Chapter 3: How a Breath Is Delivered

The Equation of Motion and the Basic Model of Ventilation

• The Equation of Motion: This is a fundamental concept in mechanical ventilation that describes the mathematical relationships between pressure, volume, and flow during a spontaneous or mechanical breath. It is expressed by the three specific terms of the transrespiratory pressure gradient:     * Transrespiratory Pressure ($P_{TR}$): The total energy (pressure) required to establish a pressure gradient to move gas into the lungs.     * Elastic Recoil Pressure ($P_E$): The pressure required to overcome the elastic load offered by the lungs and the chest wall.     * Flow-Resistance Pressure ($P_R$): The pressure required to overcome the airway resistance load produced as gas flows through the conducting airways.

• Formula Representation:     * $P_{TR} = P_E + P_R$

Factors Controlled and Measured During Inspiration

• Pressure-Controlled Breathing:     * The ventilator maintains the pressure waveform in a specific, constant pattern.     * The pressure waveform remains unaffected by changes in lung characteristics ($\text{compliance and resistance}$).

• Volume-Controlled Breathing:     * The ventilator maintains the volume and flow waveforms in a specific, constant pattern.     * Volume and flow waveforms remain unaffected by changes in lung characteristics.

• Flow-Controlled Breathing:     * Flow and volume waveforms remain unchanged.     * The pressure waveform changes dynamically with alterations in lung characteristics.

• Time-Controlled Breathing:     * Both pressure, volume, and flow waveforms are affected by changes in lung characteristics.     * This is common in specialized equipment such as many high-frequency jet ventilators and oscillators.

• Measurement Locations: Most ventilators measure variables in one of three locations:     * The upper or proximal airway (the patient interface).     * Internally, near where the main circuit lines connect to the ventilator.     * Near the exhalation valve.     * Many modern devices utilize pressure-measuring sensors on both the inspiratory and expiratory sides of the ventilator circuit.

The Four Phase Variables of a Breath

• Trigger Variable: The factor that determines the beginning of inspiration.
• Limit Variable: Limits the value of pressure, volume, flow, or time during the inspiratory phase but does not terminate the breath.
• Cycle Variable: The mechanism that ends the inspiration phase.
• Baseline Variable: Establishes the baseline pressure during expiration before the next inspiration is triggered.

The Beginning of Inspiration: Triggering

• Time Triggering:     * The ventilator delivers a mandatory breath after a set time has elapsed.     * Calculation is based on the Total Cycle Time (TCT).     * $TCT = T_I + T_E$, where $T_I$ is the Inspiratory Time and $T_E$ is the Expiratory Time.     * This sets the rate or frequency control ($f$).

• Patient Triggering: Triggering based on patient effort. The sensitivity setting determines how much effort is required.     * Pressure Triggering: Inspiration begins when the ventilator detects a drop in pressure below the baseline.     * Flow Triggering: Occurs when the ventilator detects a drop in flow through the circuit during the expiratory phase.     * Volume Triggering: Occurs when the ventilator interprets a drop in volume as patient effort.     * Neural Triggering: Triggering based on neural signals (e.g., diaphragm activity).

• Manual Triggering: Also known as a "Manual Breath" or "Start Breath" button. When activated by a clinician, the ventilator delivers a breath based on the current set variables.

Limit Variables During Inspiration

• Definition: A limit variable allows a parameter (pressure, volume, or flow) to reach a specific maximum value but does not end the inspiratory phase.

• Pressure Limiting:     * Pressure can rise to a certain value but is restricted from exceeding it.     * Excess pressure is typically vented through a spring-loaded pressure release, or "pop-off" valve.     * Clinical Example: Infant ventilators often use pressure limiting during the inspiratory phase but are time-cycled.

• Volume Limiting: The ventilator is set to deliver a specific volume; however, reaching this limit does not always terminate the breath.

• Flow Limiting: A maximum flow value is set that cannot be exceeded before the end of the inspiratory phase.

