APRV & Volumetric CO2 w/ Joe Hylton

Birth of APRV

APRV is not new. It traces back to the work of Dr. John Downs, a University of Florida anesthesiologist, in the mid to late 1980s. Downs was intensely focused on respiratory physiology and had already influenced prehospital care through the development of early CPAP systems, such as the Downs Flow Generator. APRV evolved naturally from that same CPAP mindset.

At its core, APRV was designed as an alternative to inverse-ratio pressure-control ventilation for patients with acute lung injury and ARDS. For most clinicians, APRV belongs squarely in the hypoxic, recruitable lung injury category.

Imagine CPAP applied continuously, but every few seconds, the circuit is briefly opened, allowing partial exhalation before snapping back to pressure. That description is not perfect, but it gets close to what APRV is doing mechanically and physiologically.

The Fundamental Paradigm Shift

In conventional ventilation, PEEP is your baseline. Everything builds on top of it.

In APRV, high pressure is the baseline. That single change flips how you think about ventilation. Instead of intermittently delivering breaths above PEEP, APRV holds the lung at an elevated pressure for most of the respiratory cycle and allows brief releases for ventilation and CO₂ clearance. Mean airway pressure becomes the dominant variable, not tidal volume or respiratory rate.

Optimizing Conventional Ventilation Before Switching

Before moving to APRV, conventional ventilation should be fully optimized. Many patients improve simply by increasing inspiratory time and mean airway pressure without switching modes.

Flow waveforms are critical here. If inspiratory flow decelerates fully to zero and rides flat, the lung has accepted the entire breath. Increasing inspiratory time beyond that point will not increase tidal volume, but it will increase mean airway pressure.

This is where inverse ratios often appear. A patient may tolerate a 1:1 or even a 3:1 I:E ratio without air trapping as long as expiratory flow returns to baseline. If flow reaches zero before the next breath, auto-PEEP is not occurring. When further increases in inspiratory time push the patient into problematic inverse ratios or hemodynamic compromise, APRV becomes a logical next step.

Why APRV Excels at Recruitment

APRV allows extreme inverse ratios that are not realistically achievable with conventional modes. Ratios like 16:1 are common. That prolonged high-pressure phase gives time for slow, injured alveoli to open without blasting them with excessive peak pressures.

Unlike conventional ventilation, APRV separates peak pressure from PEEP. PEEP is created by time, not resistance; therefore, peak and baseline pressures do not stack as they do in pressure-control with PEEP.

This decoupling is one of APRV’s greatest strengths.

Initial APRV Setup: A Physiologic Starting Point

Start by identifying the patient’s mean airway pressure on conventional ventilation. Set the APRV high pressure approximately three to five centimeters above that mean.

Set time high around four to five seconds.Set time low initially around 0.5 seconds.Set low pressure to zero.Start with FiO₂ at 100 percent if the patient is already hypoxic.

This creates a total cycle time of about five seconds, which corresponds to a respiratory rate of roughly twelve.

These are not magic numbers. They are starting points that allow safe assessment and adjustment.

Time Low, RC-Exp, and Ending the Release Correctly

Time low is not about ventilation. It is about preventing derecruitment.

Rather than guessing based on percentages or doing bedside math, expiratory time constants can guide this directly. On ventilators such as the Hamilton T1, the RC-Exp value represents one expiratory time constant, corresponding to about 63 percent lung emptying.

Setting time low equal to one RC-Exp is an elegant solution. It avoids excessive lung emptying while still allowing CO₂ clearance. This approach also simplifies bedside decision making and reduces error when clinicians are under stress.

Modern APRV practice targets release termination at approximately 70 to 75 percent of peak expiratory flow. This shorter release time dramatically reduces large tidal swings that previously made clinicians uncomfortable with APRV.

Volumetric Capnography: The Missing Piece

Volumetric CO₂ monitoring fundamentally changes how APRV is managed.

Unlike end tidal CO₂, volumetric CO₂ measures how much CO₂ is actually eliminated per breath and per minute. This allows real-time assessment of recruitment and overdistension.

When PEEP or mean airway pressure is increased and volumetric CO₂ rises before plateauing, recruitment has likely improved. When volumetric CO₂ drops after an increase, overdistension and capillary compression are occurring.

This method allows clinicians to find optimal lung volume without relying on pressure-volume loops that are often misleading in pressure-driven modes.

For transport teams, volumetric CO₂ may be one of the most powerful physiologic tools available.

Spontaneous Breathing and Paralysis

Contrary to common belief, paralysis is not contraindicated in APRV.

In fact, short-term neuromuscular blockade can be extremely helpful when severe patient effort is preventing recruitment. Strong spontaneous inspiratory effort creates negative pleural pressure that can worsen alveolar collapse in injured lungs.

A brief period of paralysis allows the ventilator to recruit the lung without patient-generated counterforces. Once recruitment is achieved and CO₂ clearance improves, spontaneous breathing often decreases naturally because respiratory drive diminishes.

APRV does not require spontaneous breathing to be effective.

Troubleshooting CO₂ Changes

Low-end tidal CO₂ on APRV does not necessarily indicate hyperventilation. In many cases, it reflects severe VQ mismatch and shunt. Increasing the respiratory rate by shortening the time high often worsens oxygenation and leads to loss of recruitment. Instead, increasing high pressure by small increments is usually the correct move. If the lung is recruitable, CO₂ clearance improves as gas exchange surfaces reopen.

True hypercapnia on APRV is uncommon unless cardiac output or perfusion is impaired.

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