Advanced Modes of Mechanical Ventilation Michael Haines, MPH, RRT-NPS, AE-C Victor Valley Community College
What we will cover… • Intro to advanced modes • PRVC • Automode • Volume Support/Variable Pressure • Autoflow • Adaptive Support Ventilation • Volume assured pressure support (VAPS) • Automatic Tube compensation • Mandatory Minute Ventilation • Proportional Assist Ventilation • BUT FIRST A LITTLE REVIEW….
transairway pressure transrespiratory pressure transthoracic pressure volume elastance = Dpressure / Dvolume Lung Mechanics resistance = Dpressure / Dflow flow
Static Compliance • Cs = tidal volumecorrected for gas compression Pplat – PEEP total peep Normal 100 - 200 ml / cmH2O (PDQ) Decreased with: • Mainstem Intubation • Congestive Heart Failure • ARDS • Atelectasis • Consolidation • Fibrosis • Hyperinflation • Tension Pneumothorax • Pleural Effusion • Abdominal Distension • Chest Wall Edema • Thoracic Deformity
Principle #1: Ventilation The goal of ventilation is to facilitate CO2 release and maintain a normal PaCO2 • Minute Ventilation (Ve) • Total amount of gas exhaled per minute • Ve = Vt x f • Ve comprised of 2 factors • VA = alveolar ventilation • VD = dead space ventilation • Ventilation in the ICU setting • Increased CO2 production • Fever, sepsis, injury, overfeeding • Increased VD • Vent circuit, ET tube • Adjustments: Vt and f
Fig. 13-1. Factors that affect the partial pressure of arterial carbon dioxide (PaCO2) during mechanical ventilation. V.CO2, carbon dioxide production; V.A, alveolar ventilation; V.E, minute ventilation; V.D, dead space ventilation; VT, tidal volume; TI, inspiratory time; TE, expiratory time; f, respiratory rate. (From Hess DR, MacIntyre NR, Mishoe SC, et al: Respiratory care principles and practice, Philadelphia, 2002, WB Saunders.)
Principle #2: Oxygenation The primary goal of oxygenation is to maximize O2 delivery to the blood (PaO2) • Alveolar-arterial O2 gradient • Equilibrium between O2 in the blood and O2 in the alveoli • A-a gradient measures efficiency of oxygenation • PaO2 partially depends on ventilation but more on V/Q matching • Oxygenation in the ICU setting • PaO2/PAO2 ratio (a/A ratio) • Indicator of efficiency of O2 transport • CaO2 • Adjustments: FiO2 and PEEP
Volume vs. Pressure Control Ventilation Volume Ventilation • Volume delivery constant • Inspiratory pressure varies • Inspiratory flow constant • Inspiratory time determined by set flow and Vt Pressure Ventilation • Volume delivery varies • Inspiratory pressure constant • Inspiratory flow varies • Inspiratory time set by clinician
What’s Wrong with Volume Control Ventilation? • The limited flow may not meet the patient’s desired inspiratory flow rate • If the patient continues to inspire vigorously -- , added, unnecessary work is done • Can lead to fatigue • Can cause excessive airway pressure leading to barotrauma, volutrauma, and adverse hemodynamic effects
Pressure Control Ventilation:The Alternative • Definition • The application of clinician-set inspiratory pressure and inspiratory time. Flow delivery varies according to patient demand • The clinician sets the inspiratory pressure, I-time or I:E ratio and RR • Tidal volume varies with changes in compliance and resistance • Flow delivery is decelerating
Pressure Control Ventilation • May be used in A/C and SIMV modes • In A/C - all breaths (either machine-initiated or patient-initiated) are time-cycled and pressure-limited • In SIMV - only machine-initiated breaths are time-cycled and pressure-limited • Spontaneous breaths can be pressure-supported
Pressure Control Ventilation • Advantages • Limits risk of barotrauma • May recruit collapsed and flooded alveoli • Improved gas distribution • Uses a active exhalation valve which uses servo-control technology that allows gas to be released from the exhalation valve during the inspiratory phase if the patient makes an expiratory effort. • Disadvantages • Tidal volumes vary when patient compliance changes (i.e., ARDS, pulmonary edema) • With increases in I-time, patient may require sedation and/or chemical paralysis
Indications for PCV • Enhance patient / ventilatory synchrony • Patient determines flow • Lung protection strategy • Lower inspiratory pressure with decelerating flow may improve V/Q matching • Adjusting I-time may improve oxygenation by ↑ MAP • Alveolar diseases that produce varying time constants • May recruit alveoli by lengthening I-time
Rationale of Pressure Modes • Ventilator-induced lung injury (VILI) • Atelectrauma • Pre-existing lung damage and/or inflammation
The Cons of Pressure Control • Variable Vt as pulmonary mechanics change • Potentially excessive Vt as compliance improves • Inconsistent changes in Vt with changes in PIP and PEEP
Most Commonly used Waveforms • Pressure vs. Time • Flow vs. Time • Volume vs. Time
Pressure-Time Curve 20 Volume Ventilation Pressure Ventilation Paw Expiration cmH2O Sec 1 2 3 4 5 6
30 A B C PIP Baseline Paw Mean Airway Pressure cmH2O Sec 1 2 3 4 5 6 -10 Pressure vs. Time Curve
P aw cmH 0 2 Volume Control Breath Types 60 SEC 1 2 3 4 5 6 -20 120 INSP Flow SEC 1 2 3 4 5 6 L/min 120 EXH If compliance decreases the pressure increases to maintain the same Vt
Paw Paw Volume/Flow Control Pressure Control Inspiration Expiration Inspiration Expiration 20 20 Pressure 0 0 2 2 1 0 1 20 20 Volume 0 0 2 2 0 1 0 1 3 3 Flow Time (s) Time (s) 0 0 -3 -3
