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4 Let’s start by reviewing the indications for mechanical ventilation. Primary indicators are a result of inadequate spontaneous ventilation, and pH may be the most reliable index. It makes little sense to mechanically ventilate a patient with COPD on the basis of a PaCO 2 of 70 mm Hg if their normal PaCO 2 is 60 to 70 mm Hg. In these patients, renal compensation usually occurs, and the pH is within normal range. The most reliable index of respiratory failure in chronic hypercarbic patients is the severity of the respiratory acidosis. On the other hand, the same high PaCO 2 in a previously healthy person would strongly suggest that ventilation should be supported. Hypoxemia with supplemental high flow oxygen is also a primary indicator; generally this is defined by a PaO 2 less than 60 mm Hg with a FiO2 greater than 50%. Arterial blood gas results will usually indicate respiratory failure. There are many disease states that can lead to respiratory failure.
5 These include inadequate lung expansion due to chest wall deformity or neuromuscular diseases; respiratory muscle fatigue, which is usually caused by excessive work of breathing; postoperative prophylaxis; closed head injury where the objective is to reduce PaCO 2 ; and flail chest. We have briefly discussed some of the clinical indicators for mechanical ventilation, but let’s be a little more definitive.
6 Clinical indicators would include but are not limited to: respiratory rate greater than 35 bpm, which in some chronic patients may be their normal rate; negative inspiratory force of less than 25 cm H 2 O; vital capacity of less than 10 ml/kg, and a minute ventilation less than 3 lpm or greater than 20 lpm. Gas exchange indicators would include; PaO 2 less than 60 mm Hg with a FiO2 greater than 50%, and a PaCO 2 greater than 50 mm Hg with an pH less than or equal to 7.25. Now that we have identified the clinical indications for mechanical ventilation, we want to examine our goals.
7 The goals of mechanical ventilation would include: adjusting alveolar ventilation to bring pH and PaCO 2 within normal range, improving oxygenation to acceptable levels given the age of the patient and disease process, and decrease the work of breathing. Before we move on with our discussion of mechanical ventilation, we should take a look at how spontaneous breathing occurs.
11 There are two types of mechanical ventilators: negative-pressure ventilators and positive-pressure ventilators. Negative-pressure ventilators started as an early attempt to mimic spontaneous ventilation. The polio epidemic led to widespread use of the “iron lung.” The first volume ventilators were introduced in the 1950s, but it was the introduction of microprocessor-based ventilators, which occurred in the early 1980s, that revolutionized the application of mechanical ventilation. We will discuss negative-pressure ventilators first.
12 Negative-pressure ventilation mimics spontaneous ventilation. A negative extrathoracic pressure applied to the chest wall increases the volume of the thoracic cage, which results in a negative intrathoracic pressure gradient and causes air to enter the lungs. One of the advantages of negative-pressure ventilation is that it requires no need for endotracheal intubation. It is used mainly for chronic care of patients with neuromuscular disorders such as ALS, MS, etc. Some adult polio victims and kyphoscoliosis patients still use the iron lung. Other examples of negative-pressure ventilators are the pulmowrap and chest cuirass. Almost all ventilators currently being used in hospitals today are positive pressure ventilators.
13 Air is applied under positive pressure at the patient’s airway, forcing gas flow into the lungs. In positive-pressure ventilation, intrathoracic pressure changes are the opposite of spontaneous breathing. Pressure remains positive throughout the inspiratory phase, as opposed to the negative pressure generated during spontaneous breathing . Because of this, venous return may be impeded and the patient may require intravenous volume support.
14 In positive-pressure ventilation, intrathoracic pressure remains positive throughout respiration. Gas flow takes the path of least resistance, and is distributed to the non-dependent, less perfused lung regions. You are more likely to have ventilation/perfusion mismatch.
