e-Learning: Essential variables and mechanical breath types
This module is the third in a series on the basics of mechanical ventilation and ventilators. You should first complete the two modules: 1 Lung ventilation, natural and mechanical 2 Ventilator system concept Review the two modules if you have not. In this module, we will learn about five essential variables and eight mechanical breath types. The essential variables serve as the foundation of mechanical breaths, while mechanical breaths are the foundation of ventilation modes. The essential variables are directly related to the major ventilator control parameters. They belong to the core of intermittent positive
pressure ventilation (IPPV) regardless of ventilator brands and models. After completing this module, you should be able to answer the following questions correctly: What breath cycle time, inspiratory time and expiratory time are. What triggering, cycling, controlling, targeting and baseline pressure, and their mechanisms are. What a mechanical breath is. What the properties of eight mechanical breath types are. It will take you 30 minutes to complete. Mechanical breath and timing Mechanical ventilation can be viewed as a process composed of a series of mechanical breaths. A mechanical breath is defined as any breath realized through a ventilator system. There are different mechanical breath types. To carry out mechanical ventilation, a ventilator must receive commands from its operator for: When inspiration should start and stop. When expiration should start and stop. How the delivery of inspiratory gas should be controlled. How big the mechanical breath should be. How high the baseline pressure should be. Every mechanical breath takes a certain length of time to complete. The duration is referred as breath cycle time (BCT), which always contains inspiratory time (Ti) and expiratory time (Te). Typically, Ti comes first. During inspiration, a ventilator delivers the gas into a ventilation system, resulting in the airway pressure to rise and lungs are inflated. During expiration, the ventilator stops gas delivery and allows the gas exit the ventilator system, resulting in the airway pressure to drop
and the lungs are deflated. BCT, Ti and Te are specified with two time events, triggering and cycling. Both are essential variables. Triggering Triggering refers to the point in time when inspiration starts. Cycling refers to the point in time when inspiration ends. So, Ti is the duration from a triggering point to the following cycling point. Te is the duration from a cycling point to the following triggering point. BCT is the duration from a triggering to the next triggering. Triggering can be realized with different mechanisms. The common ones are: Time triggering Pressure triggering Flow triggering Both pressure and flow triggering are regarded as patient triggering. Time triggering is also known as machine triggering, and is based on BCT, which is defined with the set rate. Once the “rate” is set, the ventilator automatically converts the rate to BCT with a simple equation: Breath cycle time in second equals 60 divided by the set rate in breaths per minute. In other words, every set rate has a corresponding BCT. For instance, For instance, if you set the rate to 10 b/min, the resulting BCT is 6 seconds. If you set the rate to 20 b/min, the resulting BCT is 3 seconds. The scale shows the commonly used rates and the corresponding BCTs. A BCT is the interval between any two consecutive triggering points. Whenever the defined BCT is over, the ventilator delivers inspiratory gas. Currently, all ventilation modes allow the patient to trigger unless patient trigger is deliberately disabled. If so, an active patient can dominate the actual rate, which is often higher than the set rate. If the patient is passive, however, the actual rate and the set rate are equal. Therefore, the set rate usually serves as the minimum or backup rate. Pressure triggering Pressure triggering is a form of patient triggering. It relies on circuit or airway pressure monitoring. One way to understand pressure triggering may be that you suck an empty wine bottle. Your effort generates a negative pressure inside the bottle but little gas flow. The stronger you suck, the more negative the pressure is. If pressure trigger is activated, the breathing circuit serves as the “wine bottle”. The patient’s inspiratory efforts cause the airway pressure to drop from the current baseline. If the pressure drop reaches a set threshold, the ventilator is triggered and starts delivery of inspiratory gas. To utilize pressure triggering, you must activate pressure triggering and set the pressure trigger sensitivity. The sensitivity is a negative value in cmH2O, such as -0.5, -2 or -5 cmH2O. The value represents a pressure threshold below the current PEEP. For instance, if the set PEEP is 5 cmH2O, and the trigger sensitivity is set at -2 cmH2O, the ventilator is triggered if the actual airway pressure drops to or below 3 cmH2O. The smaller the absolute value of the sensitivity, the more sensitive the triggering, and vice versa. So, pressure trigger of -0.5 cmH2O is more sensitive than that of -2.0 cmH2O. Flow triggering Flow triggering is another form of patient triggering. It relies on circuit or airway flow monitoring. To make understanding easier, let us first specify three flow types: Flow A: the gas flow measured at the inspiratory limb of the circuit. Flow B: the gas flow measured at the expiratory limb of the circuit. Airway flow: the gas flow measured at the airway. It can be either inspiratory or expiratory. Further, let us divide expiratory time into early expiration and later expiration. Early expiration At early expiration, the