The evolution of pediatric mechanical ventilators
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The evolution of pediatric mechanical ventilators
- 1. The Evolution of Pediatric Mechanical Ventilators Robert L. Chatburn, RRT, FAARC University Hospitals of Cleveland Case Western Reserve University
- 13. 0 1 20 0 0 1 2 3 -3 0 20 0 2 1 20 0 0 1 2 3 -3 0 20 0 2 Inspiration Expiration 20 0 20 0
- 37. Dual Control Within Breaths Pressure Augment (Bear 1000), VAPS (Bird)
Notas del editor
- This lecture will give a brief historical review of pediatric ventilators over the last 20 years. To understand how and why ventilators evolved the way they did, we will cover two key ideas; the mechanics of patient-ventilator interaction and the concept of a ventilation mode. Ventilator manufacturers have started to depart from the practice of creating separate infant and adult ventilators in favor of machines that can ventilate any type of patient. With this trend has come the application of volume controlled ventilation to pediatrics, breaking a long history of exclusive use of pressure control. In addition, the new ventilators have made use of advanced single and double loop control schemes to create the latest modes of ventilation.
- The 1st generation of infant ventilators were controlled by simple analog electronics. They provided only one mode of ventilation: pressure controlled intermittent mandatory ventilation. Mandatory breaths were time triggered, pressure limited and time cycled. CPAP was also available, essentially IMV with a rate of zero. The ventilators of this generation had only simple control circuit alarms (eg, inverse I:E ratio or low battery). There was no provision for monitoring disconnection from the ventilator, let alone monitoring airway pressure, volume, and flow waveforms.
- Improvements in the first generation infant ventilators were centered on the alarms. Specifically, it became clear that users wanted integrated airway pressure (ie, disconnect) alarms.
- The next generation of infant ventilators was marked by the use of microprocessor electronics. At first they were not used to improve ventilator control schemes, but rather to increase the number and sophistication of alarms. In fact, the Infant Star ventilator was created to specifically address the concerns of the FDA that some infants had been injured because of insufficient safety features of earlier ventilators.
- It did not take long for manufacturers to exploit the power of microprocessors and create new modes of infant ventilation. Pressure triggering brought SIMV to infants well after it had become standard for adults. The Infant Star took advantage of the interest in high frequency ventilation by becoming the only ventilator to this day to offer that mode along with conventional modes in one machine.
- But the ventilator’s microprocessor could do even more. With the proper sensors, it now became practical to monitor and display pressure, volume, and flow waveforms.
- Not only did monitoring allow better ventilator management, it opened the possibility to trigger mandatory breaths on a variety of signals like volume, flow, and chest wall movement. By now, patient triggered ventilation had become commonplace for neonates with a number of studies showing its advantages. Manufacturers began to sell add-on monitors for ventilators that did not have the capability built in. Unfortunately, this led to cumbersome setups and the lack of integration among several add-ons was at best inconvenient and at worst, prone to malfunction.
- The current, 4th generation is marked by the use of advanced microprocessors in conjunction with sophisticated computer-like user interfaces. The power of the internal computers and the sophistication of the pneumatic controls allows one machine to ventilate any size patient, from neonate, to child, to adult. Having more versatile machines allows a department to have a smaller inventory of ventilators with the result of less capital and operating expense. Fourth generation devices are also characterized by advanced single loop modes of ventilation and dual loop control modes. Manufacturers have also tended to design the operator interface as a “virtual machine”. In other words, instead of a control panel made up of real buttons and knobs, a computer screen (in some cases, a touch screen) displays pictures of knobs and buttons along with displays. The obvious advantage of this approach is that upgrades become simply a matter of changing software rather than drilling new holes into the panel.
- To get a deeper knowledge of how the new ventilator operate, it is essential that you understand three key concepts. First is the equation of motion. It is a mathematical model of the interaction between the ventilator and the patient. The next concept is that all there are only two types of breaths: mandatory and spontaneous. It follows that any mode of ventilation can be described as a pattern of mandatory and spontaneous breaths, each with their own characteristics.
- The equation of motion says that the pressure necessary to deliver a breath has two components; the pressure to overcome elastic recoil of the lungs and chest wall and the pressure to cause flow through the airways. The left hand side of the equation can be expanded to show that ventilating pressure may be made up of muscle pressure and/or airway pressure generated by the ventilator. The right hand side of the equation can be expanded to show that elastic recoil pressure is the product of elastance (inverse of compliance) times volume while resistive pressure is the product of resistance and flow.
