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 must first complete the two modules: 1 Pulmonary ventilation, natural and

mechanical

2 Ventilation system concept Review both modules if you have not already done so. In this module, we will learn about five

essential

variables

and eight

types

of mechanical respiration. Essential

variables

serve as the basis for mechanical respirations, while mechanical respirations serve as the basis for ventilation modes. The

essential

variables are directly related to the main control parameters of the fan. They belong to the core of intermittent positive pressure ventilation (IPPV) regardless of the brands and models of ventilators.

After completing this module, you should be able to correctly answer the following questions: What are respiratory cycle time, inspiratory time, and expiratory time? What are activation, cyclical, control, focusing and baseline pressure, and their mechanisms. What mechanical respiration is. What are the properties of eight

types

of mechanical respiration. It will take you 30 minutes to complete. Mechanical respiration and timing Mechanical ventilation can be seen as a process composed of a series of mechanical respirations. A mechanical respiration is defined as any

breath

performed through a ventilation system. There are different types of mechanical respiration. To carry out mechanical ventilation, a ventilator must receive commands from its operator about: When inspiration should start and stop.

When the expiration should begin and end. How the delivery of inspiratory gas should be controlled. How big the mechanical

breath

should be. How high the initial pressure should be. Each mechanical breath takes a certain amount of time to complete. The duration is called the respiratory cycle time (BCT), which always contains the inspiration time (Ti) and the expiration time (Te). Normally, You come first. During inspiration, a ventilator delivers gas to a ventilation system, causing airway pressure to increase and the lungs to inflate. During expiration, the ventilator stops the gas supply and allows gas to exit the ventilator system, causing airway pressure to drop and the lungs to deflate.

BCT, Ti and Te are specified with two timing events, activation and cycle. Both are essential variables. Trigger Trigger refers to the moment at which inspiration begins. Cycling refers to the moment when inspiration ends. So, Ti is the duration from one trigger point to the next cycle point. Te is the duration from one cycle point to the next trigger point. BCT is the duration from one activation to the next activation. Shooting can be done with different mechanisms. The most common are: Time Activation Pressure Activation Flow Activation Both pressure and flow activation are considered patient activations. Time activation is also known as machine activation 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: Breathing cycle time in seconds equals 60 divided by the set rate in breaths per minute. In other words, each established exchange rate has a corresponding BCT. For example, if you set the speed to 10 b/min, the resulting BCT will be 6 seconds. If you set the speed to 20 b/min, the resulting BCT is 3 seconds. The scale shows commonly used rates and corresponding BCTs. A BCT is the interval between two consecutive trigger points. Every time the set BCT ends, the ventilator delivers inspiratory gas.

Currently, all ventilation modes allow the patient to activate, unless the patient activator is deliberately disabled. If so, an active patient can dominate the actual rate, which is usually higher than the set rate. However, if the patient is passive, the actual rate and the set rate are the same. Therefore, the established rate usually serves as a minimum or backup rate. Pressure Activation Pressure activation is a form of patient activation. It is based on monitoring the pressure of the circuit or airways. One way to understand the triggering of pressure can be to suck on an empty wine bottle.

Your effort generates negative pressure inside the bottle but little gas flow. The harder you suck, the more negative the pressure. If the pressure release is activated, the breathing circuit serves as a “wine bottle.” The patient's inspiratory efforts cause the airway pressure to decrease from the current baseline value. If the pressure drop reaches a set threshold, the ventilator activates and begins delivering inspiratory gas. To use pressure trigger, you must enable pressure trigger and set the pressure trigger sensitivity. 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 example, if the PEEP setting is 5 cmH2O and the trigger sensitivity is set to -2 cmH2O, the ventilator activates if the actual airway pressure drops to 3 cmH2O or less. The lower the absolute value of the sensitivity, the more sensitive the trigger will be and vice versa. Therefore, the pressure trigger of -0.5 cmH2O is more sensitive than that of -2.0 cmH2O. Flow Activation Flow activation is another form of patient activation. It is based on monitoring the flow of the circuit or airways. For ease of understanding, let's first specify three types of flow: Flow A: The gas flow measured in the inspiratory branch of the circuit.

