By this point you have probably developed a healthy skepticism for the ventilator. While remaining an undeniable cornerstone in the support of critically ill patients, we have gone into the myriad of limitations the ventilator has in severe respiratory failure, when compliance worsens and it becomes progressively difficult to oxygenate/ventilate.
Now we will progress to some principles for management of the ventilator while on extracorporeal membrane oxygenation (ECMO). It should be noted that this is one of the areas where there is probably the most variation in practice patterns, with some practitioners being very conservative with ventilator weaning, and others being very aggressive. Rather than advocating one approach over the other, we will try to focus this chapter on principles that can be applied in order to minimize the damaging effects of the ventilator for patients on ECMO. This will not be a comprehensive overview of all strategies of mechanical ventilation, but rather a very focused review of principles that are commonly applied to patients on ECMO.
We will focus this discussion on patients with primary respiratory failure, however the principles discussed will apply to all types of support.
Common ventilator settings in ECMO support
Imagine you are caring for a patient who was just initiated on ECMO. As support was initiated, they had a phenomenal response – their saturations increased from 77% to 100%, their heartrate decreased from 120 to 98, and their blood pressure improved from 89/55 to 105/70.
Mode AC/VC. Tidal Volume 500. 100% FiO 2 . PEEP 14. Rate 34. Peak pressures measured at 42.
Your eyes now turn to the ventilator. It reads as follows:
Just as you are pondering the ventilator and the settings that you can transition this patient to, the respiratory therapist asks, “Do you want me to change the patient to our typical vent settings?”
Mode AC/PC. Inspiratory pressure 12. 50% FiO 2 . PEEP 10. Rate 26. Tidal volume measured at 145 mL.
What is the difference between these two settings? What is the rationale behind these changes?
Mode of ventilation: Spontaneous versus controlled ventilation
The mechanism of the ventilator is relatively straightforward – it pushes a set amount of air into the lungs. As such, you can imagine that there really are three primary variables that effect how this air is delivered: what initiates the breath (referred to as trigger ), what is delivered with each breath (referred to as limit ), and what ends the breath (referred to as cycle ).
If the ventilator is delivering a set number of breaths regardless of whether the patient is initiating the breath or not, this is controlled ventilation. Contrast this to spontaneous ventilation, where every breath that is taken has to be initiated by the patient ( Figs. 19.1 and 19.2 ).
The other difference between these two modes is how the breath ends. In spontaneous mode, as you can see earlier, the breath can be long or short and is only stopped when a specific flow has been obtained. Compare this to controlled ventilation, in which the breath is stopped after a set amount of time. This may result in more variation in the size of the breath for spontaneous ventilation, which is similar to how we breathe without the ventilator, but may or may not be well tolerated in the setting of severe lung dysfunction.
In spontaneous ventilation, a certain amount of positive end-expiratory pressure (PEEP) is set followed by a set amount of pressure that is delivered above PEEP with every breath. The volume of breath that is generated is dependent on the flow that the patient is able to generate, based on the strength of the diaphragm.
This reliance on the diaphragm forms the main advantages and disadvantages of spontaneous ventilation. On one hand, this mode can condition the diaphragm, employing it in every breath similar to breathing without the ventilator. However, if there is severe decompensation, this may lead to overexertion and weakness.
How do you make the distinction between selecting spontaneous versus controlled ventilation? This is a challenging question to answer due to the wide range of approaches taken by various practitioners, as well as the varied response that individual patients can have. As a general rule, controlled ventilation may be needed early in the course, with the aim of allowing for spontaneous ventilation as the patient continues to wake up and recover. This can be over the course of several days or several hours, depending on how the patient is responding to weaning. Wean as much as is tolerated clinically.
What happens when spontaneous breathing is not feasible clinically? Let’s go over a couple of considerations for controlled ventilation.
Controlled Ventilation: Volume Versus Pressure Control
The two main modes for controlled ventilation are pressure control and volume control. The difference between the two is as follows:
Volume control: delivers a set amount of volume with each controlled breath (flow limited/volume cycled)
Pressure control: delivers a set amount of pressure with each controlled breath (pressure limited/time cycled)
Minimizing both pressure and volume delivered is ideal, since as you remember, both can cause damage to the lungs in the form of barotrauma (pressure) and volutrauma (volume). The challenge comes when compliance decreases, as occurs when lungs get progressively stiff due to edema, acute respiratory distress syndrome (ARDS), pneumonia, hemorrhage, or any of the host of conditions that are associated with severe cardiac/respiratory failure. In this case, you are left with either tolerating higher pressures or lower tidal volumes, as denoted below:
This is the tradeoff that often has to occur with worsening respiratory failure that leads to a failure to ventilate: either you have to tolerate higher CO 2 levels due to lower tidal volumes or you have to tolerate higher airway pressures in order to achieve a target tidal volume.
ECMO obviates the need for this choice. Since the circuit is able to remove CO 2 with great efficiency, you can significantly lower both the pressure and volume delivered with each breath and afford better lung protection from the damaging effects of barotrauma and volutrauma.
Either mode is acceptable as long as you are providing lung-protective ventilation.
Lung Protective Variable #1: Tidal Volume
Tidal volume is one of the mechanical ventilation parameters with the most well-established association to improved outcomes. We know that high tidal volumes are damaging to the lung parenchyma and that this damage has been linked to a higher mortality.
The generally accepted target is 6 mL per kg of ideal body weight. When compared to higher tidal volumes, tidal volumes of less that 6 mL/kg have been associated with improved mortality. What about 5 mL/kg? 4 mL/kg? 2 mL/kg?
6 mL/kg is usually the target because lower tidal volumes often leads to difficulty in removing CO 2 , due to lower minute ventilation (tidal volume × breaths per minute). However, CO 2 removal is less of an issue in ECMO, allowing for lower tidal volumes if desired.
Thus, many practitioners aim for tidal volumes on ECMO less than 4 mL/kg . This is sometimes referred to as “ultra–lung-protective ventilation.” There is some evidence to support this strategy, as 4 mL/kg was the lower limit of normal in many trials.
Lung Protective Variable #2: Driving Pressure
Let’s introduce this concept of driving pressure. Driving pressure is defined as follows: