OVERVIEW OF �MECHANICAL VENTILATION

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Dr. Do Ngoc Son

Center for Critical Care Medicine – Bach Mai Hospital

Outlines

1. Basics of mechanical ventilation
2. Types of mechanical ventilation
3. Mechanical phases and controls
4. Modes of ventilation

Basics of mechanical ventilation

Ventilation and gas exchange

Ventilation and gas exchange

The airway-lung-chest wall assembly can be mimicked by modified fireplace bellows

Characteristics of mechanical ventilation

• It is a life-supporting respiratory therapy;
• It requires a ventilator system;
• It is powerful and effective;
• It is highly risky;
• It is highly complicated;

• It is labor intensive;
• It is error prone;
• It is typically applied by a group of clinicians on different shifts;
• It is extraordinarily costly;
• It is widely applied the world over.

The knowledge required for successful mechanical ventilation

Clinical application of mechanical ventilation

• Is mechanical ventilation necessary for this patient?
• Should the patient be ventilated invasively or noninvasively?
• Must this patient be intubated now?
• Which ventilation mode is optimal for this patient now?
• What are the optimal control settings for this patient, such as respiratory rate, tidal volume, inspiratory pressure, positive end-expiratory pressure (PEEP), and fraction of inspired oxygen (FiO2)?
• How can I optimize the patient-ventilator interaction?
• Is sedation necessary?
• What can I do to minimize ventilator-induced lung injury?

The therapeutic equipment

• What therapeutic equipment is needed for mechanical ventilation?
• What are the essential components of a ventilator system?
• How is a ventilator system assembled?
• How is intermittent positive airway pressure generated?
• What are the required conditions for a ventilator system to function?
• How does a ventilator control delivery of inspiratory gas?
• What are the differences between volume modes, pressure modes, and adaptive modes?
• What do the various ventilator controls mean and how do they work?
• What do the ventilator monitoring parameters mean?
• What do the ventilator alarms mean and how do they work?
• How can I prevent and troubleshoot equipment problems during mechanical ventilation?
• How can I warm and humidify the inspiratory air properly?

Three major origins of mechanical ventilation problems

The required conditions

• All required parts are present and functioning properly;
• The system is assembled correctly and securely;
• The supplies of compressed air and oxygen are continuous and appropriate in pressure and flow;
• The electrical supply is continuous and appropriate in voltage and frequency;
• The entire system is gas tight;
• The entire system is free from occlusion;
• The patient’s lung volume is capable of changing normally as the applied airway pressure changes;
• The operator has sufficient expertise to define and adjust the ventilator mode, controls, and alarm limits when the patient’s condition changes during mechanical ventilation.

How is variable P generated?

The foundation of intermittent positive pressure ventilation (IPPV) is the alternation of Pao. The desired Pao change is created by opening and closing the two tubes with hands A and B in the balloon model.

How is variable P generated?

A complete ventilator system and its operator

1. Pressurized O2 and air supplies;
2. An electrical supply;
3. A ventilator;
4. A breathing circuit;
5. An airway;
6. A patient’s lungs or a test lung to mimic the lungs.

A ventilator system has two pressure areas that are connected by an airway

The forces to inflate and deflate the lungs

The six physical forces involved in mechanical ventilation

Functional residual capacity (FRC), tidal volume, and dead space

The relationship between minute volume, tidal volume, rate, and dead space

Effect of increased arterial PCO2 and decreased arterial pH on the rate of alveolar ventilation

Pressures

• Pressure (P): A force exerted against resistance or a force applied uniformly over a unit area of surface. Atmospheric pressure and peak airway pressure are two typical examples.
• Pressure gradient (ΔP): The pressure difference between two connected areas.

Pressures

Flow

flow (V′ or V̇): The motion of gas volume over time. Flow = volume/ time.

Compliance (C)

• Compliance (C): A parameter to describe the pressure–volume relationship in a balloon-like structure.

Resistance (R)

• Resistance (R): A force that tends to oppose or retard gas movement
• Resistance is flow dependent. Whenever a gas flows through a tube, a given resistance is generated. It is usually expressed mbar/L/min or cmH2O/L/min:
• Flow rate. The higher the flow rate, the greater the resistance, and vice versa. If the flow rate drops to zero, resistance disappears.
• Physical properties of the tube, such as length, internal diameter, inner surface, and curvature.
• Physical properties of the gas, such as density and viscosity.

Time constant (RC)

Time constant (RC): An estimation of the time needed to complete the process of lung inflation or deflation.

• After one time constant (1 × RC), 63% of the volume change is complete;
• After two time constants (2 × RC), 86.5% is complete;
• After three time constants (3 × RC), 95% is complete;
• After four time constants (4 × RC), 98% is complete;
• After five time constants (5 × RC), 99% is complete.

The time constant is used to estimate the time required to complete a flow process (e.g. expiration) with the current compliance and resistance in a passive model.