Flow B: The gas flow measured in the expiratory branch of the circuit. Airway flow: The flow of gas measured in the airways. It can be inspiratory or expiratory. Furthermore, let's divide the expiration time into early expiration and later expiration. Early expiration In early expiration, the ventilator closes its inspiratory valve and fully opens its expiratory valve to obtain maximum expiratory flow. The high pressure of the circuit expels the gas and the airway pressure drops sharply. Both B flow and expiratory airway flow increase rapidly to the maximum and then gradually decrease as the circuit pressure falls. Late expiration In late expiration, the ventilator attempts to rebuild and maintain the set base pressure by reducing the expiratory valve opening for high resistance against expiratory flow and slightly opening the inspiratory valve for a constant base flow, which is crucial for flow activation. .

If the patient does not inhale, both flow A and flow B are equal to the base flow and the airway flow is zero. If the patient inhales, a portion of the base flow goes to the patient, resulting in a decreased inspiratory airway flow and a decrease in flow B. If the inspiratory airway flow detected or the difference between flow A and B reaches a defined threshold, the fan turns off. motivated. To use flow triggering, you must enable flow triggering and then set the flow trigger sensitivity. Sensitivity is expressed in liters per minute, such as 0.5, 1, 2 or 5 liters per minute.

The lower the set value, the more sensitive the flow trigger is and vice versa. For a ventilated patient, flow triggering is generally easier than pressure triggering under the same conditions of use. Abnormal Patient Activation Unlike time activation, patient activation, both pressure and flow, can fail in two ways. The first way is that a ventilated patient has inspiratory effort, but the ventilator is not responding. Common causes are that the patient's effort is too weak or the patient's activation environment is not sensitive enough, or both. The second way is automatic activation, which means that the ventilator activates when the patient does not inhale.

Normally, automatic activation appears as a series of rapid, rhythmic mechanical breaths. Automatic activation occurs when the ventilator is activated by pneumatic devices, but not by the patient's expected inspiratory effort. Artifacts are usually the result of a gas leak, water condensed in the circuit or even cardiac oscillation. Another possible cause of automatic tripping is that the pressure or flow trigger is set too sensitive. The best remedy for automatic activation is to eliminate the root cause. If this is not possible, you can carefully decrease the sensitivity of the patient trigger until the self-timer disappears. Note that this makes triggering more difficult for the patient.

Cycling Cycling refers to the end of inspiration. Determines the duration of the 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 loops and flow loops are commonly used loop mechanisms. Cycle Time With cycle time, you can define Ti in several ways, including: Ti I:E Ratio Peak Flow Cycle Time by Setting Ti In this case, an operator directly sets Ti in seconds. The ventilator switches from inspiration to expiration when the set Ti ends. This method applies to both volume and pressure breaths.

Cycle Time Setting the I:E Ratio Each respiratory cycle time (BCT) has two portions: Ti and Te The relationship between Ti and Te is the I:E ratio. The number to the left of the colon represents the Ti portion and the number to the right, the Te portion. Suppose we set the rate at 10 breaths per minute for a 6 second BCT. If the I:E ratio is set to 1 to 2, the resulting Ti is 2 seconds and Te is 4 seconds, respectively. The advantages of I:E ratio are: A change in I:E ratio or set rate causes corresponding changes in both Ti and Te and the relationship between Ti and Te is clearly shown.

The disadvantage is that you don't get Ti and Te second without mental calculation. Note: In an active patient, the actual respiratory cycle time, Ti and Te, may differ from the expected values. The I:E ratio method applies to both volume and pressure breaths. Time cycles setting maximum flow It's a little confusing: How can Ti be defined by setting maximum flow? Peak flow refers to the maximum inspiratory flow. This method applies only to volume breaths with a square flow pattern, meaning that a ventilator delivers the gas at a constant rate during inspiration. In this condition, Vt is the product of Ti and the maximum flow.

Therefore, Ti increases when the set tidal volume increases or the maximum set flow decreases, or both. Ti decreases when you do the opposite. Flow Cycles Flow cycles are another way to end 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 pressure breathing, the inspiratory flow is not controlled. Normally, the inspiratory flow increases rapidly to the maximum at first and then gradually falls to zero. The flow cycle works with the descending part of the inspiratory flow. The maximum inspiratory flow is considered 100% regardless of its absolute value.

A ventilator switches from inspiration to expiration if the inspiratory flow drops to a preset percentage level. If the flow cycle is adjustable on your fan, you may find the flow cycle control. On all Hamilton Medical ventilators, this control is labeled Expiratory Trigger Sensitivity (ETS). You can set it to any value between 5% and 80%. This control is not available if the flow cycle is set at the factory. Flow cycling allows you to influence the Ti of support breaths. The lower the percentage set, the longer the Ti and vice versa. Flow cycling is very useful to improve patient-ventilator synchrony.

Flow cycling may fail if the system has a massive leak and the ETS is set too low, because the inspiratory flow cannot drop to the set cycling level. The consequence is infinite inspiration. This is clinically unacceptable. To avoid this possibility, a fan has a backup cycle time, which can be adjustable by the user. If so, you can find a control for that. On Hamilton Medical ventilators, this control is called maximum inspiratory time or Ti max. Control The third essential variable is control. Control is defined as the mechanisms by which a ventilator controls gas delivery during inspiration.

There are only two types of control: Volume control Pressure control At any given time, a fan can only control volume or pressure, but not both. Pressure control has a variant called adaptive control. Finally,some fans offer hybrid control. We will explain them to you one by one. Volume control or flow control A better term for volume control is flow control, because during inspiration a ventilator actually controls the flow of inspiratory gas delivered to the circuit. At the end of inspiration, the set tidal volume is delivered. Volume control is provided by three main controls: Vt Ti Peak Flow The ventilator automatically calculates the third control setting.

The volume control may have a secondary control: flow pattern (inspiratory). The square flow pattern is the most common. Some ventilators provide only a square flow pattern for volume breaths. However, some other fans provide more flow patterns. For example, the HAMILTON-G5 and GALILEO fans provide square, downward and sine flow patterns. The main perceived advantage of volume control is the stable tidal volume, as well as the minute volume that we can feel comfortable with. However, volume control has four inherent disadvantages: With volume control, 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 control has slowly but steadily given way to pressure control. With volume control, the tidal volume received by a ventilated patient is always less than that delivered to the circuit by a ventilator. This is due to the compression of gas in an elastic breathing circuit. The tidal volume difference is an invisible volume loss. For example, if you set the tidal volume to 500 mL, the patient may only receive 450 mL. This invisible volume loss must be corrected by circuit compliance compensation. With volume control, leak compensation is impossible because a fan supplies exactly the set volume to the circuit.

With volume control, the maximum pressure is variable, depending on the set Vt, maximum flow, the patient's respiratory resistance and compliance, and respiratory efforts. Constantly high maximum pressure can damage the lungs. Pressure Control With pressure control, a ventilator first draws a target airway pressure profile based on 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 far below the target pressure. Decreases inspiratory flow if the monitored pressure is slightly below the target pressure.

Stops inspiratory flow if the monitored pressure matches the target pressure. Pressure control is made up of two main controls: Ti and inspiratory pressure. Inspiratory pressure refers to the expected positive pressure applied above the PEEP. It propels the gas to reach the lungs. On Hamilton Medical ventilators, the inspiratory pressure is labeled Pressure Control for Controlled or Assisted Breathing. Or as pressure support for assisted breathing. The pressure control can have a secondary control: Pressure Ramp (Pramp) or Rise Time. It is defined as the time required for airway pressure to reach a target pressure at the beginning of inspiration.

A short Pramp means rapid pressurization and vice versa. If you compare the volume control to a dictator, the pressure control is somewhat liberal because the Vt and inspiratory flow can vary according to the patient's demand. Because of this important property, patient-ventilator asynchrony occurs much less frequently. When the system leaks, causing a drop in circuit pressure, the ventilator responds by increasing inspiratory flow. This is how leak compensation works. With pressure control, a fan can effectively compensate for a moderate leak. With pressure control, the tidal volume is variable, depending on the set inspiratory pressure, the patient's respiratory resistance and compliance, and the patient's respiratory efforts.

Under unfavorable conditions, the resulting tidal volume may be too large or too small. It is important to correctly configure tidal volume alarms to protect the patient. Adaptive Control Adaptive control represents the result of efforts to exploit the advantages and minimize the disadvantages of volume and pressure control. Adaptive control is a variant of pressure control. With pressure control, the inspiratory pressure is set, the maximum pressure remains stable, and the resulting Vt can vary. With adaptive control, the inspiratory pressure is automatically regulated breath-by-breath to match the monitored Vt with the target Vt set by the operator. The ventilator does the following: Increases inspiratory pressure if the monitored Vt is below target.

