Optimizing Critical Care Ventilation: What can we learn from Ventilator Waveforms?

Optimizing Critical Care Ventilation- What Can We Learn from Ventilator Waveforms Luca Bigatello, MD Department of Anesthesia and Critical Care, Massachusetts General Hospital Associate Professor of Anesthesia, Harvard Medical School

Outline   Understanding the basic physiology of ventilation   Breath delivery:   Basic choices   Effects of patient’s changes   Effects of ventilator changes   Specific situations:   ALI / ARDS   Weaning

The Equation of Motion of the Respiratory System PAPPL = V × R + VT × E PAPPL = V × R + VT / C PAPPL = PMUS + PVENT = V × R + VT / C

From the Equation of Motion PAPPL = PMUS + PVENT = VT / C + V × R 1.  The basic mechanics of ventilation do not differ during spontaneous and mechanical breathing 2.  The results of mechanical ventilation depend not only on what we set on the vent., but also on the patient’s physiology- mechanics and effort

From the Equation of Motion PAPPL = PMUS + PVENT = VT / C + V × R 3.  For any independent variable set on the vent. (‘control’, e.g., PVENT), for any mechanics (C, R) and effort (PMUS), there is only one possible value of the dependent variable (VT ) 4.  If we know two ventilating variables, we can calculate the patient’s physiologic variable: C = VT / Paw

Breath Delivery: the simple view (the pt. is not breathing…)   Volume- controlled   Pressure- controlled The Emerson Critical Care Ventilator

Volume- Controlled Ventilation   Constant flow   Set: VT, flow   Airway pressure (Paw) will increases with:   VT, flow, ⇓’d C, ⇑’d R Paw   Will also decrease with:   ⇑’d pt. effort Volume Courtesy of Claudia Crimi, MD

V- CV Flow 30 l/min, VT 0.5 l   The effect of changing flow settings Flow 60 l/min, VT 0.5 l Courtesy of Dean Hess, RRT, PhD

V- CV Flow 60 l/min, VT 0.5 l   Theeffect of changes in respiratory mechanics Decreased compliance

Volume- Controlled Ventilation Paw   The effect of patient Flow effort   Insufficient flow   Vigorous insp. effort

Volume- Controlled Ventilation   Descending Ramp   Peak flow is reached early, then decreases linearly   Set pressure is reached earlier, is lower, and ≈ plateau   For the same set flow, the insp. time has to be longer⇒   better PaO2 (may be)   auto-PEEP

Volume- Controlled Ventilation   Descending ramp: the effect of time

Pressure- Controlled Ventilation   Set: insp. pressure, time   Flowis variable, and VT will change with:   patient’s mechanics: C, R   patient’s effort   capability of the vent.

Pressure- Controlled Ventilation   Changing ventilator settings: the effect of time Lucangelo et al., Respir Care 2005;50:55

P- CV Slow time-   Changes of resp. mechanics constant Increasing airways resistance Decreasing lung compliance Fast time- constant time

P- C, Inverse Ratio Ventilation Chan et al., Chest 1992;102:1556 Mercat et al. AJRCCM 1997;155:1637   Higher mean Paw⇒ better lung recruitment for the same plateau pressure   Higher mean Paw⇒ hemodynamic compromise, auto- PEEP, increased need for sedation

V- CV vs. P- CV   VCV   PCV   It assures the delivery   It assures a limit to of a desired minute insp. pressure (neonates, B-PF, etc.) ventilation ⇒ PaCO2   It allows further   It limits the size of the adjustment of breath VT (e.g., ARDS-Net) delivery (‘rise time’)   It’s simple   It may favor pt.-vent.   It allows bedside synchrony measurement of resp.   It may cause mechanics hypoventilation

Bedside Measurement of Respiratory Mechanics pressure PIP resistance Pplat compliance PEEP time

Bi-Level, Bi-PAP… (includes inverse-ratio, APRV)   Two different levels of pressure (high & low) are applied at a certain rate, and spontaneous breathing is allowed at both levels   In the absence of spont. breaths, this is P-CV!   The spont. breaths may be assisted by pressure support, generally only at the low level

Bi-Level, Bi-PAP

P- C, Airway Pressure Release Ventilation- APRV High Pressure Low Pressure time High pressure time Low pressure time

APRV Putensen et al., AJCCM 1999;159:1241 APRV Habashi, Crit Care Med 2005;33:S228   Without spont. breathing, ⇒ PCIRV   Enhanced lung recruitment at lower pressures, by allowing spont. breathing CPAP   Comfortable?   Important: think trans- pulmonary pressure!

