Basic Principles of Mechanical Ventilation

Basic Principles ofMechanical Ventilation

IntroductionThe ventilatory needs of a patient depend largely on the mechanical properties of the respiratory system and the type of abnormality in gas exchange.

Pulmonary Mechanics A. The mechanical properties of the lungs

are a determinant of the interaction between the ventilator and the infant.

B. A pressure gradient between the airway opening and alveoli drives the fl ow of gases.

C. The pressure gradient necessary for adequate ventilation is largely determined by compliance and resistance.

ComplianceCompliance describes the elasticity or distensibility of

the lungs or respiratory system (lungs plus chest wall).

Compliance in infants with normal lungs ranges from 3 to 5 mL/cm H2O/kg.

Compliance in infants with RDS is lower and often ranges from 0.1 to 1 mL/cm H2O/kg.

ResistanceResistance describes the ability of the gas-conducting parts of the

lungs or respiratory system (lungs plus chest wall) to resist airflow.

Resistance in infants with normal lungs ranges from 25 to 50 cmH2O/L/sec.

Resistance is not markedly altered in infants with RDS or other acute pulmonary disorders but can be increased to 100 cmH2O/L/sec or more by small endotracheal tubes.

Normal lungs: 20-40 cm H2O/L/secRDS: 20-40 cm H2O/L/secIntubated infant: 50-150 cm H2O/L/sec

Time ConstantTime constant is the time (expressed in

seconds) necessary for the alveolar pressure (or volume) to reach 63% of a change in airway pressure (or volume).

Time ConstantA duration of inspiration or expiration

equivalent to 3 to 5 time constants is required for a relatively complete inspiration or expiration.

The time constant will be shorter if compliance is decreased (e.g., in patients with RDS) or if resistance is decreased.

The time constant will be longer compliance is high (e.g., large infants with normal lungs) or if resistance is high (e.g., infants with chronic lung disease).

Time ConstantPatients with a short time constant ventilate

well with short inspiratory and expiratory times and high ventilatory frequency,

whereas patients with a long time constant require longer inspiratory and expiratory times and lower rates.

If inspiratory time is too short (i.e., a duration shorter than approximately 3 to 5 time constants), there will be a decrease in tidal volume delivery and mean airway pressure.

If expiratory time is too short (i.e., a duration shorter than approximately 3 to 5 time constants), the result will be gas trapping and inadvertent (auto) PEEP.

Goals of MechanicalVentilation

Achieve and maintain adequate

pulmonary gas exchange Minimize the risk of lung injury

Reduce patient work of breathing

Optimize patient comfort

VentilationThe goal of ventilation is to facilitate CO2 release and maintain normal PaCO2

• Minute ventilation (MVE)• Total amount of gas exhaled/min.• MVE = (RR) x (TV)• MVE comprised of 2 factors

• VA = alveolar ventilation• VD = dead space ventilation

• VD/VT = 0.33• VE regulated by brain stem, responding to pH and PaCO2

• Ventilation in context of ICU• Increased CO2 production

• fever, sepsis, injury, overfeeding• Increased VD

• atelectasis,ALI,RDS,pulmonary embolism• Adjustments: RR and TV

variables and pulmonary mechanics that determine minute ventilation

during time-cycled, pressure-limited ventilation

OxygenationThe primary goal of oxygenation is to maximize O2 delivery to blood (PaO2)

• Alveolar-arterial O2 gradient (PAO2 – PaO2)• Equilibrium between oxygen in blood and oxygen in alveoli• A-a gradient measures efficiency of oxygenation• PaO2 partially depends on ventilation but more on V/Q

matching• Oxygenation in context of ICU

• V/Q mismatching• Patient position (supine)• Airway pressure, pulmonary parenchymal disease, small-airway

disease

OxygenationWhat (other than FiO2) determines

oxygenation? Mean Airway Pressure (MAP) MAP is the average pressure to which the

lungs are exposed during the respiratory cycle.

Mean Airway Pressure (MAP)For the same increase in MAP, changes in PIP and

PEEP increase PaO2 more than Changes in I:E ratio.

Very high MAP may cause over distention of alveoli leading to right-to-left intraparenchymal shunting.

The amount of MAP transmitted to intrathoracic structure is inversely related to lung compliance.

If very high MAP is transmitted to intrathoracic structures, CO may decrease.

Thus, despite adequate oxygenation, O2 delivery may decrease

Ideal Mode of VentilationDelivers a breath that:Synchronizes with the patient’s spontaneous respiratory effortMaintains adequate and consistent tidal volume and minute ventilation at low airway pressuresResponds to rapid changes in pulmonary mechanics or patient demandProvides the lowest possible WOB

Ideal Ventilator DesignAchieves all the important goals of mechanical ventilation Provides a variety of modes that can ventilate even the most challenging pulmonary diseases Has monitoring capabilities to

adequately assess ventilator and patient

performance Has safety features and alarms that offer lung protective strategies

Classifying Modesof Ventilation

A. StartTrigger mechanism:What starts the breath?B. LimitsWhat is controlled and what is variable?C. EndCycle mechanism:What causes thebreath to end?