Decreases inspiratory pressure if the monitored Vt is above the target. It maintains the inspiratory pressure unchanged if both tidal volumes are equal. Adaptive control may be mistakenly perceived as volume control 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 respirations show the similarities and differences. The main advantages of adaptive control are: The actual Vt can be quite stable, especially in passive patients. Maintains most of the advantages of pressure control. For optimal performance, adaptive control has a difficult, but rarely mentioned, requirement for the operator: the target volume must always adapt to the patient's current ventilatory demand, which may change over time.

An unfavorable scenario is when the target volume is set below the current demand. The patient has to breathe harder to meet the demand. When the monitored Vt exceeds the set target, the ventilator reduces the inspiratory pressure. In the end, the patient does all the work of breathing and the ventilator does little. Hybrid control Hybrid control is defined as the application of both: pressure control and volume control within the same mechanical respiration. In Chatburn's taxonomy of mechanical ventilation, hybrid control is called "dual control." Typical examples include the Volume Assured Pressure Support (VAPS) mode of the Bird 8400 STi ventilator and the volume control mode of Maquet ventilators.

We will not delve into hybrid control because it is complicated to understand and has not become popular until now. Focus Focus 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 combined with the type of control. The target parameter is: Tidal volume with volume control Inspiratory pressure with pressure control Target tidal volume with adaptive control When the set target is reached, the ventilator stops supplying more gas to the circuit. However, achieving the goal does not necessarily mean an immediate cycle from inhalation to exhalation.

PEEP stands for positive end expiratory pressure and is the initial pressure above which positive pressure is applied intermittently. This is the fifth and last essential variable. PEEP is expressed in cmH2O and counts from zero or from atmospheric pressure. PEEP is adjustable on all ventilators. PEEP: initial pressure PEEP is generated by the 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 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.

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 biphasic modes where PEEP automatically switches between two set levels. From variables to mechanical respirations So far we have learned the five essential variables. With this knowledge, we are now ready to define the types of mechanical respiration. This task requires only three variables: Activate Cycles Control PEEP is applicable to all types of breathing and targeting is combined with control. Classification of mechanical respiration types Here is a 3 x 3 matrix for the classification of mechanical respiration types: One dimension is based on activation and cycling.

Depending on the selection, a mechanical breath can be a control breath, an assisted breath, or a support breath. The other dimension is based on control. Depending on the selection, a mechanical breath can be a volume breath, a pressure breath, or an adaptive breath. Combining the two dimensions we obtain eight types of breathing as shown. Volume-controlled breathing, which is time-triggered, time-cycled, and volume-controlled. Pressure-controlled breathing, which is time-triggered, time-cycled, and pressure-controlled. Adaptive supportive breathing, which is time-activated, time-cycled, and adaptively controlled. Volume-assisted breathing, which is activated by pressure or flow, has time cycles and volume control. Pressure-assisted breathing, which is pressure- or flow-activated, time-cycled, and pressure-controlled.

Adaptive assisted breathing, which is activated by pressure or flow, has time cycles and adaptive control. There is no breathing with volume support because flow cycling is impossible with volume control. Note: There is a mode called "Volume Support". Do not confuse breathing type and ventilator mode. Pressure support breathing, which is pressure or flow activated, flow cycled, and pressure controlled. Adaptive support breathing, which is activated by pressure or flow, cycles flow and is adaptively controlled. What is shown here is a summary of the 8 types of breathing with their properties. These types of mechanical breathing are vitally important because they form the basis of conventional and adaptive modes.

When faced with an unfamiliar ventilation mode, if you can correctly identify its essential variables and breathing types, you will be able to know what it can do and where it can be used, regardless of its given name. In this module, we have learned five essential variables: Cycling Activation PEEP Target Control Knowing the essential variables is very useful because the variables are directly related to the ventilator control parameters. Applying this knowledge, we identified eight types of mechanical respiration. Types of mechanical respiration are the basis of ventilation modes. Now we are well prepared to move on to the next module "Mechanical ventilation modes".