Types of mechanical ventilation

Definition

Mechanical ventilation can be realized with one of three operating principles:

• Intermittent positive pressure ventilation (IPPV),
• Intermittent negative pressure ventilation (INPV), and
• High-frequency ventilation (HFV).

IPPV is currently the most popular and is the basis for most commercially available ventilators.

Pressure waveform in intermittent positive pressure ventilation (IPPV)

Some popular ICU ventilators based on the intermittent positive pressure ventilation (IPPV) principle

Pressure waveform in intermittent negative pressure ventilation (INPV)

a) An iron lung, b) A cuirass ventilator

During high-frequency ventilation (HFV), positive pressure or positive-negative pressure is applied to the airway opening at a very high rate

3000A high-frequency oscillatory ventilator from CareFusion

Mechanical phases and controls

Intermittent positive pressure applied to airway opening

Ventilation variables

Manual of Neonatal Respiratory Care pp 87-91

Paw

Risetime

Cycling

Limited

Triggering

Time

Typical P (orange), P (dotted white), and airway flow change during a pressure breath (a), and volume breath (b), in a passive patient.

The type of mechanical breath is determined by five essential variables

1. Triggering: defines when inspiration begins;
2. Cycling: defines when inspiration ends;
3. Controlling: defines how delivery of inspiratory gas is controlled;
4. Targeting: defines the size of a mechanical breath;
5. Baseline: defines the baseline pressure at which mechanical breaths occur.

Essential variables and their common mechanisms

The relationship between breath cycle time (BCT), inspiratory time (T ), and expiratory time (T )

The principle of pressure triggering

The principle of flow triggering with base flow

Common root causes of abnormal triggering

Cycling refers to the end of inspiration

Relationship between peak flow (height), Ti (width), and tidal volume (total area)

Flow cycling works with the descending part of inspiratory flow (between a and b)

Volume breaths have four common patterns of inspiratory flow

The mechanism to generate inspiratory pressure in a pressure breath

The meaning of rise time and pressure overshoot.

Pressure and flow waveforms of a volume breath with constant flow (left) and a pressure/adaptive breath (right) in a passive model. The white dotted curves indicate alveolar pressure

 Flow waveforms of three mechanical breaths with hybrid controlling

Dynamic regulation of expiratory valve to maintain a stable baseline at the desired level

Depending on the triggering and cycling mechanisms applied

• A control breath is a mechanical breath that is time triggered and time cycled.
• An assist breath is a mechanical breath that is patient (pressure or flow) triggered and time cycled.
• A support breath is a mechanical breath that is patient (pressure or flow) triggered and flow cycled.

Depending on the controlling mechanism applied

• A volume breath is a mechanical breath with volume controlling.
• A pressure breath is a mechanical breath with pressure controlling.
• An adaptive breath is a mechanical breath with adaptive controlling.

Characteristics of mechanical breath types

Ventilation modes

Mode of ventilation�The true position of ventilation modes in the big picture

Categories of ventilation mode. The dotted lines indicate the inherent relationships between pressure modes and adaptive modes

Terminology of common ventilation modes

Graphic symbols for breath types

Volume assist/control (A/C) mode

Settings: Tidal volume; Rate; T (or I:E ratio or peak flow); Patient trigger type and sensitivity; PEEP (positive end-expiratory pressure); FiO2; Flow pattern (possibly).

Pressure assist/control (A/C) mode

Settings: Inspiratory pressure (often called pressure control); Rate; Ti (or I:E); Patient trigger type and sensitivity; PEEP; FiO2; Rise time (possibly).

Pressure support ventilation (PSV) mode

Settings: Inspiratory pressure (also known as pressure support); Patient trigger type and sensitivity; PEEP; FiO2; Flow cycling criteria; Rise time (possibly).

Volume SIMV mode

Settings: Tidal volume; Rate (also known as SIMV rate); T (or I:E); Psupport; Patient trigger type and sensitivity; Flow cycling; Rise time (possibly); PEEP; FiO2.

Volume SIMV mode

Typical pressure-time waveform for volume SIMV mode with a high set rate

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Typical pressure-time waveform for volume SIMV mode with a low set rate

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Pressure SIMV mode

Settings: Pcontrol; Rate (also known as SIMV rate); Ti (or I:E); Pressure support; Patient trigger type and sensitivity; Flow cycling g. Rise time (possibly); PEEP; FiO2.

Pressure SIMV mode

Typical pressure-time waveform for pressure SIMV with a high set rate

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Typical pressure-time waveform for pressure SIMV with a low set rate

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 In practice, a ventilator system is often dynamic

Closed-loop ventilation control

1. The operator’s command to define the target (e.g. target tidal volume).
2. The controller: A software algorithm that takes in the monitoring input, compares it to the set target, determines how to respond, and sends instructions to the executer.
3. The executer: The ventilator system, which carries out the mechanical ventilation according to operator settings and autoregulation.
4. The subject: The patient’s pulmonary system.
5. The output: The result of interaction between the executer and the subject. It changes if either or both sides change.
6. Monitoring: The component that closes the loop. With it, the output can directly influence or change the behavior of the ventilator system without the operator’s involvement.