A

B C

What Starts the Breath?Time (IMV)PressureFlowChest impedance

Abdominal movement

How does the ventilator know when to give a breath? (Trigger)

•Time • CMV, paralyzed patient on A/C or SIMV• Often combined with flow or pressure

•Flow – patient achieves set flow•Pressure – patient achieves set negative pressure•Can be issues with Auto-PEEP

Which Parameters areLimited or Controlled?

Pressure limitedPressure is controlled, volume is variableVolume limitedVolume is controlled, pressure is variable

What Ends the Breath?Cycling MechanismsTimeVolumePressureFlow

Phase Variables

Ventilator Parameters

Peak Inspiratory Pressure (PIP) maximum pressure measured(or set) by the ventilator during

inspiration.An increase in PIP will increase tidal volume, increase

CO2 elimination, and decrease PaCO2.An increase in PIP will increase mean airway pressure

and thus improve oxygenation.An elevated PIP may increase the risk of barotrauma,

volutrauma, and bronchopulmonary dysplasia/chronic lung disease.

It is important to adjust PIP based on lung compliance and to ventilate with relatively small tidal volumes (e.g., 3 to 5 mL/kg).

Positive End-Expiratory Pressure (PEEP)pressure present in the airways at the end of

expiration.PEEP in part determines lung volume during

the expiratory phase, improves ventilation-perfusion mismatch, and prevents alveolar collapse.

A minimum “physiologic” PEEP of 2 to 3 cm H2O should be used in most newborns.

Positive End-Expiratory Pressure (PEEP)Gas exchange effects

1. An increase in PEEP increases expiratory lung volume(FRC capacity) during the expiratory phase and thus improves ventilation-perfusion matching and oxygenation in patients whose disease state reduces expiratory lung volume.

2. An increase in PEEP will increase mean airway pressure and thus improve oxygenation in patients with this type of disease.

3. An increase in PEEP will also reduce the pressure gradient during inspiration and thus reduce tidal volume, reduce CO2 elimination,and increase PaCO2.

Positive End-Expiratory Pressure (PEEP)Side effects

1. An elevated PEEP may overdistend the lungs and lead to decreased lung compliance, decreased tidal volume, less CO2 elimination, and an increase in PaCO2.

2. Although use of low to moderate PEEP may improve lung volume, a very high PEEP may cause overdistention and impaired CO2 elimination secondary to decreased compliance and gas trapping.

3. A very high PEEP may decrease cardiac output and oxygen transport.

Frequency (or Rate)breaths per minute.The ventilator frequency (or rate) in part determines

minute ventilation, and thus, CO2 elimination.Spontaneous breathing rates are inversely related to

gestational age and the time constant of the respiratory system. Thus, infants with smaller and less compliant lungs tend to breathe faster.

Use of very high ventilator frequencies may lead to insufficient inspiratory time and decreased tidal volume or insufficient expiratory time and gas trapping.

Inspiratory Time (TI), Expiratory Time (TE), and Inspiratory-to-Expiratory Ratio (I:E Ratio)

The effects of TI and TE are strongly influenced by their relationship to the inspiratory and expiratory time constants.

A TI as long as 3 to 5 time constants allows relatively complete inspiration , A TI of 0.2 to 0.5 sec is usually adequate for newborns with RDS, Infants with a long time constant (e.g., with chronic lung disease) may benefit from a longer TI (approximately 0.6 to 0.8 sec).

A very prolonged TI may lead to ventilator asynchrony,

A very short TI will lead to decreased tidal volume.

Inspired Oxygen Concentration (FiO2)Changes in FiO2 alter alveolar oxygen pressure, and

thus, oxygenation.Because both FiO2 and mean airway pressure

determine oxygenation, the most effective and less adverse approach should be used to optimize oxygenation.

When FiO2 is above 0.6 to 0.7, increases in mean airway pressure are generally warranted.

When FiO2 is below 0.3 to 0.4, decreases in mean airway pressure are generally preferred.

A very high FiO2 can damage the lung tissue, it was determined that toxic levels of FiO2 are that above 0.6.

FLOWvolume of gas per time.Inadequate flow may contribute to

air hunger, asynchrony, and increased work of breathing.

Excessive flow may contribute to turbulence, inefficient gas exchange, and inadvertent PEEP.

Effect of Ventilator Settings changes on Blood Gases