Pressure- Regulated, Volume Control- PRVC (Auto-Flow, VC+) Dual Mode   PCV + a guaranteed VT Effect of decreased compliance   In each breath, the insp. pressure is regulated ⇑ or ⇓, to target a set VT   To maintain a set VT (VCV) while meeting pt. demand (PCV) Branson, Respir Care 2005;50:187

Breath Delivery: When the Patient Breathes   Assist- control   SIMV   Pressure support   CPAP

  SIMV   SIMV+ Pressure Support

Assist- Control Ventilation

What’s Happened to IMV ?   The principle of IMV- ventilator and patient can share the work in a fair proportion, does not pan out   Physiologically, it is a difficult principle to accept

Continuous Positive Airway Pressure- CPAP   Set: a pressure at the airway, throughout the resp. cycle- how???   The source provides a flow higher than the Paw patient’s own:   continuous high flow with a valveless system   just enough flow to match the pt.’s with a ventilator

CPAP- Uses   To recruit the lung:   obesity, OSA, postop. atelectasis   To increase intrathoracic pressure:   pulmonary edema   To offset auto-PEEP:   asthma/COPD   Zero CPAP for SBTs Antonelli et al., CCM 2002;30:602

Pressure Support Ventilation   Set: insp. pressure   VT, time change with:   patient’s effort   patient’s rate   pt.’s mechanics: C, R   Important: the effect of pt. effort on the trans- pulmonary pressure!!

Pressure Support Ventilation Flow 25%   The ‘cycling’ variable is unique to PSV:   Thefirst part of the breath is equal to PCV   Thebreath ends when Paw flow reaches a set low value, or % of the peak

Progressive Withdrawal of PSV Paw PSV= 0 PSV= 5 Peso PSV=10 PSV=15 Yamada et al., J Appl Physiol ’94;77:2237

Pressure Support Ventilation   PSV: effect of resp. mechanics   Pts. with a fast time-constant (low compliance- ALI/ARDS) have a sharp decline in insp. flow,⇒ low VT, low Paw, possible dyssynchrony   Pts. with a slow time-constant (high compliance, high resistance- COPD) have a slow decline in insp. flow⇒ large VT, auto-PEEP, dyssynchrony

PSV with Low Compliance: the Sigh   PCV+, BiLevel:   You can ‘trick’ the vent. by adding 1, 2 PC breaths during PSV Patroniti et al., Anesthesiology 2002;96:788

PSV with High Resistance: Dissynchrony   Backup cycling criteria: pressure, time Branson, Resp Care 1998; 43:1045

Volume- Assured Pressure Support- VAPS Dual Mode   Combines the initial flow pattern of PSV, with the constant flow of a VC breath   Important to learn the proper settings to take advantage of the dual mode Branson, Respir Care 2005;50:187

Proportional Assist Ventilation- PAV Younes M, ARRD 1992 PAPPL = PMUS + PVENT = VT x E + V × R PAPPL = PMUS + PVENT = K1 x E + K2 × R   PAV supports a % of ‘the patient effort’   In reality, a % of E and R   More ‘physiological’?

  The support PAV increases with the pt. effort   Needs an intact ventilatory drive   Generates a variable breathing pattern   May be more interesting physiologically than clinically Marantz et al., J Appl Physiol ‘96

PAV   In the US version, the support is expressed as % of the work of breathing, calculated from periodical measurement of R and E

What We Have NOT Discussed   Bedside measurement of respiratory mechanics   Loops   Triggering   Missed triggering   Tube compensation   NAVA Emerson Iron Lung

Conclusions   There is a gazillion modes of ventilation   Rather,there is a gazillion names of modes of ventilation   Understanding the physics of ventilation (equation of motion….) greatly simplifies understanding mechanical ventilation

Conclusions   Ventilatorwaveforms display helps guiding the use of the many modes of ventilation   Very little evidence exists that using one mode over another improves outcome   Patient outcome is affected more by how a mode is used than by the mode itself

Dyssynchrony   Missed triggering from dynamic hyperinflation

PSV: the ‘Rise Time’   PSV, PCV: the rate of rise of the insp. flow can be adjusted:   a high drive, a fast time-constant may require a high rate of rise   Quiet breathing, bronchospasm may benefit from a slower rate of rise Chiumello et al., Eur Respir J. 2001;18:107

PSV+ Tube Compensation   ATC:   To overcome resistive WOB imposed by the ETT: PSV   The vent. applies additional pressure to the airway pressure (cm H2O) throughout the resp. cycle, based on known resistance of ETTs and measured flow, resulting in more even ATC tracheal pressure Fabry, Intensive Care Med 1997; 23:545

PSV   Expiratory sensitivity Tokioka et al., Anesth Analg. 2001;92:161

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Optimizing Critical Care Ventilation: What can we learn from Ventilator Waveforms?

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Optimizing Critical Care Ventilation: What can we learn from Ventilator Waveforms? — Presentation Transcript