Ventilator Review in the NICU – What Does Positive Pressure, Volume-Targeted, and High Frequency Do?

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Introduction

Are you ready for a review of ventilators in the NICU? Have you been wondering what on earth positive pressure, volume-targeted, and high frequency ventilators do? Once your baby is admitted to the NICU, there are so many pieces of information thrown at you, it is so hard to absorb it all! And, to top it off, for the majority of parents, it is all brand new information that you have never heard of before!

Our goal for this podcast is to provide you with basic information to help you understand different aspects of the care being provided to your baby in the NICU. Today, we break down the different modes of invasive ventilation commonly used in the NICU. 

First, we briefly review respiratory distress syndrome as well as some key concepts including atelectasis, lung compliance, and functional residual capacity to help you understand why a particular mode of respiratory support may be chosen versus another. 

Next, we will review conventional ventilation modes including SIMV, assist control, and volume guarantee as well as 2 different types of high frequency modes of ventilation. The mechanics can get very complicated especially factoring in the pathophysiology behind it all, but my ultimate goal is to give you a little bit more information and education to help empower you as a NICU parent, but not overwhelm or confuse you. 

To help minimize any confusion, we put together an amazing free graphic for you! The graphic will help you grasp a much better understanding of RDS. Since many infants in the NICU require invasive ventilation due to RDS, I strongly recommend for you to go HERE and grab this free graphic now!

Okay, let’s get to it and discuss invasive ventilation in the NICU!

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Ventilator Review in the NICU – What Does Positive Pressure, Volume Targeted, and High Frequency Do?

Episode 10

 

Respiratory Distress Syndrome Review

A few weeks ago, we took a deep dive into Respiratory Distress Syndrome in Newborns. If you have an infant in the NICU, especially a premature infant with RDS, I strongly recommend that you go back and listen to episode 8. We looked at why infants, especially preterm infants are prone to RDS. Oftentimes, with significant or worsening RDS, to better manage infants, a breathing tube or endotracheal tube may be placed and the infant will be placed on a ventilator. The infant’s clinical condition, gestational age, unit preferences and response to medical interventions, will determine what type of ventilator the baby is placed on. 

Now, to briefly review, RDS or respiratory distress syndrome is caused by a deficiency in surfactant in mostly preterm infants, but it can also occur in term infants as well. Surfactant spreads like a thin layer over the alveoli (or tiny air sacs) in the lungs and ultimately prevents them from collapsing by lowering the surface tension. By keeping the air sacs open, surfactant also decreases the amount of pressure required for each subsequent inflation or breath. 

Over time, when there is an insufficient amount of surfactant, more air sacs collapse which results in what is called atelectasis – a complete or partial collapse of the lungs. Babies with atelectasis, have to work harder to breathe trying to re-inflate the collapsed airways. 

For effective gas exchange, the air sacs or alveoli must be open for oxygen to enter the blood from the lungs and carbon dioxide to be released. Once too many alveoli collapse, the lungs are unable to get oxygen to the rest of their body resulting in hypoxemia or an abnormally low level of oxygen in the blood. Carbon dioxide begins to build up in the blood and ultimately leads to an increased amount of acid in the blood or acidosis. The hypoxemia and acidosis also impair the oxygenation by causing pulmonary vasoconstriction (constriction of the arteries in the lungs) that results in poor perfusion throughout the baby’s body. 

Management of Respiratory Distress Syndrome

We also reviewed a few weeks ago that there are 3 main strategies for prevention and/or treatment of RDS in newborns. For prevention, if there is risk of a preterm delivery, antenatal steroids are given to the mother before delivery to accelerate lung growth and the release of surfactant in the baby. 

For treatment, after the baby is delivered, if they develop respiratory distress, they will be supported with exogenous surfactant therapy and respiratory support, most commonly, continuous positive airway pressure or CPAP. CPAP pushes a continuous flow of air with or without additional oxygen into the airways to keep the tiny air sacs in the lungs open. By keeping the alveoli open, the infant does not have to work so hard to manually keep them open and it ultimately reduces atelectasis. 