15 Here, we can examine the pressure and volume curves typical of a spontaneous and a positive-pressure breath delivered via a mechanical ventilator. On the left, which is the spontaneous breath, we can see the application of the information we just discussed; the inspiratory phase has a slight negative pressure and then on exhalation shows a positive deflection. From our recent discussion of positive-pressure ventilation, notice that the pressure remains positive during the entire inspiratory phase. It is easy to see how spontaneous ventilation, which generates a negative intrathoracic pressure, aids venous blood flow to the heart. Also, positive-pressure ventilation may decrease venous blood flow, and thus some patients may require intravenous volume. We'll take a moment here to review some of the more basic ventilator settings. wrack pressure aids venous blood return to the right heart Negative intrathoracic pressure aids venous blood return to the right heart
16 Some of these settings, such as FiO 2 , respiratory rate, tidal volume, inspiratory time or I:E ratio, and mode of ventilation, are specified primarily by the physician. Trigger sensitivity, or how easily the patient can trigger the ventilator into the inspiratory phase, and peak flow are usually not physician ordered. The goal of course with FiO 2 is to try to keep it below 50%, and tidal volume at usually 7-12 cc/kg, depending upon how conservative you are or what the clinical situation dictates. In volume based-ventilation, and by that I mean that delivering the set tidal volume is what terminates inspiration, peak flow determines how fast that tidal volume is delivered. In pressure-based ventilation, having reached the set inspiratory pressure and the set inspiratory time is what normally terminates inspiration. Let’s begin our discussion about modes of ventilation.
5 5 5 5 5 12 11 AutoFlow What about new modes? There are so many modes of ventilation out there that it can get a bit confusing. The same mode can even have a different name depending on which manufacture your talking about.
11 11 11 11 11 26 26 What are your goals in ARDS? The literature suggests low alveolar pressure and more recently the concern is lung damage caused by over distention, and pressure falling below critical closing pressure.
40 Many clinicians who are not respiratory therapists are often uncertain of what sensitivity is. Sensitivity is another clinician-set variable. Sensitivity, often referred to as trigger, determines when the ventilator will recognize a patient’s spontaneous effort. When patient effort is recognized, the machine will trigger a response - either to give a mechanical breath or support the spontaneous effort. With spontaneous breaths, this change in pressure may trigger a ventilator breath, compared to mandatory machine breaths where breath delivery begins when a set time interval is reached. With spontaneous efforts, the trigger can be a change in pressure or a change in flow. Let’s take a closer look at sensitivity.
41 Recall from our earlier discussions that spontaneous breathing effort begins with contraction of the diaphragm. This effort causes a decrease in intrathoracic pressure. This drop in pressure is transmitted through the closed ventilator system. A ventilator is said to be a closed system when it’s valves shut or close completely during exhalation. To open the valves to allow flow delivery, some variable must trigger the ventilator.
In pressure triggering, when the pressure drop reaches the clinician-set sensitivity (typically -1 to -3 cm H2O), the ventilator will respond according to other established parameters. If the ventilator is in A/C mode, the ventilator will always trigger a mechanical breath, whenever the sensitivity level is reached. There is typically a lag time between when the patient initiates spontaneous effort to when the ventilator recognizes and delivers the breath. Often referred to as ventilator response time, a prolonged lag time can cause patient discomfort and asynchrony.
43 In this pressure-time graph, the sensitivity is set at -2 cm H2O. The first two patient efforts reach the sensitivity level and the ventilator responds by delivering a mechanical breath. Notice that the last breath does not reach the sensitivity level, probably due to poor patient effort. In this example, the ventilator does not recognize the effort, so this spontaneous effort is not supported. I’m sure you can see how important it is to set the sensitivity appropriately. If the sensitivity is too high, patient efforts may go unrecognized. If sensitivity is too low, the machine may trigger a response to patient movement or even the patient’s heart rate! This phenomena is often called auto-cycling. Both situations would cause patient discomfort and patient/ventilator asynchrony.
44 In addition to pressure, on some ventilators, flow may also be selected as the sensitivity variable. In flow triggering, the ventilator delivers a low level of flow that constantly circulates. Because of this constant flow, the ventilator is said to have an opened system, meaning that no valves fully close.
As in pressure triggering, patient effort begins with contraction of the diaphragm. As the patient begins this effort, some of the constant flow in the ventilator circuit is diverted to the patient, so that less flow is returned.
46 This small amount of flow that is drawn in by the patient can help satisfy initial inspiratory demand. Once the flow from the ventilator circuit is depleted to the sensitivity setting ( typically 1 to 3 LPM), the machine recognizes the patient effort and responds by delivering a breath. Keep in mind that this is an open system and the ventilator valves remain partially opened, resulting in a fast response time. Compared to pressure triggering, flow triggering has less of a lag time, thus improving patient comfort and ventilator synchrony. Note that on this graphic all patient efforts are clearly depicted by the negative deflection. In actual pressure-time curves, this deflection is often barely visible when flow triggering is set.
