Hungry Hungry Patient-Flow Starvation

My initial mechanical ventilation knowledge focused on this obsession with memorizing specific settings for patients. Personalization of settings for each disease process, much less each patient, was unheard of. How dare a patient to dictate their mechanical ventilation! We, as medical professionals should be choosing when, how, and to what extent our patients breathe! This mantra was reinforced within my mind when researching the ARDSnet standards of mechanical ventilation. Basically, I should start my patients at a tidal volume of 8cc/kg and reduce this if I run into issues with plateau pressures (1).

If this mode of mechanical ventilation works for those patients with such sick lungs, we should of course translate this to patients with healthy lungs. I, as a responsible clinician, would never want to cause a lung injury from my mechanical ventilation. I made the mental decision to start patients at 6cc/kg, ensuring this would never be a problem.

This mental framework seemed to work well for patients under high levels of sedation or neuromuscular blockade, however it didn’t take long for me to run into issues with patients who weren’t deeply sedated. Upon switching the patient to our ventilator, I often had patients who seemed quite uncomfortable. Ventilator alarms echoed down hospital hallways as I found myself bolusing Fentanyl. “It's a different ventilator, it will just take them a second to get used to this vent,” a mantra that ran through my mind as I waited for my sedation plan to kick in.

It took some self-reflection and smart mentors for me to soon learn that maybe instead of “adjusting” the patient with high doses of sedating agents, I could adjust how I’m delivering the ventilation to improve the patient's comfort. How comfortable would you be if someone suddenly started dictating when and how much you are breathing?

A common ventilator synchrony issue we may run into is flow starvation. Before we understand what flow starvation is, we need to understand what flow is. Flow is simply the rate of gas movement to the patient. This is the flow a patient inhales at. This is described in liters per minute (Lpm) with a normal being roughly 60-80 Lpm.

Flow starvation is when the generated flow, or breath, feels inadequate to the patient making them feel as if they are “starved” of their breath. Imagine you go to take a breath, but are unable to pull in air at the speed that you want, this would be quite distressing! This dyssynchrony is more commonly seen in volume-based ventilation as flow is fixed vs pressure-based ventilation where it is altered based on patient effort (2). The nerve-wracking part, at least in my mind, is that your ventilator will not alarm for this issue. Instead, we must study the patient's presentation and ventilator waveforms to notice this problem. So, let’s review the waveforms!

First, let's look at a normal pressure waveform in a volume mode:

This is a “passive” patient who is allowing the ventilator to do all the work delivering the breath. This almost has a shark fin appearance, and unlike a shark fin in ETCO2, this is a fish we want to see!

What about flow starvation? What will that waveform look like?

Huh, that's weird, the waveform now has a slope downward. This is caused by the patient inhaling with such force against an inadequate flow causing the waveform to "pull" downward. The patient is working harder to inhale than the ventilator is willing to provide. That's all well and good, but what is the problem with this?

Outside of the obvious fact that this can be incredibly uncomfortable, the patient can also be at risk for additional lung or diaphragm injury (3). Additionally, if this problem goes undetected, you may find yourself having to provide quite a bit of analgesia and sedation to get ventilator synchrony.

The solution to this problem is patient-dependent. The first maneuver should be shortening the inspiratory time. This will allow the patient to get their breath more quickly. A side effect of this is that the gas being delivered more quickly may cause turbulence within the system and increased peak inspiratory pressures. Maintain a watchful eye on pressures and exhaled tidal volumes to ensure you are still delivering what you set. Additionally, some patients may require a larger tidal volume. If your patient is set for 6 cc/kg of IBW, it may be adventitious to increase this to 8 cc/kg while keeping an eye on your plateau pressures. Given that we cannot often increase tidal volumes in patients with sick lungs, like ARDS, we may need to turn to increased analgesia, sedation, and potentially neuromuscular blockade to maintain pressure ranges.

We as clinicians need to remember there are multiple causes for ventilator dyssynchrony, many of which can be solved by simple adjustments on the ventilator. An understanding of respiratory mechanics and ventilator waveforms can go a long way in making our patients comfortable and compliant with our ventilation strategies.

Stay tuned for future blogs on other dyssynchronies!

Citations:

Ventilator Protocol - NHLBI ARDS Network. (n.d.). Retrieved June 6, 2022, from http://www.ardsnet.org/files/ventilator_protocol_2008-07.pdf

Bulleri, E., Fusi, C., Bambi, S., & Pisani, L. (2018, December 7). Patient-ventilator asynchronies: Types, outcomes and nursing detection skills. Acta bio-medica : Atenei Parmensis. Retrieved June 5, 2022, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6502136/

CoEMV, says:, J., says:, R. C., & says:, G. werstiuk. (2018, November 30). Flow starvation. The Toronto Centre of Excellence in Mechanical Ventilation. Retrieved June 5, 2022, from https://coemv.ca/flow-starvation/