This is a subject near and dear to many a Respiratory Therapist’s heart. It is what defines our ability to manipulate the mechanical ventilator to a patient’s needs. The primary need is oxygen delivery to the tissues, removal of carbon dioxide and the maintenance of normal pH. A tall order when a patient has advance lung disease as with adult respiratory distress syndrome (ARDS).
You have already learned but maybe not mastered the basics of volume and pressure ventilation. Now we will explore the many subtle nuances of how we can manipulate these two basic elements into the modes we use every day.
The kinds of problems we will look to solve in this course are the key control interactions. For example in the mode of VC-CMV the ventilator controls are: rate, tidal volume, PEEP, flow, and FiO2. A typical question we will explore may include a statement like; if you increase the rate what would happen to minute ventilation? Another question might be what would happen to the I:E ratio when you increased the RR? What would happen if to the mean airway pressure (Paw) with this increase in RR?
The questions are probably a little intimidating at this time, but by the end of the course you will have explored all these relationships and be more confident.
In class I have been impressed that many of you are considering what does this mean for the patient? Isn’t that rate too low or minute ventilation too high? What about those ventilator pressures? All excellent questions because two elements are critical; maintaining the patient’s ABG’s to meet the clinical goals, and to minimize harm to the patient. We can never eliminate all the harm we will do to the patient but ultimately the good of keeping them alive outweighs the bad. We must always try to minimize the damage to the patient.
Here is a challenge for you to think about. Two adult patients with ideal body weights of 70kg are involved in a motor vehicle collision come into ER. They are both unconscious and require mechanical ventilation. One is set-up with volume ventilation, the other with pressure control. Now I challenge you to think about the volume ventilator, what would should the initial settings be? Initial settings Vt set for normal tidal volume about 8ml/kg or a Vt of 560 or round it to 550ml is set, Rate 12 bpm, flow 60lpm, FiO2 1.0 because we don’t know anything about their oxygen needs at this time, PEEP of 5 because we think that is a fairly normal physiological level.
Now what about patient #2? What PC level do we set? How would or could we know? We would need to know the patient’s lung compliance to be able to estimate the Vt delivered. How could we know that? The answer is I don’t know! We could guess and set a safe PC level and see what happens and sometimes we do this. Say PC level of 25, Ti 1.2 sec (longer than an expected TC*5 but we will have to check), Rate 12 bpm seems reasonable, FiO2 1.0, throw in +5 of PEEP just for fun. Okay but what about our minute ventilation. Are we meeting the patient’s needs? With this mode we would need to check the Vt delivered (this would be recorded as the ventilator would measure it for us). Then we could get an idea if the Vt with this PC level would be enough. So here I hope I have highlighted one of the dilemmas about choosing a PC mode initially in an emergency situation. There may be a few breaths or couple of ABG,s required before you can fine tune your ventilator. Mind you VC may also require fine tuning as patients demands change and are dependent upon many factors.
This brings me to the last point today that is we need to meet the patient’s needs. This means ventilating to ABG’s. We use our expertise to fine tune the ventilator to minimize the lung damage caused by pressure, volume, and oxygen. Generally speaking keep the Pplat less than 30 cmH2O, maintain a PaO2 of at least 60 mmHg (works out to about a SpO2 of 90%) Thus the art of the RT is born!
