Conventional Mechanical Ventilation: Initial Settings by B. Grenier | OPENPediatrics

Conventional Mechanical Ventilation: Initial Settings by Barry Grenier. Hello. My name is Barry Grenier. I am the education coordinator in the Department of Respiratory Care at Boston Children's Hospital. And today I would like to outline a practical approach to choosing

initial

ventilator

settings

for a variety of pediatric patients ventilated with a

conventional

mechanical

ventilator. Now, this discussion assumes that you have a ventilator configured with a circuit and accessories appropriate for your patient size, and that you have subjected the ventilator to a pre-use check according to the manufacturer's recommendations. And we will do an overview of choosing the

ventilation

mode, choosing the ventilator

settings

that define the type of mandatory breath you are using in that

ventilation

mode, choosing the other ventilator settings applicable in that mode, and finally we will do Talk a little about the use of pressure support and ventilation modes that allow spontaneous breaths.

Determination of the appropriate mode of ventilation. Now, choosing a ventilation mode can seem daunting due to the proliferation of modes and mode names that have appeared on

mechanical

fans. But it may be helpful to try to simplify the process a little by stating that all ventilation modes can fit into three main mode families: continuous mandatory ventilation, intermittent mandatory ventilation, and continuous spontaneous ventilation. And that these modes contain breaths controlled by volume or pressure. With Continuous Mandatory Ventilation, all breaths are mandatory. And these would include the so-called assisted control modes that we have traditionally used. Intermittent mandatory ventilation modes offer a combination of mandatory and spontaneous breaths, and continuous spontaneous ventilation would include those ventilation modes in which all breaths are spontaneous.

With volume-controlled breaths, the tidal volume and flow rate are set and delivered consistently from breath to breath. And to do this, the fan will vary the pressure as necessary. With pressure-controlled breaths, the ventilator pressure is predetermined and the volumes and flows are, to some extent, variable. With pressure controlled, the target pressure can be set, but sometimes in some of the newer ventilation modes, the pressure is controlled by the ventilator to achieve a target tidal volume. So in modes like pressure-regulated volume control, if you drill down into what's happening with that breath, the breath has a target volume but is delivered as a pressure-controlled breath.

So if we can use this framework, it becomes clear that there are actually five groups of fan modes, basically, that we can choose from. Continuous mandatory volume control type modes, which would include volume control assisted control, continuous mandatory pressure control modes, such as pressure regulated volume control assisted control and pressure regulated volume control assisted control, intermittent mandatory ventilation volume control and pressure control that would include volume control. SIMV control on one side, and SIMV pressure control and SIMV PRVC on the other. And finally, continuous spontaneous modes which would include CPAP or the use of independent pressure support, that is, pressure support without mandatory breaths.

When choosing a ventilation mode, you may want to choose a mode with volume-controlled mandatory breaths if the primary goal is to control tidal volume and minute volume over a very narrow range, such as in patients with acute brain injury where intracranial pressure is a consideration, or for some cardiovascular patients. Pressure monitoring may be more appropriate if you are looking to control ventilation pressures much more tightly, both in terms of mean airway pressure and peak alveolar pressure. Pressure control also offers variable gas flows. In some circumstances, this may better meet your patient's inspiratory flow demands. Another reason to use pressure control is that, by convention, we typically ventilate newborns and infants with pressure-controlled type modes.

And another pressure-controlled type mode is pressure-regulated volume control, which allows you to target the volume of that pressure-controlled breath and combines the variable flow of a pressure waveform with the volume target. That said, the choice of ventilation modes, whether Assist Control or SIMV, pressure controlled or volume controlled, is largely based on tradition and comfort level. Controls that define the type of mandatory breathing. So perhaps a much more important question is: when we ventilate our patients, what is the correct tidal volume to use? We choose the tidal volume based on the patient's size and lung pathophysiology. And we should index those volumes based on patient size, typically in milliliters per kilogram and based on some measure of ideal or predicted body weight.

For a general ICU patient without pulmonary disease in the PICU, a patient with refractory seizures, or a postoperative patient without pulmonary disease, the usual target tidal volume at our institution is 8 milliliters per kilogram, while a patient with diffuse lung disease of low compliance disease, the patient with diffuse pneumonia, or the patient with acute respiratory distress syndrome, it would be much more appropriate to start with the tidal volume of 5 to 7 milliliters per kilogram. Now, evidence is emerging that a lung-protective ventilation strategy, a low tidal volume strategy, is likely to be beneficial for all patients. Therefore, it would not be bad to use 5 to 7 milliliters per kilo to ventilate all your patients.

