Assist Control (PTV/SIPPV) vs. SIMV: when should we consider these modes?
In this short article, I try to explain the key differences between Assist Control (AC/PTV) and Synchronized Intermittent Mandatory Ventilation (SIMV) in neonatal ventilation, particularly when combined with Volume Guarantee (VG).
It is important that we are comfortable with these two useful modes, and don’t hesitate to switch between them if the clinical condition requires it.
Assist Control / Patient Triggered Ventilation (PTV):
• In this mode, every single breath the baby takes is supported by the ventilator with the set pressure (PIP as directed by the volume guarantee algorithm).
•Clinical Use: This is often the preferred starting modality for many clinicians. However, if CO2 washout occurs, it is important to investigate the cause of the rapid breathing.
If the baby is breathing very fast (tachypneic), supporting every breath can lead to over-ventilation or "CO2 washout," where carbon dioxide levels in the blood drop too low. This could result in reduced respiratory drive, as well as the risk of lung overdistension from air trapping.
The tachypnoea could be related to underlying acidosis, and is often seen in the bigger babies with aspiration pneumonia. Careful evaluation of the cardio-respiratory status and CXR findings are important. Avoid sedating the baby just to capture the work of breathing-development friendly care from the entire team especially with experienced nurses would help. Consider cold light (transillumination) to rule out air leaks if associated with clinical worsening.
Synchronized Intermittent Mandatory Ventilation (SIMV):
• This mode is often used as an alternative when a baby is breathing too fast for Assist Control. It provides a set number of mandatory breaths synchronized with the baby's efforts.
It’s useful to add Pressure Support (PS) in SIMV.
If the backup rate is set low on SIMV, the baby may have to work significantly harder to take additional breaths that are not fully supported by the machine.
To reduce the "work of breathing" for the baby during those extra breaths in SIMV, clinicians can add Pressure Support.
• Flexibility: SIMV with Pressure Support allows the ventilator to provide some assistance to the non-mandatory breaths, but with a lower pressure than the main mandatory breaths.
• Baby Control: In these pressure-supported breaths, the baby retains control over how long the breath lasts (both the inspiratory and expiratory times), as both Ti and Te are baby led in PS mode (only for the PS breaths-the SIMV breaths follow the set Ti).
Clinical Summary
While Volume Guarantee (VG) is compatible with both modes, it is typically started with Assist Control (PTV). If the baby becomes too tachypneic and experiences CO2 washout, switching to SIMV with Pressure Support can help manage the baby's effort while preventing over-ventilation.
In addition to this, it is important to identify situations where volume guarantee is not suitable, and where we use the above modes without VG-old fashioned approach 😊. We will discuss that in a future article.
Do review this video on my channel where I discuss the above. Do subscribe to my Substack as well as YouTube channels, do share with your colleagues.