New Modes in Mechanical Ventilation

Manish TandonHartford Hospital

July 10, 2013

PRVC

VC +

VS

APRV

HFO

Bi-LevelPAV

PAV +

NAVAATC

ASV

PC-IRV

AC (VC)

AC (PC)

SIMV (VC) + PS

SIMV (PC) + PS

PSV

CPAP

Variable PS

Understanding differences in vent modes

• What initiates the breath• What controls the amount of airflow during

inspiration• What controls the duration of inspiration

• Philosophy of the mode

Standard Modes

• Volume Control• Pressure Control• Pressure Control Inverse Ratio Ventilation• Pressure Support• CPAP

Traditional Volume Control

Flow

Pressure

Volume

Current Volume Control

Flow

Pressure

Volume

Pressure Control

Pressure = __Volume__ + Flow * Resistance Compliance

Initially, high pressure on vent >> low volume of lung (therefore low pressure in lung) => high flowLater, high pressure on vent = higher lung volume (therefore higher pressure in lung) => low or zero flow

Flow

Pressure

Volume

Pressure Control Inverse Ratio

Philosophy of mode = control and increase mean airway pressure, by increasing ratio of time spent at higher pressure

Flow

Pressure

Volume

pO2

FiO2

MAP

PEEP

I/E ratio

pCO2

Minute volume

Rate

Tidal Volume

Dead space

Pressure Support

Flow

Pressure

Volume

CPAP

Flow

Pressure

Volume

Newer Modes

• PRVC/VC/VS• APRV• PAV• NAVA

Pressure Regulated Volume Control, Volume Control, Volume Support

• Breath is initiated by patient or elapsed time• PC used for ventilator initiated breath, to target goal

tidal volume• PS used for patient initiated breath, to target goal tidal

volume• “Set like VC, flows like PC”• Philosophy – allow more natural decelerating flow

• Fallacy – uses lower pressure to achieve the same volume

Flow

Pressure

Volume

APRV

• Sustained CPAP to ventilate the lung while recruited

• Short, infrequent releases of pressure to augment the minute volume for CO2 clearance

• Fallacy – inverse ratio ventilation

Proportional Assist Ventilation

• Ventilator provides support in proportion to patient’s effort

Neurally Adjusted Ventilator Assist

• Measures diaphragmattic activity as a proxy of phrenic nerve activity

• Breath initiated and ended based on diaphragmattic activity

• Flow proportional to amount of activity and based on ratio set by provider

• Philosophy – better synchrony with patient’s efforts

Why all of the new modes?

For the company• Latest hardware, latest

software• Modalities requested by

providers• Product differentiation• Increased switching

costs• Sell more vents

For the providers• Greatest/latest toy• Easier to achieve goals

of ventilation

Goals of mechanical ventilation

• Provide support– Not necessarily a perfect ABG

• Do no harm– Ventilator Induced Lung Injury

• Shortest required duration on ventilator

• Improve long term lung function in survivors

Too Much of a Good Thing

Tremblay L, et al. J Clin Invest 1997; 99(5): 944.

Current best practice

• Limit airway pressures• Limit tidal volumes

Limitation of current practice• How to minimize shear injuries from

collapse/re-opening

• Shear forces are increased in heterogeneous lung

• These abnormal stresses can also affect the pulmonary capillaries

Mead J, et al. JAP 1970; 28: 596.West JB, et al. JAP 1991; 70: 1731.Marini JJ (ed). Phys Basis of Vent Support (1998): p. 1226.

Heterogeneous Lung = Heterogeneous Opening &Closing Pressures

Marini JJ (ed). Acute Lung Injury (1997): p. 240.

Recruiting with time – HFO and APRV

Froese AB, Bryan AC. Anesthesiology 1974; 41: 242.

Recruiting with Spontaneous Breaths

Evidence for APRV• Improves oxygenation• Improves distribution of ventilation• Improves renal blood flow• Improves mesenteric perfusion• Decreases development of ARDSLimitation – unknown if decrease in mortality or

less time on ventPutensen C, et al. AJRCCM 1999; 159: 1241.

Putensen C, et al. AJRCCM 2001; 164: 43.Hering R, et al. ICM 2002; 28: 1426.

Hering, R, et al. Anesthesiology 2003; 99(5): 1137.Roy, S, et al. Shock 2013; 39(1): 28.

As promising as HFO?

Liberating from ventilator

Current best practice• Fix underlying problem• Daily assessment of need for ventilator• Correct underlying reason for needing supportAdditional goals• Use vent mode which allows lower sedation– Decrease dysynchrony

• Allow the patient to do some, but not too much, work of breathing

Studies of PAV and NAVA

• Evidence of better synchrony with patient• More natural variation in tidal volumes– Better oxygenation

• No proven benefit so far of shorter duration on ventilator

Fastest way to liberate off vent

• Daily spontaneous breathing trial– Remove the ventilator to eliminate dysynchrony

• Limit/avoid sedation• Fix underlying problem each day the patient

needs the vent– e.g. Infection, Fluids, Debilitation

• Automated Weaning Trials– Eliminate our variability in removing support

Potential value of PAV and NAVA

• NIV• May allow more patients to be supported

without intubation

The Future of the Ventilator?

Conclusion

• Keep it simple• Don’t intubate the patient if you don’t have to– Use NIV in appropriate settings

• Provide support, but not so much that it will harm the patient

• Look each day to see if the patient still needs the vent• Determine why it is needed, and fix the underlying

issue• Future – avoiding the ventilator altogether?– NIV, ECMO