In most institutions, non-invasive modes of ventilation are attempted initially if the infant meets the requirements to prevent lung trauma. The goal is to find the balance needed for each baby to achieve adequate gas exchange and ventilation that minimizes the potential lung injury or chronic lung disease. 

But, some infants in the NICU do require intubation and additional support with invasive ventilation. The decision to intubate the baby is based on their clinical status and/or blood gas results. The size of endotracheal tube placed is determined by the infant’s weight initially. Once the infant is intubated, the provider will determine which method of invasive ventilation will be most beneficial for the baby. 

Each NICU will have their own preferences, practices, and protocols that they typically follow. If the care is different than what I am discussing here, it does not mean that it is incorrect. The information provided here is meant to educate you with basic information on different modes of ventilation so you are more empowered and can continue to be a valuable member of your baby’s care team!

Key Concepts

Spontaneous inspiration is an active process, the lungs are naturally expanded by forces generated by the diaphragm and the intercostal muscles. As the pressure increases, gas flows from the atmosphere into the lungs. But, in newborns, due to their anatomy, the intercostal muscles provide minimal support during inspiration. Therefore, newborns with respiratory distress ultimately develop respiratory muscle fatigue and respiratory failure. 

Expiration is more passive and driven mostly by force from elastic recoil. Elastic recoil is dependent upon surface tension, lung tissue, and structure of the rib cage. In infants with RDS, the most significant contributor to elastic recoil is elevated surface tension due to surfactant deficiency in the alveoli and terminal airways. 

An infant with RDS, or surfactant deficiency, will have unstable alveoli that continually collapse at the end of expiration. So with each breath, normal functional residual capacity, or the volume that remains in the lungs after a normal, passive exhalation will not be fully established. Also, the pressure that is needed to inflate the lungs with each breath will be high. 

Lung compliance, or the ease with which the lung can be distended is also affected by surfactant deficiency. Without surfactant replacement therapy, surfactant deficient lungs will remain stiff with low compliance.  

Once exogenous surfactant is given to an infant with RDS, it will reduce the surface tension and allow alveoli to re-expand easier with inspiration. After surfactant is given to the infant, the functional residual capacity improves. The baby’s work of breathing also improves since there is a decrease in the airway resistance and a vast improvement in the lung compliance. But, even with surfactant replacement, infants with RDS, still typically need respiratory support.

The decision of which strategy or mode of ventilation to use on each infant depends on an interplay of several factors including the progression of the underlying condition, the state of the baby’s lungs, gestational age, and chronological age.

Positive Pressure Ventilation 

Historically, positive pressure ventilation was the preferred ventilation mode in neonates, but recently, volume targeted ventilation has gained popularity. The positive pressure modes delivery pressure according to parameters set on the ventilator. Within positive pressure ventilation, there are different modes that can be used based on the infant’s clinical condition.

Synchronized Intermittent Mechanical Ventilation

First, we will discuss Synchronized Intermittent mandatory ventilation or SIMV. With SIMV, a rate or a predetermined number of breaths per minute will be set by the provider, but the breaths are triggered by the infant’s spontaneous breathing efforts. The ventilator breaths synchronize or match once the baby initiates their own breaths. The baby can also take additional spontaneous breaths in between the set rate of ventilator-assisted breaths.

With each synchronized breath, the ventilator provides the ordered amount of PIP or peak inspiratory pressure. PIP is the predetermined peak pressure reached at the end of inspiration. The PIP typically ranges from 18 up to 30, but the goal is to keep the PIP as low as possible to prevent barotrauma in the lungs. In general, if a PIP in the upper 20s is consistently needed, then the NICU care team may consider a different mode of ventilation.

