In this video, we will review the basics of mechanical ventilation. Some concepts will be simplified for learning. By the end of this presentation, you will be able to 1. Choose a basic ventilator mode 2. Set initial ventilator settings for a patient 3. Change your initial ventilator settings based upon your patient's requirements And four, this will facilitate communication with intensive care practitioners and respiratory therapists about your patient's mechanical ventilation and will also facilitate learning of more advanced mechanical ventilation management. Basic settings on a mechanical ventilator include mode, tidal volume in cc's, respiratory rate, inspired oxygen concentration or FiO2, and positive end-expiratory pressure or PEEP. Let's start by going over these basic settings and some adjustments before we discuss different ventilator modes.
Tidal volume is the volume of air in the lung that is displaced between normal inhalation and exhalation. It is based upon ideal predicted body weight, which is based on height. Tidal volume is usually set between 6 to 8 cc.
per kilogram for ideal predicted body weight. We use height to predict ideal body weight because height correlates more with lung volume. As an example, the lung volume of a 5'9 male at 70 kilograms is approximately equal to the lung volume for a 5'9 male at 150 kilograms. In other words, the soft tissue mass does not affect lung volume.
The formulas for these estimations are, for males, predicted body weight in kilograms equals 50 plus 2.3 times height in inches minus 60. For females, it is predicted body weight in kilograms equals 45.5 plus plus 2.3 times height in inches minus 60. A good way to predict a patient's weight is to start with estimating. A good set of numbers to keep in mind is the following. For a 5'9 or 175 centimeter male, a tidal volume of 500 cc is equivalent to 7 cc per kilogram. Using these numbers, you can adjust the tidal volume up or down based upon height until you have time to cross-check by formula or table. Respiratory rate is the number of breaths taken per minute.
In a patient without other physiologic derangements, respiratory rate is approximately 10-14 breaths per minute. Remember the following equation. Tidal volume multiplied by respiratory rate gives us a minute ventilation.
A normal minute ventilation for you and me is approximately 4-6 liters per minute. FiO2, or inspired fraction of oxygen, is the percentage of oxygen in the air that is delivered to a patient. When first placing a patient on a ventilator, it is reasonable to start with an inspired fraction of oxygen at 100% and titrate down to 0. later. The goal is to use a minimum FiO2 required to have a PaO2 greater than 60 millimeters of mercury or an oxygen saturation greater than 90%.
Positive end expectations Pressurized respiratory pressure, or PEEP, is a concept that is often initially confusing. PEEP, applied by the ventilator, is called extrinsic PEEP since it is produced by the ventilator. PEEP is a positive pressure, measured in centimeters of water, that is constantly applied to the airway, not only at the end of expiration.
Therefore, the pressure in the lungs is never allowed to go below your set PEEP level. In this graph, the minimum pressure is 5. PEEP pressure generated with inspiration will be added on top of your PEEP value. With normal exhalation, the end expiratory pressure following end exhalation on a ventilator should be equal to your set PEEP value, here a value of 5. The role of PEEP is to help alveolar recruitment. Some alveoli are easier to inflate with a breath than others. PEEP helps keep some marginal alveoli open so that they are available to participate in gas exchange.
Many of you are familiar with or have heard of the concept of CPAP. CPAP is continuous positive airway pressure applied to a non-intubated patient via an external face mask. CPAP is the same as PEEP, except CPAP is applied via face mask and PEEP is applied via an endotracheal tube. CPAP is most often used in patients with a obstructive sleep apnea who have collapsed of their airway soft tissues during sleep, leading to airflow obstruction. CPAP has been described as an airway stent to keep these tissues from collapsing.
Ventilator settings are changed based upon a patient's respiratory compliance and gas exchange. Respiratory compliance, or the lungs ability to stretch or expand, is assessed by measurements of peak and plateau airway pressures from the ventilator We will discuss airway pressures later in this presentation. Gas exchange is frequently assessed in the intensive care unit by arterial blood gas analysis. Arterial blood gases estimate arterial pH. partial pressure of carbon dioxide or PaCO2, and partial pressure of oxygen or PaO2. Let's begin with looking at some arterial blood gases and how to change our basic ventilator settings based on these values.
A generic normal arterial blood gas may be 7.4, 40, and 100. Generally, once we choose an initial ventilator mode, we stay with this mode while we adjust the temperature. adjust other settings on the ventilator based upon our patient's needs. That leaves us with four potential variables to adjust. Tidal volume, respiratory rate, FiO2, and PEEP.