- A basic understanding of the equation of motion allows us to do three things: First, the equation says that any conceivable ventilator design (past, present, or future) can only control one variable at a time. For most applications, these variables are pressure, volume, and flow. Thus, any ventilator can be classified as a pressure controller, a volume controller, or a flow controller, depending on the variables it is designed to control during inspiration. The same is true for any mode of ventilation (hence pressure control, volume control or flow control). Second, the equation is build into the software of every ventilator that calculates respiratory system mechanics (ie, compliance, resistance, and time constant). Finally, if you do not understand what the equation of motion says, you will never fully understand the newer modes of ventilation such as “proportional assist”, “automatic tube compensation”, and “adaptive support”.
- It follows from the equation of motion that the ventilator can control either the left side of the equation (ie, airway pressure) or the right side (ie, volume and flow). These curves illustrate the two basic approaches to ventilator control. If the ventilator controls flow, it controls volume indirectly (by definition) and vice versa. Usually, inspiratory flow is held constant during inspiration, causing volume and pressure to rise linearly. Inspiration ends (cycles off) when a preset tidal volume is met. In contrast, with pressure control ventilation, airway pressure may be held constant during inspiration. This causes inspiratory flow to decay exponentially from its peak value towards zero as volume rises exponentially. Inspiration usually ends after a preset inspiratory time or (in the case of pressure support) after a preset inspiratory flow threshold has been crossed. If inspiratory time is long enough (usually about 5 time constants) lung pressure will equilibrate with airway pressure and inspiratory flow will cease. You will note that for passive exhalation is exponential. That mean expiratory time must be at least 5 time constants long to exhale more that 99% of the tidal volume. As expiratory time becomes shorter than 5 time constants, gas trapping (ie, autoPEEP) occurs.
- To understand any mode of ventilation, especially the more complex patterns, it is helpful to realize that there are only two basic types of breaths. A spontaneous breath is one that the patient has full control over. It is started (triggered) and ended (cycled) by the patient’s efforts or lung mechanics.
- From the two basic types of breaths, we can build three basic patterns. These are the basis of all modes of ventilation. The first possibility is that all breaths are mandatory. That is called, logically enough, Continuous Mandatory Ventilation (CMV). Another possibility is that all breaths are spontaneous. That is called Continuous Spontaneous Ventilation (CSV). The only other possibility is that both mandatory and spontaneous breaths occur. That is called Intermittent Mandatory Ventilation (IMV). As an aside, most ventilators allow the patient to trigger a mandatory breath, thus synchronizing the ventilator with the patient’s inspiratory effort. This is called Synchronized Intermittent Mandatory Ventilation (SIMV).
- We can now use the understanding gained by the equation of motion and the basic definitions of breath types and patterns to give a complete description of a “mode of ventilation”. First, according to the equation of motion, we state the variable being controlled during a mandatory inspiration (ie, pressure, volume, or flow). For most purposes it is easier to restrict this to simply pressure or volume (because control of pressure implies control of volume and vice versa). Next, we state the pattern of breaths, CMV, IMV, or CSV. Then, if more detail is needed, we can describe the specific phase variables the ventilator uses to trigger (start), limit (maximum value during inspiration),and cycle (end) the mandatory and spontaneous breaths. Finally, we can describe the rules the ventilator uses to switch between breath types and phase variables. Any mode, no matter how complex, can be describe in this simple fashion.
- So far we have used the word “control” without definition. Ventilators use three types of control, open loop, closed loop, and double (dual) loop.
- The most basic type scheme is open loop control. An open loop control mechanism simply turns on and off according to preset values. Lawn sprinkler systems and street lights are examples. You can also imagine a furnace in your house that simply turns on for 5 minutes every hour.
- The advantage of open loop control is that it is simple and inexpensive. The disadvantage is that the system is completely ignorant of its environment and thus its level of output may be inappropriate. The lawn sprinkler may give too much or too little water. The street light might come on after dark instead of at dusk. And nobody would want to stay in a room with such a crude furnace control.
- Early model infant ventilators and some current transport ventilators use open loop control of inspiratory pressure. This makes them small, inexpensive, and easy to operate. Unfortunately, they may under-ventilate the patient if there are leaks in the system or the operator changes the continuous flow rate.
- Closed loop control was invented to overcome some of the disadvantages of open loop control. Closed loop control is also called “feedback control” or “servo control”. The idea here is to set the control circuit to some desired output level and then measure the actual output. The measured value, or “feedback signal” is then compared to the preset value by the controller. If there is a significant error, the controller adjusts itself to compensate. A familiar example is the thermostat on your home furnace. You preset the desired room temperature and furnace stays on until the thermostat senses that the temperature has been reached. When the room cools down enough, the sensor tells the furnace to turn on again and the cycle repeats.