The PEEP or positive end-expiratory pressure, which is the end pressure at the end of expiration will also be set. The PEEP is typically set around 4-6, but some infants may need 7-8. The PEEP is the positive pressure that is maintained throughout expiration to prevent the alveoli from collapsing and to minimize the resistance against the increased surface tension. With SIMV, as you wean, the provider will gradually decrease the PIP or peak inspiratory pressure, the PEEP, and the rate.  

Assist Control

Another mode of positive pressure ventilation used is called patient triggered ventilation or assist control. With this mode, each time the infant takes a spontaneous breath, it triggers the ventilator to deliver a breath. Each breath delivered will be at the preset pressure or PIP. Therefore, the rate delivered is determined by the infant. If the infant becomes apneic and does not spontaneously breathe, the ventilator will deliver the set back-up rate at the preset PIP. With weaning on assist control, the pressure or PIP will be gradually decreased since the rate is ultimately controlled by the infant. And with the preset PIP in place, the aim is to lower the pressure as possible based on the infant’s condition and blood gases to prevent barotrauma. 

Volume-Targeted Ventilation

Another common mode of ventilation used in the NICU is volume-targeted ventilation or volume guarantee. With volume guarantee, a preset tidal volume is delivered by using the lowest possible pressure necessary to reach the set volume. Tidal volume is the volume of gas that enters into the lungs in one breath and is expressed in milliliters. The goal of volume ventilation is to minimize the risk of barotrauma and volutrauma.

The PIP needed to deliver the preset volume also takes into consideration the baby’s airway resistance and the lung compliance. So, the peak inspiratory pressure or PIP will vary with each breath as the baby’s lung compliance changes or how easy or not it is to expand the lungs. Therefore, if the lung compliance improves after exogenous surfactant is given, less pressure or a decreased PIP will be needed to generate the desired tidal volume which is why it minimizes barotrauma. The provider is also able to monitor the trend over time of the average PIP needed for the present volume to assess for improving lung conditions and readiness for weaning. 

The typical tidal volume for infants is 4-6 ml/kg, but may vary based on the infant’s clinical condition. If the infant has pneumonia or bronchopulmonary dysplasia, more volume may be needed. With volume ventilation, a rate will also be set by the provider to deliver a set number of breaths per minute. 

High-Frequency Ventilation

Next up is high frequency ventilation. High-frequency ventilation uses breath rates much greater than normal physiologic breaths.  The 2 different types of high-frequency ventilation typically used in the NICU are high-frequency jet ventilation and high-frequency oscillating ventilation. 

High-Frequency Jet Ventilator (HFJV)

High-frequency jet ventilation (HFJV) is used in the NICU on extremely premature infants for more gentle ventilation or for those infants who do not respond to other conventional ventilation. High frequency jet ventilation introduces small pulses of gas under pressure into the airway through the endotracheal or breathing tube at a very fast rate. Using a very small amount of tidal volume, it produces less distal airway and alveolar pressures than conventional mechanical ventilation. The jet actively pulsates gas into the baby’s lungs down the center of the breathing tube and exhalation on the jet ventilator is passive with carbon dioxide spiraling up and around the expiratory circuit.

The HFJV may also be used to minimize flow through leaks with conditions like pulmonary interstitial emphysema (PIE) or a pneumothorax. It can also be helpful with meconium-aspiration syndrome where gas trapping often occurs.

A key principal with the jet ventilator, is finding the optimal PEEP. The only true way to determine the optimal PEEP is by evaluation with an X-Ray. With too little PEEP, an X-Ray may display some atelectasis or poor expansion. With too much PEEP, the X-Ray will show overdistention. Since the jet ventilator uses less tidal volume and overall mean airway pressure, a higher PEEP may be used when compared to the other forms of mechanical ventilation. 

A rate is also set on the high frequency jet ventilator. Again, the key is to find the optimal rate for the baby – too low of a rate may result in an increase in carbon dioxide, too high can cause gas trapping. The rate on the jet ranges from 240 up to 660 bpm but typically averages around 420 bpm.

