Let’s assume that the patient is being ventilated with assist control ventilation.
Some of the main settings we can change in this mode of ventilation are:
Changing either of these will affect the carbon dioxide levels of the patient.
Changing either of these will affect the oxygen levels of the patient.
So the question is, when needed, do you increase the rate or the tidal volume?
One of the first things to understand when trying to make that decision is that every breath comes with some dead space. Dead space is that part of the respiratory circuit that does not play a part in gas exchange. The gas simply moves along these areas and gas exchange does not occur. Some of this is created by the ventilator tubing and some of it is physiological dead space.
So let’s increase the rate. If you do that then the ventilation of the patient will increase as every minute you are giving them additional breaths. The problem however is that you are also increasing the amount of dead space ventilation you are giving them, as each additional breath also carries the same amount of dead space. This process can increase the patients propensity to retain carbon dioxide.
If, however, you just increase the patients tidal volume with each breath, you do not increase the dead space at all.
There maybe some situations where this rule needs some modification.
Those patients who are suffering with Chronic Obstructive Pulmonary disease (COPD), because of the physiological problems they are suffering, have problems getting the air out of their lungs. So they might not be able to get the air out of their lungs before the next breath comes along when being mechanically ventilated.
This is very relevant if they are being given tidal volumes that are too large, or a respiratory rate that is too fast. This can then lead to air trapping. The air from the previous breath has not been exhaled fully before the next breath, so breaths start to stack on top of one another.
So increasing the tidal volume or respiratory rate could actually reduce the minute ventilation because of the breath stacking potential.
You also need to be aware that larger tidal volumes will lead to greater stretching of the alveoli as each breath goes in and out. This stretching is possibly harmful to the patient with the compromised lung, particularly in those patients with Acute Lung Injury (ALI) or Adult Respiratory Distress Syndrome (ARDS).
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