However, there are a couple of possible exceptions to this general rule when ventilating newborns. Even in newborns with normal lungs, we will typically have a target tidal volume of 6 to 7 milliliters per kilogram. This can usually be achieved with a fairly low vent pressure. But in premature patients or patients with diffuse lung disease, our target tidal volume range would be 3 to 5 milliliters per kilogram. In patients with obstructive airway disease, we would typically choose a strategy of using a higher tidal volume and lower rate. And this is to minimize gas buildup. And in this case, we would be using tidal volumes of 8, 10 or even more milliliters per kilo.

And for cardiac surgery patients, we would also adopt a higher tidal volume and lower rate strategy, but in this case, the strategy would be employed to improve hemodynamics. If you choose a mode that uses mandatory breaths with volume control, and it is volume control, assist control or volume control, SIMV type modes, we would set the tidal volume directly. But you will also have to choose a flow rate, which determines how quickly the volume is delivered. The flow rate can be adjusted directly, usually as maximum flow rate. In this case, the ventilator will calculate the inspiratory time necessary to deliver that tidal volume at that flow rate.

Or the flow rate can be set indirectly by setting an inspiratory time, in which case the ventilator will calculate the flow rate needed to deliver that volume in that set time interval. The established flow rate, in any case, must be adequate to satisfy the patient's inspiratory flow demands. Because even ventilators where the flow is set directly will often now calculate and display the inspiratory time, the inspiratory time can be used as a guide for flow rate choice. A variety of appropriate inspiratory times will be discussed a little later in the presentation. If you choose a ventilation mode in which pressure-controlled breaths are the mandatory breath type, and synchronized intermittent mandatory ventilation type modes are pressure-controlled, assist-controlled, or pressure-controlled, the breath will be defined depending on the target ventilation pressure. and for a certain inspiratory time.

We will establish an appropriate inspiratory pressure to achieve the tidal volume that we have determined is appropriate. On some ventilators, the maximum inspiratory pressure is adjusted directly. On some ventilators, the ventilation pressure is set as a pressure higher than PEEP, sometimes called delta pressure. And in this case, the maximum inspiratory pressure is the delta pressure plus the set PEEP. A reasonable starting range for setting maximum inspiratory pressure is probably between 18 and 25, usually. You will use a number at the lower end of that range with patients with normal lungs. And you probably want to be near the high end of that range for patients with lung disease.

Although for patients with significant lung disease, it may be necessary to start with an

initial

maximum inspiratory pressure greater than 25. In any case, we also want to avoid maximum inspiratory pressures greater than 30 centimeters of water, unless we have evidence of a decrease in chest wall compliance. . When setting the inspiratory time, the inspiratory time will be directly related to the size of the patient. And in our smallest patients, we can choose an inspiratory time as low as 0.3 or 0.35 seconds, while in our large and adolescent patients, those time intervals can be on the order of 1 second or even 1.2 seconds .

It is important to understand that when you set the Inspiratory Time in the Pressure Control, the Inspiratory Time does not control the Inspiratory Flow Rate. Inspiratory flow rate is a function of the compliance and resistance of the respiratory system and the magnitude of the set ventilation pressure. We will want to set an inspiration time long enough to allow the lungs to fill adequately. And we're going to try to adjust the inspiratory time so that the inspiratory flow approaches zero sometime near the end of the inspiration. So if you have access to flow graphs, you can use the inspiratory flow waveform to adjust the inspiratory time.

Choose other settings. So we describe how to choose a ventilator mode, how to set settings around the mandatory respiratory rate that you chose to use. Let's now discuss the settings that will be common to all ventilation modes with mandatory breaths, that is, the fraction of inspired oxygen concentration or FiO2, or in some ventilators it is set as % O2, the mandatory respiratory rate, positive end. expiratory pressure or PEEP and the sensitivity of the trigger. Unless contraindicated, for example by prematurity or certain cardiac injuries, we will use an FiO2 of 1 or 100% oxygen as the initial setting. But we will be ready to eliminate it quickly based on some objective measure of oxygenation, such as oxyhemoglobin saturation.

The mandatory breathing rate you choose will depend on the amount of spontaneous breathing the patient is doing and the degree of lung disease. Again, we have ranges of typical initial settings for respiratory rate that depend on the size and age of the patient. For newborns, that range could be between 20 and 40 years or older. For our older patients, older children and adolescents, that rate will be lower, probably on the order of 12 to 20 as a starting setting. However, the point is that you want to choose a respiratory rate that, in combination with your tidal volume, ensures adequate minute ventilation to achieve an acceptable level of carbon dioxide removal.

