Jul 7 2013 in Education by Todd West
For our inaugural edition of “Why Do We Do That?” I thought we could address something that many people are curious about: Targeted Tidal Volume Ventilation.
Historically, ventilators for neonatal patients have mainly relied on pressure regulation for delivering breaths to the patient. You set things like PEEP, delta P and rate, and try to synchronize the ventilator to what the baby is trying to do. The problem is that babies, like all patients, can vary their “receptiveness” to the ventilator depending on many things, such as being awake or asleep, agitated or calm. With these changes come great variability in the effectiveness of the ventilator.
We know that managing a baby on the ventilator successfully depends on giving the “just right” amount of support. Think of it as what I scientifically call The Goldilocks Theory: we don’t want Papa Bear or Mama Bear, just Baby Bear. Too much or too little pressure, breath size, or even oxygen is not good for the baby. The trick is finding “just right.”
Theoretical Clinical Practice
Now we have a tool that makes finding “just right” quite a bit easier. By using targeted tidal volume on our current ventilator, we can let the ventilator constantly monitor and adjust the breaths delivered to the patient. On our Hamilton G5 ventilators, that mode is called APV. We now choose the PEEP, rate and the desired tidal volume, instead of the delta P. This allows the ventilator to choose the minimum pressure needed to maintain the same size breath. Better yet, the ventilator monitors every breath and adjusts itself in real-time to keep the baby’s ventilation as consistent as possible.
This adaptability allows the ventilator to compensate for all kinds of clinical changes the baby experiences. Baby getting agitated? The pressure goes up to keep the breaths consistent, then goes back down as soon as the baby relaxes. Baby sleeping comfortably? The ventilator follows along with minimal pressure to support the baby’s own effort.
Practical Clinical Practice
Choosing the correct tidal volume for breath size is based on the knowledge of lung physiology for babies. We are using a starting volume of 6ml/kg for the tidal volume, then adjusting as needed based on blood gases. We chose that number based on the middle of the range for babies across all gestations in the NICU. Larger term babies usually need 4-6ml/kg, and preemies need more like 5-8ml/kg based on gestation. While it may seem strange that smaller babies need a larger tidal volume, keep in mind that they have relatively more dead space (the volume of trachea and airways) compared to their small lungs. Practical experience since starting tidal volume ventilation in our NICU has proven that 6ml/kg is a good starting point across the board.
Once we find the right tidal volume for the baby, we generally don’t need to adjust it much because this represents the physiologic need for that particular patient. Usually the only things that lead to changes are clinical developments that alter the metabolism of the baby, like developing sepsis. We also don’t need to decrease or wean the tidal volume as the baby moves toward extubation, we just watch the peak pressures improve over time. We will talk in more depth about weaning and readiness to extubate in a future installment of “Why Do We Do That?”
Questions? Just ask below!