- The advantage of closed loop control are fairly obvious. Because of the sensor, the controller now has a rudimentary knowledge of its environment. It can thus maintain a more stable output in the presence of negative environmental influences. The relative disadvantage is that the controller must now be more complex and expensive. Reaction time becomes an issue.
- As it pertains to mechanical ventilation, the control circuit is the part that compares the set value to the measured value and generates a signal to the pneumatic system. Most often, the preset value is either an airway pressure limit or an inspiratory flow limit set by the clinician. Once the ventilator is triggered on, it attempts to adjust its output so that the preset value is maintained. The result is either pressure control (eg, constant inspiratory pressure) or volume control (eg, constant inspiratory flow) of the mandatory breath.
- The advantage of closed loop control for mechanical ventilation is that the clinician can either assure a constant level of gas exchange (ie, volume control) or a constant level of risk due to barotrauma (ie, pressure control). The ventilator will maintain its preset value in the face of changing lung mechanics or even small circuit leaks.
- An advanced form of pressure control has been developed by Draeger. It is called “Proportional Pressure Support”. Conventional pressure support is a form of pressure control in which the ventilator attempts to maintain a preset level of pressure above PEEP during a spontaneous inspiration. Proportional pressure support attempts to maintain a preset amount of pressure necessary to overcome respirator system elastance and resistance. But as the equation of motion predicts, the pressure necessary to overcome elastance and resistance are proportional to volume and flow respectively. And since this is a spontaneous breath, the volume and flow are not preset but change from breath to breath. Thus, an advanced control algorithm is necessary to measure volume and flow during inspiration and adjust inspiratory pressure accordingly.
- Proportional Pressure Support is the trade name Draeger has given to a general mode invented by Dr. Magdy Younes, called “Proportional Assist”. This mode is based completely on the equation of motion of the respiratory system. As you will recall, the equation says that the pressure necessary to deliver a breath has two components; the pressure to cause flow (the resistive load) and the pressure to expand the lungs and chest wall (the elastic load). Under normal conditions, the ventilatory muscles can generate the total pressure needed for inspiration. But disease may increase resistance and elastance and thus the muscle pressure needed to breathe. Proportional assist is a form of pressure controlled ventilation that provides the extra pressure necessary to overcome the abnormal elastic and resistive loads. This assumes you have estimates of the abnormal elastance and resistance from previous measurements. The equation of motion provides both the means to estimate these parameters and the means to control the ventilator’s pressure output. The operator sets the “flow assist” and “volume assist” levels. The ventilator measures the flow and volume generated by the patients initial inspiratory effort. It then increases the flow and volume above the patient’s demand until it starts to sense back pressure at the airway. As soon as pressure rises above the baseline pressure (ie, PEEP) the ventilator is doing work on (ie, assisting) the patient. How much pressure it generates depends on how high the assist levels are set along with the instantaneous flow and volume levels according to the equation of motion.
- Taking a closer look at this mode, we see that inspiration is triggered by the patient (eg, by pressure or flow). Inspiratory pressure is limited but not to a particular constant pressure level as in, say, a pressure support breath. Rather, it is limited to a constant proportion of the resistive and elastic loads. Inspiratory pressure is proportional to the volume and flow signals (hence the name “proportional assist”), where the constants of proportionality are the (abnormal) elastance and resistance estimated from previous measurements. For example, if “flow assist” is set to support half of the patient’s resistive load, then the ventilator will generate an inspiratory pressure that rises and falls with the inspiratory flow demanded by the patient’s inspiratory effort. This pressure will at every instant be just half of the pressure needed to cause flow through the airways, with the patient generating the other half. If instead, “volume assist” is set to support, say, 75% of the patient’s elastic load, then inspiratory pressure will rise continually throughout inspiration as the tidal volume is delivered. But at each instant there will be only enough pressure above PEEP to deliver 75% of the volume, with the patient having to generate enough muscle pressure to deliver the other 25%. When both of these “assist” levels are set above zero, the resulting pressure waveform has no characteristic shape, but depends on the flow and volume waveforms. Inspiration is terminated like a pressure support breath, when inspiratory flow decays below a preset threshold.
- This slide illustrates what the pressure, volume, and flow waveforms might look like for two breaths. In the first, the patient makes a large effort and thus generates a large inspiratory flow and volume. As a result, the ventilator generates a relatively large inspiratory pressure. In the next breath, the patient’s effort is smaller, generating a smaller inspiratory flow and volume. In response, the ventilator generates a smaller inspiratory pressure at each moment in time. The actual shape of the pressure waveform depends on the flow assist and volume assist settings and the flow and volume demand of the patient.