17 1 The most widely used and best understood modes of ventilation are Assist Control, SIMV or Synchronized Intermittent Mandatory Ventilation, Pressure Control Ventilation and Spontaneous. Spontaneous breathing can be Pressure Supported, and CPAP is often utilized to support oxygenation. We will start with a discussion of Assist Control.
18 2 In Assist Control ventilation, all breaths are delivered at a clinician-set tidal volume, flow rate and waveform, and a set respiratory rate. Machine-initiated and/or patient initiated breaths are all delivered at these set parameters. What that means is that every time the machine delivers a breath. regardless of whether the machine or the patient triggered the breath, it is exactly the same in every aspect. If you look at the pressure waveform shown here, you can see that each breath is the same, whether it is a patient-triggered breath like the one on the right, or a ventilator-initiated breath shown on the left. Each breath has the same tidal volume, same flow rate and same flow delivery waveform. Like most things, A/C has it’s advantages and disadvantages.
6 6 6 6 6 18 17 In reality, all breath types can really be classified by which variable they hold constant, pressure or volume. PAV is really the first “new breath type” in that it doesn’t hold either pressure or volume constant, rather it is governed by the percent work the ventilator is told to do. We will discuss PAV later in this presentation.
21 21 21 21 21 37 37 The next goal of ventilation we’re going to discuss is patient ventilator synchrony. To do this we need to meet patients variable flow demands and variable inspiratory times. PCV allows the patients to have any flow they want but we control I-time. PS allows patients to have any flow they want and any I-time they desire. Do we really want to control Vt in these patients? Perhaps there are other capabilities that could lead to greater synchrony.
19 4 On the advantages side, A/C provides full ventilatory support for patients who require it. Additionally, patients can control their own rate of breathing. The disadvantages are that the settings may not match the patient’s ventilatory demands, and since every breath is the same, this can lead to patient ventilator dysychrony. As the patient increases their respiratory drive and begins to trigger more breaths, minute ventilation will increase proportionately. This can result in hyperventilation, because remember that every breath is delivered at the clinician-set tidal volume. CO 2 can drop dramatically. To alert the clinician that the patient has increased his respiratory rate, set the high respiratory rate or high minute ventilation alarm. When the patient is ready to be weaned off mechanical ventilation, the mode will need to be changed. Why? . You can turn the set respiratory rate down, but remember that each time the patient triggers a breath it is a machine breath with a set V T that is delivered. At this point, the patient needs to be switched to a mode that allows for spontaneous breathing between machine breaths, which brings us to our discussion of SIMV.
20 6 SIMV, or Synchronized Intermittent Mandatory Ventilation, is a combination of machine and spontaneous breaths. SIMV has a set rate of mandatory breaths, typically delivered when patient effort is sensed, which is the synchronized part; and then the patient is free to breath spontaneously in-between the machine breaths. The patient determines the tidal volume and the number of the spontaneous breaths. In this way, the set respiratory rate can be decreased, and the responsibility for ventilation can gradually be transferred from the ventilator to the patient. SIMV is currently one of the more common modes used to wean patients off mechanical ventilation. As you can see on the pressure time curve on the bottom of the screen, it is a combination of machine and spontaneous breaths. What are the advantages of SIMV?
21 8 Synchronized breaths may improve patient comfort and reduce the competition between the patient and the ventilator. Because the patient is in full control of the spontaneous breaths, patient ventilator synchrony is enhanced. Hyperventilation is less of a concern compared to A/C. Let’s take a look at some concerns with SIMV.
There may not be enough support if the set rate or V T is too low. Tidal volume and respiratory rate should be monitored to assess patient tolerance and fatigue. SIMV may increase work of breathing due to the lag time between patient effort and delivered flow. Flow triggering should be utilized to eliminate lag time and provide exquisite sensitivity to patient effort. The resistance of the ET tube is another issue in spontaneous breathing. Properly set Pressure Support can minimize the work of breathing caused by the resistance of breathing through an artificial airway. Next, we will move into pressure modes of ventilation; but before we do lets take a moment to discuss the differences between volume and pressure based breaths.