Here's our first example. You obtain an ABG, pH of 7.28, PaCO2 of 55, and PaO2 of 100. Let's look at what this gas is showing us. At 7.28, At 1.28, the pH is acidotic.
Next, we look at the carbon dioxide level, which is elevated, 55, from a normal value of 40. We know that this is likely contributing entirely, or in part, to our patient's acidosis. The PaO2 is 100, which is acceptable and does not require adjustment at this time. We conclude that our patient has a respiratory acidosis with adequate oxygenation. Thank you. Which two variables can we change to compensate for this?
Generally speaking, for derangements pertaining to arterial CO2, we change the patient's minute ventilation, which is respiratory rate times tidal volume. Therefore, for this patient with a respiratory acidosis, we will likely start by changing his respiratory rate. We could alternatively change his tidal volume.
However, assuming that we have chosen an appropriate tidal volume for our patient's size, we usually have to change the tidal volume. usually change our respiratory rate first. In order to compensate for metabolic acidosis, we can increase the minute ventilation by selecting a more frequent respiratory rate or choosing a greater tidal volume. Let's look at another blood gas example.
You obtain an arterial blood gas on a different patient, pH of 7.4, PaCO2 of 40, and PaO2 of 40. What is this blood gas showing us? the pH is neutral. Next, we look at the carbon dioxide level, 40, which is normal and acceptable, particularly since our pH is normal.
The PaO2 is 40, which is too low and requires adjustment. We conclude that our patient is hypoxemic with adequate ventilation. Which two variables can we change to compensate for this?
Generally speaking, for derangements pertaining to arterial O2, we change the patient's F. FiO2 or PEEP. Therefore, for this patient with hypoxemia, we will likely start by changing her FiO2.
We could additionally change her PEEP. What these examples illustrate is that generally speaking, ventilation, or carbon dioxide exchange, and oxygenation are separated processes in terms of ventilator adjustments. At extremes, this does not completely hold true. However, for the majority of ventilator management, this is a good way to think about gas exchange and mechanical ventilation. In the operating room, we sometimes use arterial blood gases to help us with ventilator management.
However, more frequently we rely on n-tidal carbon dioxide or ETCO2 and the pulse oximeter. N-Tidal CO2 works by sampling the partial pressure of carbon dioxide in exhaled gases. Carbon dioxide has a very high diffusion coefficient and most of the time the n-tidal CO2 should be within 5 millimeters of mercury of the actual arterial n-tidal CO2. Some exceptions to this include patients with severely impaired alveolar diffusion or severely impaired pulmonary blood flow for example cardiopulmonary arrest or massive pulmonary embolism. In these cases, there will be a greater alveolar to arterial CO2 gradient.
Pole sub-symmetry is used to evaluate the patient's oxygenation. Here is the hemoglobin dissociation curve. Keep in mind that an oxygenation saturation of 90% correlates with a PaO2 of 60 mmHg. Below this, there is a sharp decline in PaO2.
So far, we have discussed very basic ventilator settings and basic ventilator adjustment strategies. Now, let's talk about the different modes of ventilation. Modes of ventilation mostly differ in the way your minute ventilation or tidal volume times respiratory rate is delivered to the patient.
Regardless of the mode of ventilation, adjustments of FiO2 and PEEP are the same. There are controlled modes of ventilation, hybrid modes of ventilation, and spontaneous modes of ventilation. For controlled and hybrid types of ventilation, there are two large categories of types of ventilator mode.
Modes where we set the patient's tidal volume, known as volume control mode, modes and modes where we set an inspiratory pressure, known as pressure control modes. Within each mode category, volume and pressure, we have two subcategories, controlled or mandatory intermittent. This refers to whether we have control over every single breath delivered or not.
We will start with volume control modes. There are two main types of volume control modes of ventilation. Assist control volume control modes are the two most common control or ACVC and synchronized intermittent mandatory ventilation volume control or SIMVVC.
Let's start with ACVC as our mode. Since this is a volume controlled mode of ventilation, we will first set the tidal volume. We will then set our respiratory rate and then our FIO2 and PEEP. Let's say our settings are ACVC with a tidal volume of 500 cc and respiratory rate of 10. If our patient has no drive to breathe over the setting or is paralyzed and therefore unable to breathe over the setting, then every six seconds or ten times per minute the ventilator will deliver a breath to our patient of 500 cc's.