So a general rule of thumb is that, at least with normal physiology, 100 milliliters per kilo per minute of volume should achieve a reasonable level of CO2 removal. However, it is important to understand that this rule will underestimate the need for minute ventilation with increased dead space due to cardiopulmonary disease or increased carbon dioxide production, as might occur with a febrile patient. It is common, or has become common, to use low levels of PEEP in almost all ventilated patients to stabilize lung volume or functional residual capacity. So at our institution we will use 3, 5, or maybe even 7 centimeters of PEEP water, even without lung disease.

Although, again, if you have a patient with already established low-compliance lung disease, you will probably have to use 8 or more centimeters of PEEP water pressure, even as an initial setting. Trigger sensitivity is that threshold for the ventilator to recognize and respond to a patient's spontaneous respiratory effort. On many new ventilators, that is set as an inspiratory flow rate, but it can also be set as a negative pressure that the patient creates during inspiration that will be set relative to a baseline pressure or PEEP. We configure the sensitivity of the trigger to minimize the patient's work withoutcause an automatic trigger, that is, a trigger on some artifact other than the patient's spontaneous breathing.

Causes of self-triggering include leaks in the system, such as a leak around the endotracheal tube, movement of accumulated condensate in the ventilator circuit, or even oscillations created by a hyperdynamic heart. If you are setting up a flow trigger for a smaller patient, infants, or toddlers, 0.25 to 0.5 liters per minute is probably a reasonable starting point. In older children, adolescents, 1 to 2 liters per minute is probably a reasonable starting point. With the pressure trigger, for our smallest patients, we probably do not want to set more than 1 centimeter of water pressure as a threshold, although for children, older children, and adolescents, 1 to 2 centimeters of water pressure will be appropriate.

Use of pressure support. With ventilation modes that allow spontaneous breathing, such as synchronized intermittent mandatory ventilation mode, we have the option of supporting the patient's spontaneous respirations with pressure support. And we generally start with a minimum level, 6 to 10 centimeters of water pressure, for example, to overcome the resistance of the endotracheal tube. But once the patient is breathing spontaneously, we will titrate that level until we achieve a tidal volume of 5 to 7 milliliters per kilo, which we consider an almost spontaneous tidal volume. With pressure support, since pressure support breaths are flow terminated or flow cycled, most ventilators have a setting for that flow cycling criterion.

And normally that setting is set as a percentage of the maximum inspiratory flow rate, that is, the inspiration will end when the inspiratory flow drops to a percentage of the maximum flow rate that was used during that breath. For example, if the inspiratory flow criterion is set to 10% and the maximum flow is 10 liters per minute, inspiration will end when the inspiratory flow drops to 10% of the maximum flow or 1 liter per minute. 10% to 25% of maximum flow is a reasonable initial setting for flow cycling criteria. And that's usually the default setting for many of the fans available today.

Pressure support can also be used as a standalone mode, sometimes called direct pressure support. Again, there would be no mandatory breaths or mandatory breathing settings. You may choose direct pressure support if the patient is breathing spontaneously and you want to allow him or her to have more control over respiratory rate and minute ventilation. Again, we would typically start with a minimum level of 6 to 10 centimeters of water pressure, titrating that level to achieve a tidal volume of 5 to 7 milliliters per kilogram. And additional settings that will need to be set on direct pressure support would be FiO2, PEEP level, and trigger sensitivity.

Many ventilators now use backup ventilation, which would require establishing some mandatory settings that would ensure the patient receives adequate support should apnea occur. Case. To illustrate the process we have just described, let us use as an example a thin 30 kilo patient without lung disease in the PICU, being ventilated postoperatively. By convention at our institution, we would probably choose Pressure Control-SIMV as our mode. But I'm going to keep in mind the target volume I want to achieve with this patient, which will be approximately 8 milliliters per kilo, or 240 milliliters. Because the patient does not have lung disease, I will start with a relatively low peak inspiratory pressure, 20.

Again, once the patient is on the ventilator, I will adjust it to achieve the target tidal volume. I'm going to choose an inspiratory time of 0.8 seconds, because there is no contraindication. I'm going to choose an oxygen concentration of 100%. I'm going to choose 5 PEEP, just as a starting pressure, and a mandatory respiratory rate of 15 breaths per minute. And those 15 breaths, combined with the 8 milliliters per kilogram, should produce a minute ventilation of just over 3 liters per minute. Anticipating that this patient will at some point wake up and begin breathing spontaneously, I will also set a flow trigger sensitivity that is appropriate, in this case, 1 liter per minute.

And I'm also going to set a minimum pressure support level of 6 and a flow cycle threshold of 15. In summary, the choice of ventilation mode is probably less important than how that mode is applied to our individual patients, which It means establishing control environments that ventilate our patients adequately and safely. And one of the main concerns here is choosing a tidal volume appropriate for the patient's size and pathophysiology. Thank you. Help us improve the content by giving us some feedback.