Bilevel machines use the proprietary names BiPAP (Philips) and VPAP (Resmed). The ST machine stands for Spontaneous/Timed. This simply means that it is s fixed bilevel machine with a set exhale pressure (EPAP) and fixed inhale pressure IPAP. In its simplest form this machine either triggers IPAP and cycles to EPAP using the patient's spontaneous respiratory effort, or if the patient fails to initiate a breath, it produces a timed trigger to IPAP. ST machines have become somewhat more intelligent and now can target aveolar volume using a mode called intelligent Volume Assured Pressure Support (iVAPS). This machine is capable of causing a breath when central apnea would otherwise cause the patient to miss a breath, and it can supplement breaths in hypopnea to increase the volume through pressure support.
The problem with the ST machines is that they are not intended or targeted to complex and central apnea patients or individuals suffering from Cheyne-Stokes Respiration, but instead are for patients with neuromuscular diseases, severe COPD and obesity hypoventilation that results in respiratory insufficiency. In this target group which needs constant pressure support to perform a significant part of the "work" of breathing, the ST works great; but in central and complex apnea patients ST is usually a colossal failure due to the constant presence of high pressure support (the difference in pressure between IPAP and EPAP) which will actually make the central apnea and periodic breathing worse!
The ASV (adapative servo ventilator) was specifically designed to treat complex, central apnea and Cheyne-Stokes Respiration. The ASV can use auto-adjusting EPAP pressure to stabilize the airway against obstructive apnea during exhale cycle, and can trigger intelligent, VARIABLE pressure support when needed, as needed to support inspiration and volume. In other words, the pressure support can be zero or 2-3 cm for comfort, but can rise as high as 15 cm to cause a breath during central apnea or support a breath during hypopnea or Cheyne-Stokes. This levels out the respiratory volume, stabilizes periodic breathing and resolves the kinds of issues you have. The problem is that a large study (SERVE-HF) found a significant risk of sudden cardiovascular death in subjects of the study that had left-ventricular ejection fraction less than 35%. The recommended practices for prescribing ASV then added a safety factor, and the current guidelines for prescribing ASV require patients to be screen for LVEF% and that they have greater than 45% LVEF. The SERVE-HF machine was done using a prior generation of ASV devices that did not have Auto EPAP and zero-PS capability; rather they more closely approximated current ST machines. The most recent evaluation of the SERVE HF study is reevaluating findings in light of the technological advances and some problems in the original study design.
https://aasm.org/resources/practiceparameters/asv.pdf Currently there is no ASV therapy for individuals that fall in the risk group (<45% LVEF), however new studies are underway to try to evaluate if the original study can be verified or refuted based on changes in the technology or by modifying parameters of the original cohort compliance and titration efficacy.
So I've given you a lot to chew on, and will also leave a link to the Resmed Titration Protocol which describes the intended purpose of the machines and methods for establishing titration pressures.
https://www.resmed.com/us/dam/documents/...er_eng.pdf Good luck, and feel free to ask any questions.