If our patient wants more minute ventilation than this, if he or she tries to breathe more than ten times per minute, then on ACVC with our current settings, the ventilator will allow the patient to take additional breaths, but we will control the volume of these additional breaths. Say our patient wants to breathe 14 times per minute. He or she will now be breathing 14 times per minute with every single delivered tidal volume staying at 500 cc according to our settings. For SIMV-VC, we will again set our tidal volume as this is still a volume controlled mode.
We will set our respiratory rate, the FIO2, and PEEP. And additionally, we will set a pressure support. Let's say our settings are S-I-M-V-V-C with a tidal volume of 500 cc, respiratory rate of 10, and pressure support of 5. If our patient has no drive to breathe over the setting, or is paralyzed and therefore unable to breathe over the setting, then every 6 seconds, the ventilator will deliver our 500 cc tidal volume to our patient.
If our patient wants more minutes, ventilation than this. This is where you will see the difference between ACVC and SIMVVC. Now, the ventilator will again see that the patient is trying to take a breath on his own.
But instead of delivering an extra 500cc tidal volume every single breath like we do in ACVC, in SIMV, the ventilator will now let the patient take whatever tidal volume he can. With a given amount of pressure support we have set for any breath over 10 that we have set per minute. Pressure support can be thought of as an assist or boost to the patient's own effort.
So, our patient will continue to receive 500 cc tidal volume breaths at 10 times per minute, which will be synchronized to our patient's respiratory efforts. But, the additional breaths will have unknown tidal volumes depending on our patient's effort and strength, with the assistance of our pressure support. With these two volume control modes of ventilation, we have set the tidal volume and respiratory rate. Unknown variables include the actual respiratory rate and airway pressures generated.
When you are reporting values on a patient with the volume control mode, you should report your ventilator settings. and also the measured variables including patient respiratory rate, peak airway pressure, and plateau pressure. These measured pressure variables will be easier to understand after we talk about pressure control modes of ventilation next. Now let's talk more about the pressure control modes of ventilation. There are two main types of pressure control modes of ventilation, assist control pressure control, or ACPC, and synchronized intermittent mandatory ventilation.
pressure control or SIMVPC. If ACPC is our mode, we will first set an inspiratory pressure since this is a pressure controlled mode of ventilation. We will set our respiratory rate are FiO2 and PEEP.
The inspiratory pressure applied will give us an associated tidal volume dependent on the patient's current lung compliance. Let's say our settings are ACPC with an inspiratory pressure of 20 centimeters of water and a respiratory rate of 10. If our patient has no drive to breathe over this setting or is paralyzed and therefore unable to breathe over the setting, then every 6 seconds or 10 times per minute, the ventilator will will deliver a breath to our patient via an inspiratory pressure of 20 centimeters of water. Note that this inspiratory pressure will be a pressure applied on top of our background constant PEEP pressure. If our patient wants more minute ventilation than this, say he or she tries to breathe more than 10 times per minute, then on ACPC with our current settings, the ventilator will allow the patient to take additional breaths, but we will control the positive pressure of these additional breaths.
So, if our patient wants to breathe 14 times per minute, he or she will now be breathing 14 times per minute with every single breath delivered via an inspiratory pressure of 20 centimeters of water. according to our settings. For our SIMVPC, we will again set our inspiratory pressure, as this is still a pressure-controlled mode. We will set our respiratory rate, the FIO2, and PEEP.
And additionally, we will set a pressure support. Let's say our settings are SIMVPC with an inspiratory pressure of 20 centimeters of water, respiratory rate of 10 with a pressure support of 5. If our patient has no drive to breathe over this setting or is paralyzed and therefore unable to breathe over this setting, then every 6 seconds the ventilator delivers a breath to our patient via an inspiratory pressure of 20 centimeters of water. If our patient wants more minute ventilation than this, this is where you will see the difference between ACPC and SIMVPC. Now, the ventilator will again see that the patient is trying to take a breath on their own, but instead of delivering an extra breath with 20 cm of water per our set inspiratory pressure, like in ACPC, now, in SIMVPC, the ventilator will now let the patient take whatever tidal volume he can With the given amount of pressure support, we have set for the breast over 10 per minute. So, our patient will continue to receive breast at 20 centimeters of water inspiratory pressure at 10 times per minute, which will be synchronized to the patient's respiratory efforts, but the additional breast will have an unknown tidal volume.
dependent on our patient's effort and strength with assistance of our pressure support. With these two pressure modes of ventilation, we have set inspiratory pressure and respiratory rate. Unknown variables include the actual respiratory rate and the tidal volume generated.