These curves illustrate the two basic approaches to ventilator control. If the ventilator controls flow, it controls volume indirectly (by definition) and vice versa. Usually, inspiratory flow is held constant during inspiration, causing volume and pressure to rise linearly. Inspiration ends (cycles off) when a preset tidal volume is met. • In contrast, with pressure control ventilation, airway pressure may be held constant during inspiration. This causes inspiratory flow to decay exponentially from its peak value towards zero as volume rises exponentially. Inspiration usually ends after a preset inspiratory time or (in the case of pressure support) after a preset inspiratory flow threshold has been crossed. If inspiratory time is long enough (usually about 5 time constants) lung pressure will equilibrate with airway pressure and inspiratory flow will cease. • You will note that for passive exhalation is exponential. That mean expiratory time must be at least 5 time constants long to exhale more that 99% of the tidal volume. As expiratory time becomes shorter than 5 time constants, gas trapping (ie, autoPEEP) occurs.
Work to Trigger 30 Paw cmH2O Sec 1 2 3 4 5 6 -10
Analysis of Compliance Waveforms • Compliance waveforms simultaneously display volumes and the amount of pressure necessary to deliver these volumes. Volume normally is plotted on the “Y” axis and pressure on the “X” axis. • The curve to the right depicts a compliance curve from a patient with normal compliance and airway resistance. The arrow pointing down and to the left is on the expiratory side of the curve.
Analysis of Compliance Waveforms • One of the clinical indications for the addition of positive-end expiratory pressure (PEEP) is low lung compliance. If PEEP is added, the baseline pressure would then be elevated and the curve would shift to the right.
Altering Compliance with PEEP • The curve drawn in a heavy, non-dashed line represents an improved lung compliance due to the addition of PEEP. Notice that the tidal volume is the same (note the “Y” axis) while the PIP has fallen (note the “X” axis). Since the same volume is delivered with less of a pressure difference (PIP-PEEP), compliance has increased.
Compliance Curves • The following series of compliance curves reflect a steady fall in lung compliance, as would occur with the development of cardiogenic or noncardiogenic pulmonary edema. The first curve (1) reflects the patient’s baseline condition. As his compliance falls, higher pressures are needed to deliver the same tidal volume (2) (second set of curves with the initial curve indicated in gray). As the patient’s condition deteriorates further, the final compliance curve is obtained (3).
Assist/Control Mechanical Ventilation • Notice that the third mechanical breath was preceded by a drop in airway pressure (indicating a spontaneous inspiratory effort). In addition, note that the TCT had not elapsed prior to the initiation of this breath. Although only one of the breaths was initiated spontaneously, all breaths had the same tidal volume.
Synchronized Intermittent Mandatory Ventilation (SIMV) • Notice that the fifth breath was a mechanical breath that was initiated by a spontaneous inspiratory effort. If this effort had occurred before the sensitivity window began, the patient would have only had a spontaneous, unassisted breath (circled). In addition, notice that the therapist selected a constant flow pattern for this patient.
Support Ventilation (PSV) • Salient features of the flow graph: • The amount of inspiratory flow may vary from breath to breath based on patient inspiratory effort (V1<V2). • Duration of each breath may vary. • http://www.youtube.com/watch?v=Rwr5ZjJI1ZQ
Support Ventilation (PSV) • Salient features of the volume graph: • The tidal volume may vary from breath to breath based on patient inspiratory effort (V1<V2). • Duration of each breath may vary.
Pressure, Flow, and Volume Curves • A clearer picture of the dynamics of plateau pressures and inflation holds is obtained when pressure curves are viewed along with their corresponding flow and volume curves. Notice that flowrate drops to zero during the plateau interval, separating expiratory flow from inspiratory flow. In addition, even though flow is not occurring during the inflation hold, the inflation hold is still considered to be part of inspiratory time or Ti. Since no flow is occurring the volume does not change during the pause.
Pressure vs. Volume Ventilation(From Branson, R., Bird product literature)
Within-breath Adjustment Automatic Tube Compensation (ATC) Volume-Assured Pressure Support New Modes: Dual Modes • Between-Breath Adjustment • Volume Support (VS) • Pressure-Regulated Volume Control
Why use newer modes of ventilation? • Newer ventilators can be set to modes other than the pressure-control and volume-control modes of older machines • The alternative modes of ventilation were developed to prevent lung injury and asynchrony through patient adaptation, promote better oxygenation and faster weaning, and be easier to use. • However, evidence of their benefit is scant. • Remember: weaning is a dynamic process requiring frequent intervention and adjustments, best performed by the RT!
