Mechanical Ventilators



Mechanical Ventilators


Edward Hoskins



Objectives


Upon the completion of the chapter activities and content review topics, the student should be able to:


1. Prepare a mechanical ventilator for use.


2. Place the ventilator in the various modes of ventilation.


3. Identify various modes using wave forms.



Key Terms












Mechanical ventilators have been in use for many decades. The recent advances in technology have provided the respiratory therapist (RT) with highly sophisticated computerized life support devices with capabilities we could only have dreamed of 40 years ago.


Management of a patient on ventilatory support is one of the most difficult tasks a respiratory therapist will have to execute. An understanding of the information provided by wave forms will help the RT recognize problems with the patient–ventilator system and make quick changes. It is essential for the RT to be knowledgeable in anatomy, physiology, pathophysiology, clinical assessment, pharmacology, science, and technology. Also, it is important to remember the patient’s life and safety are our most important responsibility, and the ventilator is a tool to help achieve that goal. This chapter will focus on the science and technology aspects involved in the proper setup of a mechanical ventilator, as well as modes of ventilation with wave form analysis.



image Reality Check As a RT, you must be current on all aspects of professional practice relating to your clinical responsibilities. No one else in the clinical environment will have the depth of knowledge to provide the patient the best chances of survival once placed on mechanical ventilation. The health care system demands that our practice be evidence based. The best sources to maintain currency and provide evidence-based practices are peer-reviewed journals.



» Skill Check Lists


20-1 Preparing a Mechanical Ventilator for Use


It is critically important to ensure that each mechanical ventilator is cleaned, that a new circuit is applied, and that the ventilator has passed an internal diagnostic test before its use and it is available and ready to go when needed. When a patient is in need of ventilatory support, delays in the initiation of ventilatory support increase the patient’s risk of complications. It is imperative that the RT understands and knows how to complete this process correctly. The following is the step-by-step process for preparing a mechanical ventilator for use.







20-2 Wave Form Analysis


Several different modes of ventilation exist, and it is important for the RT to select the appropriate mode of ventilation based on the patient’s condition and the physician’s orders. The most common wave forms seen together on the screen of critical care ventilators are pressure, volume, and flow. RTs must also be able to recognize the output wave form associated with each mode and how the wave forms are affected with change in compliance and resistance. We will be using scalar graphics with three different wave forms: (1) volume-time, (2) flow-time, and (3) pressure-time to represent each breath of the patient. The beginning of inspiration, the end of inspiration, the beginning of exhalation, and the end of exhalation can be seen on each of the waveforms.



Spontaneous Modes



Continuous Positive Airway Pressure

Continuous positive airway pressure (CPAP) is a spontaneous mode of ventilation.



image Reality Check In CPAP it is very important to set the apnea alarm appropriately. In addition, the low tidal volume and low respiratory rate alarms should be set closely to the patient’s actual tidal volume and rate so that deviations or patient failure on CPAP are detected sooner rather than later when apnea ventilation sets in.


In this mode, the RT sets a pressure level that is maintained by the ventilator throughout breathing, and the patient’s tidal volume and respiratory rate are totally dependent on his or her effort. Therefore, the baseline on the pressure-time scalar graphic will be above atmospheric pressure. The baseline pressure is raised to cause an increase in the patient’s functional residual capacity (FRC). It should look similar to the graphics in Figure 20-1, which shows an illustrations of eight breaths. At the beginning of each breath on the graphic, a slight drop in pressure is seen. This is the patient’s inspiratory effort. The pressures during this mode ideally will fluctuate plus or minus 2 or 3 cm H2O in relation to the set baseline pressure.


image
Figure 20-1 With continuous positive airway pressure (CPAP) a caregiver-defined level of pressure within the ventilatory circuit is maintained. Changes in the airway pressure with inspiration and expiration are constantly compensated for by pressure generated by the ventilator. PEEP, positive end-expiratory pressure. (From Spiro SG, Silvestri GA, Agustí A: Clinical respiratory medicine, ed 4, Philadelphia, PA, 2012, Saunders.)


Pressure Support Ventilation

Pressure support ventilation (PSV) is often used in conjunction with CPAP and provides assistance to a spontaneous breath. A pressure support (PS) level is set by the RT, and once the patient’s inspiratory effort surpasses the trigger threshold, the PS is delivered. Parameters that the RT may adjust in PSV are the PS level, the trigger sensitivity threshold, the inspiratory rise time, and, occasionally, the terminal flow value. As in CPAP, the patient in PSV can initiate a breath as often as desired, and the PS breath will augment the patient’s inspiratory effort. This is illustrated in Figure 20-2. The breath is controlled by the patient, and inspiration ends or cycles off as a result of terminal inspiratory flow. The terminal inspiratory flow is set to an amount based on a percentage of the patient’s peak inspiratory flow. Some ventilators have a fixed value of 25% of the patient’s peak inspiratory flow. In addition, some ventilators will allow the operator to adjust the percent of terminal inspiratory flow to higher or lower values, depending on the patient’s needs. The changes in the parameters of a PSV are illustrated in Figure 20-3. A PS breath results in a descending flow pattern.