ventilator closes its inspiratory valve, and fully opens its expiratory valve for a maximum expiratory flow. The high circuit pressure drives the gas out, and the airway pressure drops sharply. Both the flow B and the expiratory airway flow rise quickly to maximum and then decrease gradually as the circuit pressure drops. Late expiration At late expiration, the ventilator tries to rebuild and maintain the set baseline pressure by reducing the expiratory valve opening for a high resistance against the expiratory flow, and opening slightly the inspiratory valve for a constant base flow, which is crucial for flow triggering. If the patient does not inhale, both the flow A and flow B are equal to the base flow, and the airway flow is zero. If the patient inhales, a part of the base flow goes to the patient, resulting in an inspiratory airway flow and a decreased flow B. If the detected inspiratory airway flow or the difference between flow A and B reaches a defined threshold, the ventilator is triggered. To use flow triggering, you must activate flow triggering and then set flow trigger sensitivity. The sensitivity is expressed in liters per minute, such as 0.5, 1, 2 or 5 liters per minute. The smaller the set value, the more sensitive the flow trigger is, and vice versa. For a ventilated patient, flow trigger is generally easier than pressure trigger under the same conditions of use. Abnormal patient triggering Unlike time triggering, patient triggering, both pressure and flow, can fail in two forms. The first form is that a ventilated patient has inspiratory effort, but the ventilator does not respond. The common causes are either the patient’s effort is too weak, or the patient triggering setting is not enough sensitive, or both. The second form is auto-triggering, which means that the ventilator is triggered when the patient does not inhale. Typically, auto-triggering appears as a series of quick and rhythmic mechanical breaths. Auto-triggering occurs when the ventilator is triggered by pneumatic artifacts, but not by the expected patient’s inspiratory effort. The artifacts often result from gas leak, condensed water in the circuit, or even cardiac oscillation. Another possible cause of auto-triggering is that pressure or flow trigger is set to an overly sensitive level. The best remedy for auto-triggering is to remove the root cause. If this is not possible, you may carefully decrease the patient trigger sensitivity until auto-triggering disappears. Bear in mind that this makes triggering harder for the patient. Cycling Cycling refers to the end of inspiration. It determines the length of inspiration. BCT is the sum of Ti and Te , if BCT is given, an increase in Ti causes a corresponding decrease in Te , and vice versa. Time cycling and flow cycling are commonly used cycling mechanisms. Time cycling With time cycling, you can define Ti in several ways, including: Ti I:E ratio Peak flow Time cycling by setting Ti In this case, an operator directly sets Ti in seconds. The ventilator switches from inspiration to expiration when the set Ti is over. This method applies to both volume and pressure breaths. Time cycling by setting I:E ratio Every breath cycle time (BCT) has two portions: Ti and Te The ratio between Ti and Te is I:E ratio. The number on the left of colon represents the Ti portion and the number on the right the Te portion. Assume that we set the rate to 10 breaths per minute for a BCT of 6 seconds. If the set I:E ratio to 1 to 2. The resulting Ti is 2 seconds, and Te 4 seconds, respectively. The advantages of I:E ratio are: a change in either I:E ratio or the set rate causes corresponding changes in both Ti and Te and the relation between Ti and Te is clearly shown. The disadvantage is that you do not get Ti and Te in second without mental calculation. Note: In an active patient, the actual breath cycle time, Ti and Te may differ from the expected values. The I:E ratio method applies to both volume and pressure breaths. Time cycling by setting peak flow It is a bit confusing: How can Ti be defined by setting peak flow? Peak flow refers to peak inspiratory flow. This method applies only to volume breaths with a square flow pattern, meaning that a ventilator delivers the gas at a constant flow rate during inspiration. Under this condition, Vt is the product of Ti and peak flow. Therefore, Ti increases when you increase the set tidal volume or decrease the set peak flow, or both. Ti decreases when you do the opposite. Flow cycling Flow cycling is another way to terminate inspiration based on the change in inspiratory flow. It is designed for active patients and is the key
property of support breaths that we will discuss later. In a pressure breath, the inspiratory flow is uncontrolled. Typically, the inspiratory flow rises quickly to the peak at beginning and then drops gradually back to zero. Flow cycling works with the descending part of the inspiratory flow. The peak inspiratory flow is taken as 100% regardless of its absolute value. A ventilator switches from inspiration to expiration if the inspiratory flow drops to a preset level in percent. If the flow cycling is adjustable on your ventilator, you can find the control of flow cycling. In all Hamilton Medical ventilators, this control is labeled as expiratory trigger sensitivity (ETS). You can set it to any value between 5% and 80%. This control is unavailable if flow cycling is fixed at the factory. Flow cycling enables you to influence the Ti of support breaths. The lower the set percent, the longer the Ti is, and vice versa. Flow cycling is very useful for improving patient-ventilator synchrony. Flow cycling can fail if the system has a massive leak and the ETS is set very low, because the