- The biggest advantage of proportional assist is that it should provide the most synchrony between patient demand and ventilator response. It has been likened to power steering on an automobile. The biggest disadvantage is that it is a continuous spontaneous mode of ventilation, like pressure support, and would provide no ventilation if the patient’s respiratory drive ceased.
- Automatic tube compensation is a simplified version of flow assist. The operator specifies the endotracheal tube size and the ventilator estimates the pressure necessary to cause flow through it.
- While tube compensation may allow the patient to avoid the sensation of breathing through the ET tube, it may also fool the clinician into a false sense of security. If there is significant swelling in the upper airway after extubation, the patient may be unable to handle the extra work of breathing it imposes.
- Double loop control is a way to give the control mechanism a crude ability to change its own control settings in response to changes in the environment. An example from everyday life is the setback thermostat used to save energy in the winter. The control circuit maintains one set point during the day and a lower one during the night. This saves the operator the trouble of having to reset the thermostat twice a day.
- On a ventilator, dual control currently takes two forms. In one form, inspiration is pressure controlled for each breath and the pressure limit automatically adjusted over time to achieve a target tidal volume. In the the other form, each breath starts out as a pressure controlled inspiration but may switch to a flow controlled inspiration if the ventilator thinks the target tidal volume will not be met in a reasonable inspiratory time.
- The idea here is to get the best of both worlds. That is, the stable minute ventilation of volume controlled ventilation along with the improved patient synchrony of pressure controlled ventilation.
- For dual control between breaths, the ventilator can adjust the pressure limit a few centimeters of water each breath in an attempt to achieve the preset tidal volume. Inspiration is terminated when flow decays to a preset inspiratory flow as for pressure support. The operator can usually set a maximum inspiratory pressure above which the ventilator cannot automatically go without alarming. In this slide, you can see that in the first breath, the inspiratory muscle pressure and ventilator pressure limit combine to produce a tidal volume below the desired target. For the next breath, the patient makes the same effort but the ventilator increases the pressure limit and the tidal volume is met.
- Dual control within breaths is a little more complicated. The operator sets an inspiratory pressure limit, a tidal volume, and an inspiratory flow limit. For the first breath, the sum of muscle pressure and ventilator pressure is sufficient to produce the desired tidal volume. Thus, inspiration is flow cycled when it decays to a sufficiently low level. For the next breath, the patient effort is much less. Now the preset pressure limit is insufficient to produce the tidal volume before the flow decays to the preset inspiratory flow level. Therefore, once flow hits the preset level inspiration switches to flow control at that level until the tidal volume is met. As a result, inspiratory pressure rises above the preset level.
- Just as proportional assist is an advanced form of single loop control, “adaptive support” is an advanced form of dual control. In the simplest case, the operator has only to set the patient’s ideal body weight, the FiO2 and the PEEP level.
- Adaptive support is a form of dual control between breaths. But instead of simply adjusting the pressure limit to achieve a target tidal volume, the ventilator controls ventilatory frequency, pressure limit and inspiratory time to achieve a normal minute ventilation predicted on the basis of the body weight. The ventilator attempts to simulate the body’s own control mechanism by selecting the tidal volume and frequency that minimizes the work of breathing for a particular minute ventilation. In other words, it selects the pattern it thinks the patient would use if not connected to a ventilator.
- Taking a closer look at this mode, we see that breaths can be either spontaneous or mandatory, depending on what the patient does.
- As with all of the newer modes, there is not much data in the literature to help us understand when and how to use them and if they improve patient outcomes. And as with all modes that rely on the equation of motion or other mathematical models, any situation that violates the mathematical assumptions of the models could make the mode inappropriate in that situation. For adaptive support in particular, there is an assumption of normal deadspace and normal minute ventilation. Increased deadspace or increased metabolic rate (eg, with fever) would require a minute ventilation setting above 100%.
- An added feature of adaptive support is that “lung protective” strategies are built in. In other words, while the ventilator may vary tidal volume and frequency, it will not select a volume that is either too large (risking lung rupture) or too small (risking atelectasis). Likewise, it will not select a frequency that is too high (risking autoPEEP) or too low (forcing tidal volume to be too high). To help the operator visualize what the ventilator is doing, there is a screen that presents the curve representing all combinations of volume and frequency that satisfy the predicted minute ventilation. A circle on the curve represents the ideal combination to minimize the work of breathing, based on measured lung mechanics, at each instant. A cross indicates the actual values of frequency and volume at any moment. As the ventilator adapts to the patient’s demands, the cross and circle approach each other circle until steady state is achieved, and they are at the same place on the curve.