image
Figure 20-2 Pressure support ventilation (PSV). The delivered tidal volume (VT) is calculated by integrating the area under the flow curve (shaded area). The inspiratory rise time defines how fast the maximal airflow (100%) is achieved. Thereafter, the inspiratory airflow continuously decreases because once the pressure target is reached, maintaining this level requires progressively less air to flow into the lungs. As soon as the cycling-off air flow threshold (i.e., a preset percentage of maximal air flow) is reached, the ventilator ceases to deliver inspiratory flow, and the expiratory valve is opened to allow passive exhalation. (From Spiro SG, Silvestri GA, Agustí A: Clinical respiratory medicine, ed 4, Philadelphia, PA, 2012, Saunders.)

image
Figure 20-3 Changes in the parameters of PSV result in characteristic changes of the pressure and volume curves. Panels “A” to “D” demonstrate isolated changes of parameters, assuming that all other ventilatory parameters and the mechanical characteristics of the respiratory system remain unchanged. A, PSV breath with an inspiratory rise time of 0.20 second. B, After reducing the inspiratory rise time to its minimum, the peak inspiratory flow and consequently also the cycling-off air flow threshold are both reached earlier. Decreasing the inspiratory rise time results in a shorter inspiratory time, while the tidal volume (VT) remains unchanged. Note that although the inspiratory rise time is decreased to 0 seconds, the peak inspiratory flow is reached with a small delay. C, Increasing the cycling-off air flow threshold from 30% to 50% similarly shortens the inspiratory time; however, VT decreased in this case. D, A combination of a maximal decrease in the inspiratory time and a moderate increase in the cycling-off air flow threshold shortens the inspiratory time, whereas the loss of VT is only minimal. Such an approach may be used to achieve a prolongation of the expiratory time in patients at risk for dynamic hyperinflation because of the expiratory flow limitation (e.g., patients with chronic obstructive pulmonary disease [COPD]). (From Spiro SG, Silvestri GA, Agustí A: Clinical respiratory medicine, ed 4, Philadelphia, PA, 2012, Saunders.)


Controlled Modes



Volume Control

Volume control ventilation is a mandatory mode, in which the tidal volume and flow rate remain constant breath to breath, and the airway pressure applied varies, depending on the patient’s airway resistance and lung thorax compliance. This mode is represented in the scalar graphic in Figure 20-4. For the graphic “A,” the tidal volume peaks at 0.5 liters (L) and does not vary. For the graphic “B,” the peak flow is 400 liters per second (L/s) and does not vary. This inspiratory flow time wave form is most often described as square. The inspiratory time (I-time) is 1.3 seconds and does not vary. Graphic “C” reflects a peak airway pressure of 25 cmH2O. This wave form is described as exponential, accelerating, or a shark-fin pattern. Remember that volume and flow will not vary, causing an increase in peak airway pressure, if the patient’s airway resistance increases, lung compliance decreases, or both. Conversely, the peak airway pressure will decrease if the patient’s airway resistance decreases or lung compliance increases. During volume control, the base line pressure may be increased, which will, in turn, increase the patient’s FRC. This effect may be seen on graphic “C.” At the end of each breath, the baseline pressure is held to 5 cmH2O. This indicates the amount of PEEP set.


image
Figure 20-4 Volume control ventilation with PEEP.

In assist control–volume control–ventilation (AC/VC), a minimum respiratory rate is set on the ventilator, but the patient can initiate and receive more than that set rate. This is done by patient effort reaching the trigger sensitivity. Each subsequent breath will be the set mandatory breath, as in the example on the scalar graphic in Figure 20-4, and each and every tidal volume will be 0.5 L.



Pressure Control Ventilation

Pressure control ventilation is a mandatory mode, in which the amount of pressure applied during each breath is controlled and does not vary with patient characteristics (see Figure 20-5). As a result, the patient’s tidal volume and peak inspiratory flow rate vary, depending on airway resistance, lung compliance, or both. During this mode, if the patient airway resistance increases or lung compliance decreases, the tidal volume the patient receives decreases. The peak inspiratory pressure (PIP) is considered to be constant during inspiration and is described as a square wave form. This is shown on the pressure-time scalar graphic “C” in Figure 20-5. The flow pattern is described as descending and is illustrated on the scalar graphic “B” in Figure 20-5. As in the other modes, PEEP may be added. The amount of PEEP illustrated in Figure 20-5 is 5 cm H2O. During this mode, the operator sets the PIP, respiratory rate, I-time, fractional inspired oxygen FiO2

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Jun 12, 2016 | Posted by in RESPIRATORY | Comments Off on Mechanical Ventilators

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