New Modes in Mechanical Ventilation Manish Tandon Hartford Hospital July 10, 2013
ATC PAV AC (VC) Bi-Level PC-IRV SIMV (PC) + PS ASV PRVC PAV + NAVA HFO SIMV (VC) + PS VS Variable PS PSV AC (PC) VC + APRV CPAP
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 Flow Pressure Volume Pressure = __Volume__ + Flow * Resistance Compliance Initially, high pressure on vent >> low volume of lung (therefore low pressure in lung) => high flow Later, high pressure on vent = higher lung volume (therefore higher pressure in lung) => low or zero flow
Pressure Control Inverse Ratio Flow Pressure Volume Philosophy of mode = control and increase mean airway pressure, by increasing ratio of time spent at higher pressure
FiO2 PEEP pO2 MAP I/E ratio Rate Minute volume pCO2 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
Heterogeneous Lung = Heterogeneous Opening &Closing Pressures • Shear forces are increased in heterogeneous lung • These abnormal stresses can also affect the pulmonary capillaries Mead J, et al. JAP 1970; 28: 596. Marini JJ (ed). Phys Basis of Vent Support (1998): p. 1226. West JB, et al. JAP 1991; 70: 1731.
Recruiting with time – HFO and APRV Marini JJ (ed). Acute Lung Injury (1997): p. 240.
Recruiting with Spontaneous Breaths Froese AB, Bryan AC. Anesthesiology 1974; 41: 242.
Evidence for APRV • Improves oxygenation • Improves distribution of ventilation • Improves renal blood flow • Improves mesenteric perfusion • Decreases development of ARDS Limitation – unknown if decrease in mortality or less time on vent Putensen C, et al. AJRCCM 1999; 159: 1241. Putensen C, et al. AJRCCM 2001; 164: 43. Hering R, et al. ICM 2002; 28: 1426. As promising as HFO? Hering, R, et al. Anesthesiology 2003; 99(5): 1137. Roy, S, et al. Shock 2013; 39(1): 28.
Liberating from ventilator Current best practice • Fix underlying problem • Daily assessment of need for ventilator • Correct underlying reason for needing support Additional 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
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