FREMITUS: HYPERCAPNEA & ATELECTRAUMA: MIMICKING APRV

In a previous post "APRV in the operating room is it practical?" I argue that bringing a ICU ventilator into the operating room to utilize APRV is not practical and may lead to hypoventilation and hypoxia due to administration of anesthetic agents [1].

During surgical procedures the patient is maintained in stage 3 of anesthesia known as the "surgical stage". Stage 3 is broken down into four distinct planes, "from onset of automatic respiration to respiratory paralysis" [2]. The patient is usually maintained in Plane 3 (intercostal muscle paralysis) or Plane 4 (diaphragmatic paralysis) leading to the cessation of spontaneous breaths.

One key advantage of APRV is that the patient may breathe spontaneously contributing to the overall minute volume, with the termination of spontaneous efforts the patient will become severely hypercapnic.

Below is an image (fig 1) I captured from a "Pressure Control Ventilation Simulator" [3] demonstrating an ARDS patient on APRV.

Figure 1. Pressure Control Ventilation Simulator, notice patients PaCO2 at 100.7 mmHg.

Notice in figure 1 the outcome for a patient not contributing to the overall minute ventilation the PaCO2 would be 100.7 mmHg.

Another example of how APRV maybe harmful in the operating room is when trying to mimic APRV with a anesthesia delivery system.

I posted a video "Mimicking APRV with the FLOW-i anesthesia delivery system" revealing how one could use Pressure Control- Inverse Ratio with a floating exhalation valve on a anesthesia machine to mimic APRV [4]. This technique involves multiple steps and may lead to atelectrauma from the prolonged expiratory phase associated with PC-IRV.

Again using the Pressure Control Simulator [3] and ventilator screen shots I will explain how mimicking APRV puts a patient at a higher risk for atelectrauma.

First, lets evaluate the traditional settings of a adult patient on APRV specifically the T-Low which affects the expiratory phase.

Figure 2. APRV settings screen on Hamilton C-1 ventilator, T-Low set at 0.25 second.

In figure 2 the T-Low is set at 0.25 second or one expiratory time constant (RCexp) for this example. Setting the T-Low based on one RCexp will generate a "Peak Expiratory Flow Termination Point" (T-PEFR) of 50 to 75 percent [5] preventing total alveolar collapse.

Figure 3. Resulting affect of setting T-Low on RCexp arrow shows T-PEFR ~ 50%.

The above image shows a T-PEFR of ~ 50% based on the T-Low setting of 0.25 second.

Below (fig 4.) I selected inputs on the Pressure Control Ventilation Simulator to match the settings and patient characteristics of the Hamilton C1 ventilator simulator. Notice the duty cycle is 97% which equals a 30:1 I:E ratio.

Figure 4. Physiologic simulator with same settings and patient characteristics.

Second, lets demonstrate mimicking these settings with PC-IRV. Notice in the below image (Fig 5.) for a Inverse Ratio of 4:1 the maximum duty cycle in PC-CMV the T-Low phase would be 1.5 seconds.

Figure 5. Physiologic simulator ventilator settings inputs for PC-IRV 4:1 I:E ratio or 80% duty cycle.

A T-Low or expiratory phase of 1.5 seconds allows for a patient with a expiratory time constant of 0.25 for complete alveolar emptying/collapse increasing the risk of atelectrauma. Figure 6 is a screen shot of PC-IRV with a 4:1 I:E ratio notice complete exhalation during the expiratory phase which puts the patient at higher risk for alveolar collapse. Mathematically any patient that has a RCexp of less than ~ 0.4 (e.g. resistance of 10 & compliance of 40) there would be complete exhalation using PC-IRV.

Figure 6. PC-IRV with a 4:1 I:E ratio notice purple flow waveform complete exhalation during exhalation.

The operator may try to mitigate alveolar collapse by setting a P-Low or PEEP, however this does not prevent complete exhalation and decreases the delivered tidal volume (fig. 7). Additionally, it is not known on how much PEEP to apply to prevent alveolar de-recruitment.