• Maximum Safety Pressure:     * Used to prevent barotrauma (excess pressure damaging the lungs).     * Standard setting is usually $10\,cmH_2O$ above the patient's average Peak Inspiratory Pressure (PIP).     * In most adult ventilators, reaching this safety limit will end inspiration (pressure cycling) to protect the patient.

Termination of the Inspiratory Phase: Cycling Mechanisms

• Volume-Cycled Ventilation:     * Inspiration ends when a preset volume is delivered.     * Set vs. Delivered Volume: Factors can cause the actual delivered volume to differ from the set volume.     * Tube Compressibility: There is a loss of approximately $1\text{ to }3\,mL$ of gas for every $1\,cmH_2O$ of measured airway pressure because the breathing tubes expand under pressure.     * System Leaks: Leaks in the circuit or around the airway interface reduce the volume reaching the patient.

• Time-Cycled Ventilation:     * Inspiration ends after a specific time interval ($T_I$) has passed.     * Relationship: $\text{Tidal Volume} = \text{Flow (Volume/Time)} \times T_I$.

• Flow-Cycled Ventilation:     * Commonly used in Pressure Support (PS) mode.     * Inspiration ends when the flow rate decreases to a predetermined percentage of the peak inspiratory flow.     * The clinician can typically adjust this percentage from $5\%$ to $80\%$.

• Pressure-Cycled Ventilation:     * Inspiration terminates when a set pressure threshold is reached at the mouth or upper airway.     * Advantage: Limits peak airway pressures, reducing the risk of barotrauma.     * Disadvantage: Delivers inconsistent tidal volumes if the patient's lung compliance decreases or airway resistance increases.

Inspiratory and Expiratory Maneuvers

• Inflation Hold (Inspiratory Pause):     * Maintains air in the lungs at the end of inspiration before the exhalation valve opens.     * The manometer pressure peaks and then levels off to a Plateau Pressure.     * Used to improve gas distribution and measure static compliance.

• Expiratory Phase:     * The period from the end of inspiration to the start of the next breath.     * Begins when the expiratory valve opens and flow begins.     * Auto-PEEP (Intrinsic PEEP): Occurs if the expiratory time ($T_E$) is insufficient, leading to air trapping and lung hyperinflation.

• Baseline Pressure:     * ZEEP: Zero End-Expiratory Pressure (atmospheric pressure).     * PEEP: Positive End-Expiratory Pressure (baseline pressure maintained above zero).

Advanced Expiratory Controls and Oxygenation

• Time-Limited Expiration: Clinicians control specific time durations for pressure phases.     * Airway Pressure Release Ventilation (APRV):         * $T_1$: Controls the duration that high pressure is applied.         * $T_2$: Controls the release time (duration of low-pressure application).

• Continuous Gas Flow: Many ICU ventilators provide gas flow during the late expiratory phase to minimize the resistance the patient faces when exhaling.

• Expiratory Hold (End-Expiratory Pause):     * A maneuver performed at the end of exhalation to measure end-expiratory pressure.     * Accurate readings are impossible to obtain if the patient is breathing spontaneously.

• Expiratory Retard:     * Mimics the "pursed-lip breathing" technique used by patients with obstructive diseases to prevent early airway closure.     * Adds resistance to the exhalation phase to prolong expiration.     * Currently not common in modern clinical practice.

• Refractory Hypoxemia Management: Continuous positive pressure is applied throughout the respiratory cycle to improve oxygenation.     * CPAP: Continuous Positive Airway Pressure.     * BiPAP: Bilevel Positive Airway Pressure.

Categorization of Breath Types

• Spontaneous Breaths:     * Initiated by the patient.     * Volume and flow are determined by patient demand, not ventilator settings.

• Mandatory Breaths:     * The ventilator determines the start time (time triggering) or the tidal volume (or both).

• Assisted Breaths:     * Breaths that have characteristics of both spontaneous effort (patient triggers) and mandatory delivery (ventilator controls the limit or cycle).