When you are reporting values on a patient with a pressure control mode, you should report your ventilator settings and also the measured variables, including patient respiratory rate and tidal volume achieved. In summary, for volume-controlled modes of ventilation, we have direct control over the patient's tidal volume, but we measure and don't have control of the airway pressures. For pressure-controlled modes of ventilation, we have direct control over the airway pressures, over airway pressures, which is our inspiratory pressure. However, we measure and don't have control of the patient's tidal volume. Generally speaking, we often prefer to have control over our patient's tidal volume and favor volume control modes of ventilation.
Some of the reasoning behind this is that with pressure control, if there is a sudden change in our patient's pulmonary compliance, the tidal volume could suddenly get very large or very small. Especially, small tidal volumes can compromise our minute ventilation and put our patient at risk of inadequate gas exchange. Pulmonary compliance relates to the stiffness of the lungs. Therefore, if the lungs become more stiff or more restricted by an outside force, then with pressure control ventilation, the same inspiratory pressure will now result in a smaller tidal volume. If you were using a volume control in the same situation, then the ventilator will continue to try to administer the same tidal volume, but you will now see readouts of higher airway pressures.
One advantage of pressure control comes from the mechanics of the way the breath is delivered to the patient by the ventilator. With pressure control ventilation, at the start of the ventilator-delivered breath, the patient's lungs already receive the set inspiratory pressure. Let's use our example of 20 cm of water on top of our peep of 5. At the start of a ventilator delivered breath, our patient's lung will see an inspiratory pressure of 25 cm of water and this pressure will hold the patient for a long time. hold constant throughout the duration of the inspiratory portion of the breath. With the volume control, the end inspiratory pressure, called the plateau pressure, is not achieved until much later in the breath.
Remember the alveoli recruitment we discussed earlier with PEEP? This is a similar concept. In volume control, fewer alveoli will be recruited to the breath compared to pressure control because the pressure builds instead of being constant for a longer period of time. portion of the breath in pressure control.
This can affect gas exchange and in particular oxygenation. We've now discussed controlled modes of ventilation and hybrid modes of ventilation. The last type of ventilator mode we need to discuss is the spontaneous mode of ventilation. We call this pressure support ventilation. In pressure support ventilation, we set a pressure support, a PEEP, at FiO2.
We do not set a respiratory pressure support at FiO2. Therefore, the unknown or measured variables on a purely pressure support mode of ventilation include tidal volume and respiratory rate. You cannot put a patient on pressure support who is paralyzed or has no drive to breathe or is brain dead, as no breaths will be generated. In the intensive care units, you will see assist control modes of ventilation and pure pressure support modes of ventilation. You will very rarely, if ever, see SIMV modes of ventilation.
SIMV used to be used in the ICUs as a weaning mode. However, studies have demonstrated that SIMV may actually increase work of breathing, causing fatigue and prolong the time to extubation. Sometimes, we do use SIMV in the operating room towards the end of the case while we are awaiting return of spontaneous breathing. In the ICUs and in the operating rooms prior to extubation, we place the patient on a spontaneous breathing trial, or SBT. Usually, an SBT is pressure supported with minimal pressure.
For example, a pressure support of 5 to 7 centimeters of water on top of a PEEP of 5. This is designated as PS 5 over 5. In the ICUs, this trial is performed for 30 minutes to 2 hours per hour. hours. During an SBT, you want to assess your patient's miniventilation, tidal volume achieved with this low level of support, gas exchange, and vital signs. SBTs are performed with an FiO2 of 40 to 50 percent. Some of the criteria we'll look at for extubation include a patient must be neurologically intact enough to protect his or her airway, have manageable secretions, be able to maintain good oxygenation, and and ventilation on these minimal settings, and finally, the reason that the patient was initially intubated should be resolved or resolving.
In summary, you have learned how to choose a basic ventilator mode including the basic differences between controlled and spontaneous modes of ventilation and the basic differences between volume and pressure modes of ventilation. how to set initial ventilator settings for a patient, and how to change your initial ventilator settings based upon your patient's requirements. I hope this has been helpful in demystifying mechanical ventilation for you.
I think you will find that managing patients on mechanical ventilation is interesting and fun. Now you have a good foundation in ventilator basics. Thank you.