Negative feedback regulation for stabilization purposes

Human physiology applies extensive regulation by negative feedback loops in the regulation of blood pressure, blood sugar, PaCO2 , and body temperature, among others. Adaptive ventilation modes apply negative feedback loop regulation in a limited sense. A familiar, non-medical example is the cruise control in a car.

Positive feedback regulation for amplification purposes

Human physiology also, although rarely, applies regulation by positive feedback loop. Blood clotting and childbirth are two examples. Proportional assist ventilation (PAV) and tube resistance compensation (TRC) are based on a positive feedback loop. A non-medical example is power steering in a car.

Advantages of adaptive ventilation modes

• Proper use of adaptive modes can considerably reduce the staff’s workload, improve the therapy quality, and decrease the incidence of ventilator alarms. Note that these advantages relate exclusively to ventilator setting.
• Adaptive modes are not superior in terms of nonsetting issues.

The set high pressure alarm limit and pressure regulation ceiling in an adaptive mode

Adaptive assist/control (A/C) mode

Settings: (Target) tidal volume; Rate; Ti (or I:E); Patient trigger type and sensitivity; Rise time; PEEP; FiO2.

Adaptive SIMV mode

Settings: Target tidal volume; Rate (also known as SIMV rate); Ti (or I:E); Pressure support; Patient trigger type and sensitivity; Flow cycling criterion; Rise time; PEEP; FiO2.

Adaptive SIMV mode

Typical pressure-time waveform for adaptive SIMV with a high set rate

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Typical pressure-time waveform for adaptive SIMV with a low set rate

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Volume support mode

Settings: Target tidal volume; Patient trigger type and sensitivity; Flow cycling criteria; Rise time; PEEP; FiO2.

Biphasic ventilation modes: Why?

Inspiratory pressure and baseline pressure in non-biphasic modes

Biphasic ventilation modes

• PEEP-high: the high level of PEEP;
• PEEP-low: the low level of PEEP;

• T-high: the duration of PEEP at the high level;
• T-low: the duration of PEEP at the low level.

Biphasic ventilation modes

Relationship between ventilation modes and controls

• Mode: Qualitatively defines the breath types permitted
• Controls: Quantitatively defines the variables of the breath types

How to select a ventilation mode

• Patient breathing activity
• Staff familiarity

Patient breathing activity

• Passive patients who have no spontaneous breathing activity at all. The ventilator system must do all the work of breathing.
• Partially active patients who have weak or unstable spontaneous breathing activity. The ventilator system does most of the work of breathing.
• Active patients who have strong and stable spontaneous breathing activity. The patient and the ventilator system together provide the required work of breathing. Weaning and extubation are possible only when a ventilated patient is active.

Common ventilation modes, permissible breath types, and suitable applications

Volume, pressure, or adaptive?

The changes in ventilation mode selected in a mixed intensive care unit in Switzerland over the last 25 years.

Volume, pressure, or adaptive?

• For passive patients, volume and pressure modes work equally well.
• For partially active patients, both volume modes and pressure modes are suitable. In theory, pressure modes may be a bit better.
• For active patients, only pressure modes should be used. Active patients cannot tolerate volume modes well.
• Adaptive modes and the corresponding pressure modes share the same indications. If, and only if, the target tidal volume is set and adjusted properly and promptly, an adaptive mode is superior to the corresponding pressure mode.

BiPAP and APRV?

• The BiPAP mode, when used to mimic pressure A/C, is suitable for passive patients and partially active patients. It may be a bit superior to the pressure A/C mode in partially active patients.
• The APRV mode may be a very good choice for active patients with severe restrictive lung diseases. It is contraindicated in passive patients. Be aware that both BiPAP and APRV are relatively new modes. Adequate staff education is of paramount importance before the modes are introduced into clinical practice.

Ten Basic Maxims for Understanding Ventilator Operation

(1) A breath is one cycle of positive flow (inspiration) and negative flow (expiration) defined in terms of the flow vs time curve.

(2) A breath is assisted if the ventilator provides some or all of the work of breathing.

(3) A ventilator assists breathing using either pressure control or volume control based on the equation of motion for the respiratory system.

(4) Breaths are classified according to the criteria that trigger (start) and cycle (stop) inspiration.

(5) Trigger and cycle events can be either patient-initiated or ventilator-initiated.

(6) Breaths are classified as spontaneous or mandatory based on both the trigger and cycle events.

(7) Ventilators deliver 3 basic breath sequences: CMV, IMV, and CSV.

(8) Ventilators deliver 5 basic ventilatory patterns: VC-CMV, VCIMV, PC-CMV, PC-IMV, and PC-CSV.

(9) Within each ventilatory pattern, there are several types that can be distinguished by their targeting schemes (set-point, dual, biovariable, servo, adaptive, optimal, and intelligent).

(10) A mode of ventilation is classified according to its control variable, breath sequence, and targeting schemes.

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