Also with the high frequency jet ventilator, a conventional ventilator is used in tandem and may be used to deliver periodic “sigh” breaths to help open collapsed alveoli. It is the conventional ventilator that is responsible for the majority of the alveolar recruitment and oxygenation. 

High-Frequency Oscillatory Ventilator (HFOV)

The other mode of high frequency ventilation used on neonates is high frequency oscillatory ventilation (HFOV). The oscillator delivers supra-physiological breathing rates. It is a mode used either as a first-line ventilation strategy to protect immature lungs or as a “rescue” therapy when conventional modes are ineffective. It is beneficial for infants with PPHN, meconium aspiration syndrome, pulmonary hypoplasia and/or with air leaks like pneumothorax or PIE.

The oscillator generates very low tidal volumes that are less than dead space at very high rates. It achieves the goals of ventilation, oxygenation, and carbon dioxide removal without the cost of barotrauma. With the oscillator, pressure oscillates around a constant distending pressure or mean airway pressure. Optimal oxygenation and lung inflation are achieved with gradual adjustments in the mean airway pressure.

The “art” of high frequency oscillatory ventilation is to not only achieve, but to maintain optimal lung inflation or positive end-expiratory pressure or PEEP without causing overdistention. It essentially provides CPAP or sustained inflation and recruitment of the lung volume. Gas is pushed into the lung during inhalation and unlike the jet, pulled out during expiration.

Alveolar ventilation and carbon dioxide removal are adjusted with the amplitude or Delta P. An increase in the delta P increases the tidal volume and ultimately, carbon dioxide removal. The initial amplitude or Delta P settings are typically based on the baby’s weight but are adjusted according to the amount of chest wiggle or vibration. The respiratory frequency is measured in Hertz with a range of 8-15 Hertz and will be set according to whether your infant is term or preterm. To monitor for overinflation, chest x-rays will be done.  

Clinical Management

If a baby is receiving respiratory support in the NICU, they will be closely monitored. The nurses, respiratory therapists, Neonatal Nurse Practitioners, and Neonatologists will monitor the baby clinically as well as their labs, especially their blood gas results. Blood gases use a small amount of the baby’s capillary, venous, or arterial blood and provide the providers with an inside peek of the adequacy of the baby’s respiratory support and function. It shows how well the baby is oxygenating and ventilating as well as the baby’s acid-base balance.

A combination of blood gas results coupled with the infant’s clinical appearance, FiO2 requirements, parameters on the ventilator, and Chest X-Ray results help the provider to determine how to adjust and eventually wean the ventilator. The goal is to always minimize the settings on the ventilator with the aim to move to non-invasive ventilation to minimize lung trauma without compromising the infant’s clinical condition. 

Closing

I hope this review of ventilators in the NICU was helpful for you. Every NICU and what their preferred delivery methods are for invasive ventilation will vary. I did not review every type of invasive ventilation used in the NICU, but in my experience, these are the modes that are most commonly used. The decision will ultimately be up to the baby’s care team and based on the infant’s clinical condition which mode is preferred or needed. 

Additionally, as I said, I only reviewed the main components on each type of ventilation mode. There are many more detailed parameters that go into each mode or device, that are beyond our scope to discuss here. My goal was to give you some basic information so you have a better understanding of the different modes of ventilation used on your baby or in the NICU. 

I hope we can continue to provide you with the building blocks to gain a better understanding of the care and treatment provided to your infant in the NICU so you can confidently ask questions and be an integral member of your baby’s care team. 

Remember if you have not already done so, go back and listen to Episode 8, Take a Breath – A Deep Dive into Respiratory Distress Syndrome in Newborns for a thorough review on RDS in newborns.

Do not forget to grab your FREE Respiratory Distress Syndrome graphic that correlates with Episode 8 and 10. The graphic will absolutely help you gain a better understanding of RDS in newborns.

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Remember, once empowered with knowledge, you have the ability to change the course.