimulation therapy, the stimulator comprising: at least one respiration sensor configured to be used in monitoring respiration of the patient; a neural stimulation therapy delivery module configured to generate a neural stimulation signal for use in stimulating an autonomic neural target of the patient for the chronic neural stimulation therapy; and a controller operably connected to the at least one respiration sensor and to the neural stimulation therapy delivery module to provide therapy control using a marker for efficacy of the chronic neural stimulation therapy in treating a non-respiratory disorder, wherein the controller is configured to: control the neural stimulation therapy delivery module to deliver the chronic neural stimulation therapy, wherein the chronic neural stimulation therapy is configured to treat the non-respiratory disorder of the patient and is delivered over a period of time that includes a plurality of low activity periods and a plurality of non-low activity periods; determine the marker for one or more of the plurality of low activity periods during the period of time when the chronic neural stimulation therapy is delivered, wherein in determining the marker the controller is configured to: receive a respiration signal from the at least one respiration sensor indicative of the patient's respiration; monitor the respiration signal during one of the plurality of low activity periods within the period of time; and determine a respiration variability measurement over a plurality of respiration cycles for the one of the plurality of low activity periods using the monitored respiration signal, wherein the respiratory variability measurement for the one of the plurality of low activity periods provides the marker for efficacy of the chronic neural stimulation therapy in treating the cardiovascular disorder, wherein the respiration variability measurement includes either a respiration rate variability measurement or a tidal volume variability measurement; and adjust an intensity of the chronic neural stimulation therapy using the determined respiratory variability measurement.