Physicians who find themselves caring for a patient on mechanical ventilation must be familiar with ventilator function and settings. Altering various aspects of the mechanical ventilator will allow a physician to achieve the two basic goals of mechanical ventilation:
1. Control the oxygen and carbon dioxide levels of our patient’s bodies to sustain healthy tissues and metabolic function.
2. Control inspiratory volume and pressure in order to walk the fine line between assisting the patient with breathing without allowing their respiratory muscles to atrophy.
As future physicians, medical students should understand mechanical ventilation. As discussed in the article, Basics of Mechanical Ventilation for Med Students, we control oxygen levels by altering the fraction of inspired oxygen (FiO2) and the positive end expiratory pressure (PEEP). We control carbon dioxide levels through adjustments in inspiratory tidal volume or respiratory rate.
So let us focus on patient oxygenation first. For an intubated patient, most doctors will start mechanical ventilation on a FiO2 setting of 100%. However, keeping the FiO2 on 100% will ensure a future of oxygen toxicity for your patient due to the oxidative effects of pure oxygen over time. Therefore, the actual goal is to get the FiO2 down to 60% or less.
The FiO2 goal of 60% is usually secondary to the goal of maintaining adequate positive end expiratory pressure or “PEEP”. Once the PEEP is ideal the FiO2 can be slowly (slowly!) weened down.
Most physicians will start the PEEP at 5 cm H20. This pressure is close to the natural, physiologic pressure in the lungs at the end of expiration. The upper limit possible for positive end expiratory pressure is 25 cm H2O. However, a PEEP too much higher than the physiologic PEEP of 5 cm H2O can cause barotrauma and decrease cardiac preload.
An easy way to understand barotrauma, is to imagine that when the positive pressure from air artificially forced into the lungs is too high the alveoli of the lungs pop like little balloons. The damaged alveoli release inflammatory cytokines and set the patient up for respiratory distress and decreased cardiac filling.
So in review, we start the FiO2 at 100% with a goal of 60% or less. We don’t try to achieve that FiO2 goal until after we are able to get the PEEP close to 5 cmH2O.
Now let us focus on adjusting our patient’s pCO2. Tidal volume and respiratory rate allow us to adjust pCO2. How do you know what to set the tidal volume on? Lucky you, here’s another easy thing to remember about mechanical ventilation: tidal volume is determined by the patient’s ideal body weight at a rate of 10ml/kg maximum, and once it is set it is usually not changed.
The tidal volume is usually started at somewhere around 8 to 10ml per kg of ideal body weight (IBW). Use your medcalc or medmath on your PDA or use a chart to figure out your patient’s IBW. A setting of 4ml/kg is the lowest limit on tidal volume and is sometimes used to protect against adult respiratory distress syndrome. Remember, once the tidal volume is set it is usually not changed.
This is great for you, because now all you have to remember about getting the CO2 at goal is the respiratory rate. The respiratory rate is set based on a best guess sort of scenario. How fast was the patient breathing before intubation? Do they have any factors that would increase their metabolic rate and thus increase their need for ventilation? For example, fever, sepsis, wound healing, etc.
A respiratory rate that is set too fast will increase the peak inspiratory pressure and may result in “breath stacking” (patient hasn’t exhaled all the way before the next breath is initiated). In contrast, a patient who is “riding the vent” is not taking any breaths over the number of breaths per minute that we’ve set.
So in review, we set the tidal volume as 8 to 10 ml/kg of IBW and leave it alone. We set the respiratory rate based on a best guess scenario.
Now that we have set the FiO2, PEEP, tidal volume and respiratory rate we let the patient ventilate artificially for 20 to 30 minutes. Then we check an arterial blood gas to see what we need to adjust. Once you feel that you have grasped the essentials of this article move on to the next article in this series: Knowing when to adjust mechanical ventilator settings.