inspiratory flow can not fall to the set cycling level. The consequence is endless inspiration. This is clinically unacceptable. To avoid the possibility, a ventilator has a backup time cycling, which may be user-adjustable. If so, you can find a control for that. In Hamilton Medical ventilators, this control is called maximal inspiratory time or Ti max. Controlling The third essential variable is controlling. Controlling is defined as the mechanisms by which a ventilator controls gas delivery during inspiration. There are only two types of controlling: Volume controlling Pressure controlling At any given time, a ventilator can only control either the volume or the pressure, but not both. Pressure controlling has a variant called adaptive controlling. Finally, some ventilators offer hybrid controlling. We will explain them one by one. Volume controlling or flow controlling A better term for volume controlling is flow controlling, because during inspiration a ventilator actually controls the flow of inspiratory gas delivered into the circuit. At the end of inspiration, the set tidal volume is delivered. Volume controlling is shaped with three primary controls: Vt Ti Peak flow The ventilator calculates the third control setting automatically. Volume controlling may have a secondary control: (inspiratory) flow pattern. Square flow pattern is the most common one. Some ventilator provides only square flow pattern for volume breaths. However, some other ventilators provide more flow patterns. For example, the HAMILTON-G5 and GALILEO ventilators provide square, descending, and sine flow patterns. The major perceived advantage of volume controlling is stable tidal volume as well as minute volume with which we may feel comfortable. However, volume controlling has four inherent disadvantages: With volume controlling, a ventilator dictates all important aspects of inspiratory gas delivery. This is hardly acceptable if the patient is active. This explains why patient-ventilator asynchrony often occurs in volume modes. Due to these disadvantages, volume controlling has given way slowly but steadily to pressure controlling. With volume controlling, the tidal volume that a ventilated patient receives is always less than that a ventilator delivers into the circuit. This is due to gas compression in an elastic breathing circuit. The tidal volume difference is invisible volume loss. For instance, you set tidal volume at 500 ml, the patient may only get 450 ml. This invisible volume loss needs to be corrected by circuit compliance compensation. With volume controlling, leak compensation is impossible because a ventilator delivers exactly the set volume into the circuit. With volume controlling, the peak pressure is variable, depending on the set Vt, peak flow, the patient’s respiratory resistance and compliance, and breathing efforts. A constantly high peak pressure can damage the lungs. Pressure controlling With pressure controlling, a ventilator first draws a target airway pressure profile according to the settings. During inspiration, the ventilator dynamically adjusts the inspiratory gas flow to minimize the gap between the actual airway pressure and the target pressure profile. The ventilator increases the inspiratory flow, if the monitored pressure is much below the target pressure. It decreases the inspiratory flow, if the monitored pressure is a bit below the target pressure. It stops the inspiratory flow if the monitored pressure matches the target pressure. Pressure controlling is shaped with two primary controls: Ti and Inspiratory pressure. Inspiratory pressure refers to the intended positive pressure applied above PEEP. It drives gas to move into the lungs. In Hamilton Medical ventilators, inspiratory pressure is labeled either as Pressure control for controlled or assisted breath. Or as Pressure support for supported breath. Pressure controlling may have a secondary control: Pressure ramp (Pramp) or Rise time. It is defined as the time required for airway pressure to rise to a target pressure at the beginning of inspiration. A short Pramp means a fast pressurization, and vice versa. If volume controlling is compared to a dictator, pressure controlling is somewhat a liberal because Vt and inspiratory flow can vary per the patient’s demand. Because of this important property, patient-ventilator asynchrony occurs much less frequently. When the system has a leak, causing circuit pressure to drop, the ventilator responds with an increases inspiratory flow. This is how leak compensation works. With pressure controlling, a ventilator can effectively compensate a moderate leak. With pressure controlling, the tidal volume is variable, depending on the set inspiratory pressure, the patient’s respiratory resistance and compliance, and the patient’s breathing efforts. Under unfavorable conditions, the resulting tidal volume can be too large or too small. It is important to set the tidal volume alarms properly to safeguard the patient. Adaptive controlling Adaptive controlling represents the outcome of efforts to exploit the advantages and to minimize the disadvantages of both volume and pressure controlling. Adaptive controlling is a variant of pressure controlling. With pressure controlling, the inspiratory pressure is set, the peak pressure stays stable, and the resulting Vt may vary. With adaptive controlling, the inspiratory pressure is automatically regulated breath-by-breath to match the monitored Vt to the target Vt set by the operator. The ventilator does the following: Increases inspiratory pressure if the monitored Vt is below the target. Decreases inspiratory pressure if the monitored Vt is above the target. Keeps inspiratory pressure unchanged if the both tidal