Why use newer modes of ventilation? • Technologic advances and computerized control of mechanical ventilators have made it possible to deliver ventilatory assistance in new modes. Driving these innovations is the desire to prevent ventilator induced lung injury, improve patient comfort, and liberate the patient from mechanical ventilation as soon as possible • We call these innovations “alternative” modes to differentiate them from the plain volume-control and pressure-control modes
Terminology APV- adaptive pressure ventilation ATC – Automatic tubing compensation VP- variable pressure VTPC- Volume targeted pressure control • APC—adaptive pressure control • APRV—airway pressure-release ventilation • ASV—adaptive support ventilation • HFOV—high-frequency oscillatory ventilation • MMV- Mandatory Minute Ventilation • PAV—proportional assist ventilation • PRVC – Pressure Release Volume Control • PSV—pressure support ventilation • VC+ - Volume control plus • VS- Volume Support
Patient-ventilator Asynchrony • 24% of mechanically ventilated patients exhibit patient-ventilator asynchrony in > 10% of their respiratory efforts during AVC and PS ventilation (ineffective triggering and double triggering). • Patient-ventilator asynchrony during assisted mechanical ventilationIntensive Care Med. 2006;32:1512 Arnold W. Thille, Pablo Rodriguez, Belen Cabello Francois Lellouche, Laurent Brochard
Length of Stay Asynchrony Sedation Prolonged ventilation time1 Possible muscle atrophy2 and VAP3 Weaning is delayed 1. Kollef M et al. Chest. 1998;114:541–548. 2. Levine S et al. NEJM .2008;358:1327-1335. 3. Rello J et al. Chest .2002;122:2115-2121.
Ventilator asynchrony is manifested in several forms • Common asynchrony patterns include missed efforts, double triggering and auto-cycling. • These problems typically occur when the breath parameters set on the ventilator do not match the signals from the patient’s respiratory center in the brain. • The upper graphic shows multiple missed efforts in the pressure support mode. • The lower graphic shows an asynchronous pattern called “double trigger” in the assist control mode. • Because patient conditions are constantly changing, frequent manipulation of the ventilator settings are required to manage the asynchrony. It is not uncommon for patients to be sedated as a result of asynchrony and this has been shown to prolong ventilation time.1 Furthermore, prolonged ventilation time can result in rapid disuse atrophy of the diaphragm2 and ventilator-associated pneumonia.3
Mechanical breath terminology • Control variable—the mechanical breath goal, ie, a set pressure or a set volume • Trigger variable—that which starts inspiration, ie, the patient (generating changes in pressure or flow) or a set rate (time between breaths) • Limit variable—the maximum value during inspiration • Cycle variable—that which ends inspiration
Mechanical breath terminology • Continuous mandatory ventilation—all breaths are controlled by the ventilator, so usually they have the same characteristics regardless of the trigger (patient or set rate); no spontaneous breaths are allowed • Intermittent mandatory ventilation—a set number of mechanical breaths is delivered regardless of the trigger (patient initiation or set rate); spontaneous breaths are allowed between or during mandatory breaths • Continuous spontaneous ventilation—all breaths are spontaneous with or without assistance
Mechanical breath terminology • Set point—the ventilator delivers and maintains a set goal, and this goal is constant (eg, in pressure control, the set point is pressure, which will remain constant throughout the breath) • Servo—the ventilator adjusts its output to a given patient variable (ie, in proportional assist ventilation, • the inspiratory flow follows and amplifies the patient’s own flow pattern) • Adaptive—the ventilator adjusts a set point to maintain a different operator-selected set point (ie, in pressure-regulated volume control, the inspiratory pressure is adjusted breath to breath to achieve a target tidal volume) • Optimal—the ventilator uses a mathematical model to calculate the set points to achieve a goal (ie, in adaptive support ventilation, the pressure, respiratory rate, and tidal volume are adjusted to achieve a goal minute ventilation)
Examples of the first dual modes • Volume Assured Pressure Support (VAPS) & Pressure Augmentation • Pressure Regulated Volume Control (PRVC) & similar modes • Volume Support Ventilation (VS or VSV) & similar modes
Dual Control Breath-to-Breathpressure-limited time-cycled ventilationPressure Regulated Volume Control Maquet Servo-i Servo 300
Other Names for PRVC… AutoFlow (Drager • Medical AG, Lubeck, Germany) Adaptive Pressure Ventilation (Hamilton Galileo, Hamilton Medical AG, Bonaduz, Switzerland) Volume Control+ (Puritan Bennett, Tyco Healthcare; Mansfield, MA) Volume Targeted Pressure Control, Pressure Controlled Volume Guaranteed (Engstrom, General Electric, Madison, WI).