The Police, an 80’s English rock band, had it right in there song ‘Every Breath You Take’ when they said:
- Every breath you take
- Every move you make
- Every bond you break
- Every step you take
- I’ll be watching you
As providers, we are constantly, silently, assessing our patients. How was their stability walking into the room? Are they short of breath with a small amount of exertion? Can they speak in full sentences? Are they pale? Diaphoretic? Vascular changes?
One of the most important parts of our ongoing assessment is a patient’s respiratory status. Part of that advanced assessment is understanding the difference between oxygenation and ventilation and basic ventilator management. In this blog, I hope to make you comfortable with basic ventilator management.
Let’s start with oxygenation. Oxygenation is defined as the addition of oxygen to the body. We help the patients by placing then on nasal cannula, non-rebreathers, bipap or mechanical ventilation. When a patient is ventilated we can affect the oxygenation not only with the FiO2 (fraction of inspired O2) but also with PEEP (positive end expiratory pressure).
Ventilation exchanges air between the atmosphere and lungs. Most importantly it affects the removal of CO2 from the body and in mechanically ventilated patients can be measured by Minute Ventilation. Minute ventilation is respiratory rate (RR) x tidal volume (Vt). We can affect the CO2 concentration by adjusting the RR and Vt.
Basic Ventilator Mechanics
So how does mechanical ventilation work? Physiological respirations occur via a negative pressure system. So when the diaphragm is pulled down it causes negative pressure within the pleural cavity and pulls air into the lungs. This in turn causes increased venous return by decreasing the RA pressures leading to a sucking effect on the IVC. Mechanical ventilation changes this system into a positive pressure system. When we change a patient to this positive pressure system it causes an increase in RA pressure and decrease in venous return therefore decreasing preload. This leads to a decrease in cardiac output (CO). If you have decreased preload you will have less blood flowing into the RV, which leads to less blood reaching the LV and ultimately decreasing Stroke volume, therefore decreasing your CO. Less preload is also going to cause the heart to work at a less efficient point along the Frank-Starling curve. These are some important physiological effects of mechanical ventilation to keep in mind.
So now onto the fun part. Modes of Mechanical ventilation. Most providers are most familiar with the assist control (AC) modes. This mode allows the ventilator to take over the work of breathing for the patient in either a volume controlled or pressure controlled state. In a volume control (VC/AC) the provider sets the rate, Vt (tidal volume), PEEP and FIO2. In a pressure controlled (PC/AC) mode the provider sets the driving pressure, I:E (inspiratory:expiratory ratio), rate, PEEP and FiO2. In either of these modes if the patient wants to breath above the set respiratory rate that are able to do so if the threshold trigger is met to initiate a breath. This can be a negative if you have a patient who is triggering the vent frequently and getting full tidal volumes on a respiratory rate of 30 instead of the 18 that was set. Proper sedation in this mode can help to offset the tachypneic patient.
Next let’s talk about Pressure Support Ventilation (PSV). This mode has no rate. Instead it allows the patient to breath at their own intrinsic rate with a back-up pressure that is set by the provider. This mode is typically seen as a ‘weaning’ mode or for patients who are not cardiac or respiratory failure. It is important to closely monitor patients on PSV because there is no rate set. Therefore, if a patient goes apneic it will not be found until the alarms are triggered. Patients can also fatigue rapidly on PSV.
SIMV with PSV is another mode that can be seen for patients needing full ventilatory support. SIMV with PSV allows the provider to set a rate and Vt but allows the patient to breath above the set rate. How is this mode different for the AC modes you ask? Well let me tell you! In SIMV when the patient triggers the vent to take an intrinsic inhalation, the vent allows the patient to pull whatever they can with PSV help. Not the set Vt. This allows for more comfortable spontaneous breathing efforts. This mode does allow for patient fatigue so correct patient selection is important.
There are a few unconventional modes to be aware of. You will typically only initiate or encounter these ventilator modes if you are working in the ICU or with a pulmonary group. The first is airway pressure release ventilation (APRV). The second is high frequency oscillatory ventilation (HFOV). These modes are used to treat severe refractory hypoxemia. Lung protective ventilatory strategies imply that lower Vt and high PEEP are appropriate. The lower Vt doesn’t excessively distend the alveoli, while the high PEEP keeps alveoli from collapsing therefore reducing shear stress and ‘atelectrotrauma’. HFOV utilizes Vt less than the anatomic dead space while APRV works by excessive PEEP.
Do you have any stories to share about your vent experiences? Any questions? Post below!