volume are equal. Adaptive controlling may be misperceived as volume controlling because tidal volume can be set in both cases. Other than that, they have nothing in common. Here the waveforms of volume, pressure and adaptive breaths show the similarities and differences. The major advantages of adaptive controlling are: The actual Vt can be rather stable, especially in passive patients. It keeps most advantages of pressure controlling. For optimal performance, adaptive controlling has a hard, yet rarely mentioned requirement for the operator: The target volume must be always adapted to the patient’s current ventilatory demand, which may change over time An unfavorable scenario is when the target volume is set lower than the current demand. The patient has to breathe harder to satisfy the demand. As the monitored Vt exceeds the set target, the ventilator reduces the inspiratory pressure. At the end, the patient does all the work of breathing, and the ventilator does little. Hybrid controlling Hybrid controlling is defined as the application of both:pressure controlling and volume controlling within the same mechanical breath. In Chatburn’s taxonomy of mechanical ventilation, hybrid controlling is named as “dual control”. Typical examples include volume assured pressure support (VAPS) mode of Bird 8400 STi ventilator and volume control mode of Maquet ventilators. We will not go deeper with hybrid controlling because it is complicated to understand and has not been popularized thus far. Targeting Targeting is the fourth essential variable. Targeting is also known as limiting with the same meaning. Targeting is a parameter used to define the size of a mechanical breath, and is always paired with the controlling type. The target parameter is: Tidal volume with volume controlling Inspiratory pressure with pressure controlling Target tidal volume with adaptive controlling When the set targeting is reached, a ventilator stops delivering further gas into the circuit. Reaching the target, however, does not necessarily mean immediate cycling from inspiration to expiration. PEEP stands for Positive End Expiratory Pressure and is the baseline pressure above which positive pressure is applied intermittently. This is the fifth and the last essential variable. PEEP is expressed in cmH2O, and counts from zero or atmospheric pressure. PEEP is adjustable in all ventilators. PEEP – baseline pressure PEEP is generated by interaction between the expiratory gas flow and the resistance imposed by the expiratory valve of the ventilator. PEEP alone is therapeutic as it can increase functional residual capacity (FRC), improve alveolar gas exchange, keep the lung units open, and even improve lung compliance. A moderate level of PEEP, 3 to 5 cmH2O, is generally recommended for all intubated and ventilated patients. A high PEEP may be clinically necessary for patients with restrictive lung diseases, such as ARDS. Avoid using zero PEEP although it is possible. PEEP is usually constant in all ventilation modes. An exception is in biphasic modes where PEEP alternates automatically between two set levels. From variables to mechanical breaths So far, we have learned all five essential variables. With this knowledge, we are now ready to define mechanical breath types. This task requires just three variables: Triggering Cycling Controlling PEEP is applicable for all breath types and targeting is paired with controlling. Classification of mechanical breath types Here is a 3 x 3 matrix for classification of mechanical breath types: One dimension is based on triggering and cycling. Depending on the selection, a mechanical breath can be either a control breath, an assist breath, or a support breath. The other dimension is based on controlling. Depending on the selection, a mechanical breath can be either a volume breath, pressure breath, or an adaptive breath. Combining the two dimensions we get eight breath types as shown. Volume control breath, which is time triggered, time cycled, and volume controlled. Pressure control breath, which is time triggered, time cycled, and pressure controlled. Adaptive support breath, which is time triggered, time cycled, and adaptive controlled. Volume assist breath, which is pressure or flow triggered, time cycled, and volume controlled. Pressure assist breath, which is pressure or flow triggered, time cycled, and pressure controlled. Adaptive assist breath, which is pressure or flow triggered, time cycled, and adaptive controlled. There is no volume support breath because flow cycling is impossible with volume controlling. Note: There is a mode called “Volume Support”. Do not mix up the breath type and the ventilator mode. Pressure support breath, which is pressure or flow triggered, flow cycled, and pressure controlled. Adaptive support breath, which is pressure or flow triggered, flow cycled, and adaptive controlled. What shown here is a summary of all 8 breath types with their properties. These mechanical breath types are critically important, because they form the foundation of conventional and adaptive modes. When facing an unknown ventilation mode, if you can identify correctly its essential variables and breath types, you can tell what it can do and where it can be used, regardless of its given name. In this Module, we have learned five essential variables: Triggering Cycling Controlling Targeting PEEP The knowledge of essential variables is very useful because the variables are directly related to ventilator control parameters. By applying this knowledge, we identified eight mechanical breath types. The mechanical breath types are the foundation of ventilation modes. Now we are well prepared to proceed to the next module “Mechanical ventilation modes”.