23 In volume ventilation, the volume delivered is constant; whereas in pressure ventilation, volume varies with changes in resistance and compliance. In volume ventilation, inspiratory pressure varies with changes in compliance and resistance, and in pressure ventilation, inspiratory pressure is set and remains constant. Inspiratory flow is constant in volume ventilation and varies in pressure ventilation. In volume ventilation, inspiratory time is determined by the set flow and tidal volume, but in pressure ventilation the inspiratory time is set by the clinician. Let’s move on to our discussion of pressure control ventilation.
24 The definition of PCV is the application of a clinician-set inspiratory pressure and inspiratory time. Flow delivery varies according to patient demand or inspiratory time. The clinician sets the inspiratory pressure, inspiratory time or I:E ratio, and the respiratory rate. Tidal volume will vary with changes in compliance and resistance, and the flow delivery pattern is decelerating in pressure ventilation.
25 15 Pressure control ventilation may be used in A/C and SIMV modes. In A/C, all breaths (either machine initiated or patient initiated) are time cycled and pressure limited according to the set inspiratory pressure and inspiratory time or I:E ratio. In SIMV, only machine-initiated breaths are time cycled and pressure limited; all spontaneous breaths are whatever time the patient desires. Spontaneous breaths can be pressure supported, and thus will be pressure limited also.
26 Here, we can see the pressure and flow time waveforms for different I-times. As the inspiratory time increases, the positive pressure is delivered and then held in the patient’s lung for a longer period of time. Let’s take a look at the advantages and disadvantages of pressure based ventilation.
27 17 One advantage of pressure control ventilation is a decreased risk of barotrauma caused by over distention. Also, the medical community as a whole is focused on minimizing the pressure the lung is exposed to. Longer inspiratory time may recruit collapsed and flooded alveoli and improve gas distribution. One disadvantages is that tidal volumes vary when patient compliance changes, such as with the ARDS or pulmonary edema patient. Setting a low tidal volume alarm or minute volume alarm can alert the clinician to this changing status, and the patient can be re-evaluated. Another issue that arises with increases in inspiratory time, is that the patient may require heavy sedation or chemical paralysis. Newer ventilators on the market incorporate an active exhalation valve that allows the patient to breath spontaneously during the set inspiratory time in pressure control ventilation. It will remain to be seen whether a decrease in paralysis may be the result of this active valve.
28 Patient/ventilator synchrony may be defined as how closely the mechanical ventilator can provide or support the patient’s intrinsic breathing efforts. In pressure controlled ventilation, the patient can have as much flow from the ventilator as needed. Contrasting this to volume controlled ventilation where flow is limited to what the clinician sets, pressure controlled ventilation may improve overall patient/ventilator synchrony. As alluded to in the previous slides, some researchers have shown improved patient outcomes by using pressure controlled mode with lower than normal pressures to ventilate “sick” lungs. Additionally, the decelerating flow pattern that is inherent in pressure ventilation is believed to improve gas distribution when compared to the constant pressure delivered in volume ventilation. Recall that in pressure controlled ventilation the clinician can adjust the length of inspiration. Let’s mimic this variable. I want each of you to make note of your own breathing rate. Now, take a normal breath in - keep taking more air in- now a little more. By extending the inspiratory time - what do you think happened? Well, the amount of air or tidal volume increased, but the increased time also allowed more of the air to enter the very distant alveoli. You can clearly see the potential advantage this may have in patients with lung pathology.
29 Let’s move on to pressure support ventilation. Pressure support ventilation is defined as the application of a clinician-set positive pressure to spontaneous breaths. As with all pressure modes of ventilation, the resulting flow pattern is decelerating; the flow peaks early, then gradually declines throughout the inspiratory cycle. How does this mode differ from pressure controlled ventilation? Well, in pressure support mode, the pressure is added only to spontaneous breaths while the patient determines respiratory rate and inspiratory time. Since this mode is used to support spontaneous breathing, the patient must have an intact respiratory drive to initiate the breath. Remember, patients can have spontaneous efforts between mechanical SIMV breaths. To illustrate, we can have a patient on volume or pressure controlled ventilation with an SIMV rate of 10 bpm. If we add pressure support, the ventilator will deliver 10 SIMV breaths, but any spontaneous breath will be pressure supported.
