Pressure Control 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 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 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
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
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. 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 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
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