Ventilators: Part I
Rob Orman MD and Haney Mallemat MD
▪ The three T’s of mechanical ventilation are trigger, target and termination.
▪ Assist control mode delivers a set number of breaths to the patient. SIMV and pressure support modes allow the patient to trigger their own breaths.
▪ The recommended tidal volume is 6-8cc/kg of ideal body weight.
▪ Volume is the preferred target for metabolic problems such as diabetic ketoacidosis while pressure is the preferred target for patients with stiff lungs such as ARDS.
▪ The ventilator terminates the breath in volume mode after the volume is delivered. In pressure mode, the breath terminates after a certain amount of time.
● The ventilator is not a complicated machine. It is a big bellows that squirts air into the patient.
● Why do we intubate people?
1. Airway protection. For example, the patient is not tolerating secretions, the tongue is swelling, etc. Their intrinsic drive to breath is intact.
2. Patients that are having difficulty oxygenating their alveoli or ventilating.
3. Patients that are getting tired of breathing or have a metabolic demand that requires control of their respirations. For example, patients with DKA or sepsis who are tiring, etc.
● We need to understand the three T’s of mechanical ventilation; trigger, target and termination.
1. Who triggers the vent? Is it the patient who determines when the breath is delivered or is it the machine?
o If you have a patient who is completely obtunded and has no intrinsic drive to breathe, you will have to determine their mode of breath. The ventilator will do this based on a time. If we set the rate of the ventilator to 10, the ventilator will determine that a breath needs to be delivered every six seconds. The machine will do all the work. Depending on the mode, the ventilator will do different things if a breath is initiated in between.
o If the patient is intubated for airway protection (for example, angioedema), they maintain a drive to breathe. Once the tube is placed, the patient can determine when they want a breath initiated. The ventilator has pressure sensors that detect negative pressure when a breath is pulled in by the patient.
o The new ventilators have flow cycling which is a circle of air that goes between the inspiratory and expiratory limb. When a breath is initiated by the patient, it breaks the circle and tells the ventilator to give a breath.
● What are the different modes of ventilation?
o The classic mode is the AC (assist control) mode. You determine the respiratory rate for the patient. If the ventilator is set at a rate of 10, it will deliver a breath every 6 seconds. If the patient wakes up and tries to take a breath, the ventilator will also deliver a breath at the set tidal volume. If the ventilator is delivering a breath and another breath is initiated, it will deliver another breath on top. This can lead to breath stacking and respiratory alkalosis. The patient who is awake and agitated will take more breaths than you intended and decrease their CO2. There is no maximum and you can trigger as many breaths as you want.
o SIMV. With SIMV, you are guaranteed to get the determined rate of breaths just like assist control. When the patient takes a breath, one of two things will happen. If the breath lies directly between two set breaths, the ventilator won’t do anything to assist. The person has to do all the work of breathing unassisted. If the patient-initiated breath is near the scheduled set breath, the ventilator will give them the breath at the specified tidal volume.
o Pressure support and pressure control. Pressure support can also be applied to the ventilator for the unsupported breaths in SIMV. If you want to be nicer to your patients, you can apply a little bit of pressure support to give them a little extra background support. Pressure support assists the breath. You can dial in the background pressure needed to make the breath happen.
● What is an exclusively patient-initiated ventilator setting? The pressure support mode is completely patient derived. You enter the desired pressure. This will depend on the patient. If you have patient with a standard endotracheal tube with normal lung compliance and non-rigid chest wall, you can start with pressure support of 12-15. Watch what happens with their tidal volumes with the first few breaths. If the tidal volume is too low despite the pressure support of 15 (due to very large breaths, stiff chest wall or lungs, etc), you may need to increase the pressure. Watch the tidal volumes.
● Can patients become apneic or will the ventilator always rescue them? Most ventilators have a background safety that will initiate a set rate if a specified apnea time is detected. Pressure support mode has no rate. You can adjust the apnea time.
2. What are your targets? Your two targets are volume or pressure. There is much debate in the literature regarding which is preferred. Traditionally, we target volume in the ED. We set a tidal volume based on ideal body weight with low tidal volume (6-8 cc/kg of ideal body weight). The assist control has a volume mode. The SIMV has a pressure or a volume setting. Your independent variable is volume; you need to be cognizant of your dependent variable of pressure. Look at the plateau pressure. If your plateau pressure is less than 30, great. If the plateau pressure is greater than 30, you need to consider that the tidal volume is too high.
o Should you switch to pressure mode if you are unable to control the plateau pressure by adjusting the volume? Whether you pick volume or pressure, you still need to be aware of the dependent variable. If you pick pressure, volume will be your dependent variable depending on the patient’s physiology. The volumes might still be too low with that pressure.
o Which patients should have a target of volume versus pressure? If you are trying to titrate for a metabolic problem such as an acidotic DKA patient, it is easier to follow and titrate the tidal volume and respiratory rates with volume. A patient with stiff lungs such an ARDS patient should have a target of pressure. When you are applying a pressure to a patient, you are slowly filling up that lung and inflating all the nooks and crannies. When you are applying a volume, you are just doing general flow to the lung.
o Pressure mode is useful when you don’t want to exceed a certain pressure in the lung. Volume is a good option for everyone else.
o Peak pressures are the summation of the lung stiffness, chest wall compliance and the resistance. These are not very important. It is more important to follow the plateau pressure. You can’t look at a peak pressure in isolation. What is the plateau pressure? If you have high peak pressures and high plateau pressures, you have a problem with the lungs that you need to fix. If you have high peak pressures but normal plateau pressures, you need to consider resistance. Is the tube kinked? Is there a big mucous ball in the lungs or tube? Does the patient have COPD or asthma? Do you need to give more bronchodilators? The peak pressure is a product of the resistance and the compliance of the lungs.
3. When are you going to terminate the breath? If you are using a volume mode of ventilation, you are prescribing a flow to the patient. When that flow is administered over a certain amount of time, you get your volume.
o When does the ventilator terminate the breath in volume mode? After the volume is delivered. Then, the passive phase of expiration begins.
o Pressure is different. You tell the ventilator, “Give me 20 cm H20”. And the ventilator says, “How long do you want that breath applied?” Applying that pressure over 0.5 seconds will give a certain tidal volume versus over 1 second which will give you more volume. In pressure mode, the breath terminates after a certain amount of time.