30 OK, why do we use pressure support? First, when an endotracheal tube is inserted into a patient, and a ventilator circuit attached, the patient will experience an increased resistance to air flow. You can quickly demonstrate this by breathing in and out through a straw. I guarantee that you will quickly become tired and short of breath due to the increased resistance of breathing through a thin lumened straw. When we support spontaneous breaths by additional pressure, we can overcome or at least decrease some of the resistance caused by the ET tube and ventilator circuit. Similar with pressure controlled mode, pressure support allows the patient to have as much flow as needed, which improves patient/ventilator synchrony. Lastly, the added pressure actually contributes to an increase in tidal volume. The pressure time curve here illustrates pressure support of 10cm pressure. You can see that every breath is spontaneous by the negative deflection that precedes each breath. Notice that the rate is variable, as is the length or duration of inspiration.
31 Pressure support can be used in one of 2 different approaches. Most commonly, you’ll find that physicians will often order pressure support between 5 to 10cm in addition to other mechanical modes. Low levels of pressure support, while effective at decreasing resistance to flow, may not be substantial enough to impact tidal volume. During weaning, the number of mechanical breaths may be gradually decreased, while spontaneous breaths increase. This process may continue until all mechanical breaths are discontinued and the patient is assisted with low level of pressure support. Conversely, if a patient has an intact respiratory drive but can not effectively ventilate due to high respiratory rates and low tidal volumes, pressure support can be incrementally increased to yield tidal volumes of ~ 10 ml/kg, thus supporting alveolar ventilation. Knowing that high levels of pressure support can meet the ventilatory needs of a spontaneous breathing patient has lead to another weaning methodology. Patients with intact ventilatory drives can be switched to high levels of pressure support. Then, the pressure support is gradually weaned to a level of 5 to 10cm. If the patient can sustain ventilation, extubation follows.
32 12 After the preceding discussion, I’m certain you can all list the advantages of pressure support ventilation: first, the patient maintains much control over breathing including rate, tidal volume and inspiratory time. Because of this control, patients often feel more comfortable. Also, pressure support does help overcome some resistive forces, thereby decreasing work of breathing. Potential disadvantages that you need to be aware of include that pressure support may not provide enough assistance to maintain effective alveolar ventilation, especially if the patient’s condition deteriorates. And as with any other clinician-set parameter, the support level does not change in response to changing patient drive.
33 Important assessment parameters include exhaled tidal volume. Decreases in tidal volume may indicate changes in resistance or lung compliance while increases in tidal volume may point to patient improvement. It is very important to maintain a leak-free ventilator circuit. Depending on the type of ventilator, air flow is terminated using different criteria. For example, with the 7200 ventilator, flow stops at 25% of peak flow; if the set pressure level is exceeded by 1.5cm of pressure; or if inspiration exceeds 5 seconds. In this case, an air leak could prevent the peak flow from decreasing to 25%. Therefore, flow would continue until inspiratory time exceeded 5 seconds or the patient would actively exhale, thus increasing the pressure above the set pressure. This could certainly cause patient discomfort and increase the work of breathing. Another important assessment criteria is respiratory rate. If the rate increases, this could indicate the need for additional support to meet ventilatory requirements. Potentially, most patients can benefit from pressure support, since pressure support can be combined with other modes of ventilation. However, only spontaneous breathing patients with intact respiratory drives are candidates for pressure support when used as the only mode of ventilation.
35 Let’s begin with a definition of PEEP or positive end expiratory pressure. PEEP is the application of a clinician-set positive pressure applied at end exhalation. This prevents pressure from returning to zero, or atmospheric, at the end of the breath. When positive pressure is applied at the end of a mechanical breath, it is referred to as PEEP. When positive pressure is applied throughout the spontaneous breathing cycle, it is referred to as CPAP, or continuous positive airway pressure. Let’s look at the graphic representation of PEEP.
36 On this pressure-time graph, we know that the first breath is mechanically initiated since there is no negative deflection that precedes the breath. Note that the breath does not begin at the zero base line, but instead, begins at 5 cm H2O pressure. The mechanical breath is delivered, but at end exhalation, pressure ends at 5 cm H2O. The next breath is spontaneous. Here again, pressure throughout the breath cycle is elevated to 5 cm H2O. The final breath is a patient-initiated, mechanical breath, again showing that at end-exhalation, pressure is maintained at 5 cm H2O. Well, why do we add PEEP? Once again, we’ll try to mimic this effect. Take a normal breath in, but do not exhale all the way, thus maintaining some positive pressure. What could be the benefit of positive pressure at the end of exhalation? PEEP causes an increase in functional residual capacity or FRC. FRC is the amount of air left in your lungs at the end of a normal exhalation. This increased volume can improve oxygenation; more air remains available to participate in gas exchange. In sick lungs, PEEP can also help recruit or open collapse alveoli. The additional pressure applied at end-exhalation may be sufficient to pop open collapsed alveoli. Keep in mind that with many lung pathologies, alveoli have the tendency to collapse. PEEP can be applied at pressures sufficient to overcome this tendency to collapse, keeping the alveoli patent and functional. Finally, incases of excess pulmonary water or exudate, PEEP can cause this unwanted lung water to move from the alveoli into the perivascular space.
37 Now that you understand the physiologic effects of PEEP, you can apply the same knowledge to CPAP. The only difference being that CPAP is the application of positive pressure throughout the spontaneous ventilatory cycle. Since this is a totally spontaneous mode, the patient must have an intact respiratory center.
38 This graphic depicts CPAP mode set at 10 cm H2O. Similar to pressure support, the patient determines the respiratory rate and tidal volume. Keep in mind, that CPAP and PSV are often used in conjunction. CPAP can prevent or minimize alveolar collapse, while pressure support helps overcome common resistance forces and may augment tidal volume. Either only or in combination with pressure support, CPAP is often the final mode of ventilation before extubation.
39 Although the primary indication for PEEP and CPAP should now be readily clear, you need to be aware of the potential adverse effects. The positive pressure that is applied at end-exhalation is transmitted throughout the thoracic. Positive intrathoracic pressure can cause a decrease in venous return and subsequently, decrease cardiac output. Be prepared to volume support sick patients who require incremental increases in PEEP. This increased intrathoracic pressure can also result in lung rupture or barotrauma - another adverse effect that may require immediate clinical intervention. Lastly, PEEP and CPAP can cause an increase in intracranial pressure, a potentially devastating side effect if the patient is suffering from head injury.
17 17 17 17 17 65 65 APRV is similar to BiLevel but utilizes a very short expiratory time. This short expiratory release time at low pressure allows for ventilation. Normalized ventilation is not necessary for oxygenation as is seen in brain death studies, where oxygen levels remain high, but CO 2 levels climb. We have been working with DR. John Downs for years on BiLevel. We are combining the attributes of BiPAP as used in Europe with APRV. BiLevel/APRV appears to make better physiological sense. We think that we’re moving in a direction that offers potential advantages.
13 13 13 13 13 In some countries in Europe BiPAP is used frequently which allows free breathing at both pressure levels. The clinicians will set 2 pressure levels (PEE H and PEEP L ), spontaneous breathing can occur at either level and PS is available at both pressure levels.
14 14 14 14 14 15 66 There will also be direct setting of T H and T L or T H / T L ratio. The timing of transition from one PEEP level to the other will be synchronized with patient breathing. Settings of FAP, E SENS and flow triggering are active at both pressure levels.
15 15 15 15 15 This slide illustrates spontaneous breathing at PEEP H and PEEP L . It is possible to synchronize the transition from P H to P L with the patient’s respiratory cycle (i.e. the fall to P L synchronized with the patient’s expiratory phase. Also, notice at PEEP H that there is a spontaneous breath that is pressure supported Also notice at PEEP L that the spontaneous breath that is pressure supported.
16 16 16 16 16 The spontaneous breaths at both PEEP H and PEEP L can be Pressure Supported if the PS level is high enough to exceed PEEP H . As you wean, the delta P between PEEP H and PEEP L gets smaller which allows for more support of the spontaneous breaths at PEEP H .
18 18 18 18 18 This is a graphical representation of BiLevel/APRV. Again you see PEEP H and PEEP L and T H and T L . As you can see the difference is in the short release time or T L . Weaning is accomplished the same, by changing the Delta P and/or frequency.