Mechanical Ventilation in Respiratory Care

Mechanical Ventilation

• Indications for mechanical ventilation: respiratory failure, apnea, airway protection, and cardiac arrest
  • Example: patient with severe pneumonia and respiratory failure
• Contraindications for mechanical ventilation: patient with a Do Not Resuscitate (DNR) order, those with a poor prognosis, and those who are medically unsuitable
  • Example: terminally ill patient with a DNR order
• Goals of mechanical ventilation: oxygenation, ventilation, and respiratory muscle rest
• Normal physiologic ventilation: diaphragmatic contraction and negative pressure generation
• Negative pressure ventilation: uses negative pressure to expand the chest cavity, such as with an iron lung
• Positive pressure ventilation: uses positive pressure to expand the lungs, such as with a mechanical ventilator
• Inspiratory phase variables: tidal volume, inspiratory pressure, and flow rate
• Expiratory phase variables: expiratory pressure, flow rate, and time

Modes of Mechanical Ventilation

• Assist Control/Continuous Mandatory Ventilation (AC/CMV): delivers a set tidal volume at a fixed rate, with additional breaths supported as needed
• Synchronized Intermittent Mandatory Ventilation (SIMV): delivers a set tidal volume at a fixed rate, with additional breaths allowed but not supported
• Pressure Control Ventilation (PCV): delivers a set pressure at a fixed rate, with the tidal volume variable
• Pressure Support Ventilation (PSV): provides a set pressure support during spontaneous breathing
• Continuous Positive Airway Pressure (CPAP): provides a constant pressure during both inspiration and expiration
• Airway Pressure Release Ventilation (APRV): provides a high pressure during inspiration and a low pressure during expiration

Hazards and Complications of Mechanical Ventilation

• Pulmonary barotrauma and volutrauma
• Oxygen toxicity
• Ventilator-associated pneumonia (VAP)
• Respiratory alkalosis
• Cardiac depression

Effects of Mechanical Ventilation on Lung Mechanics

• Increased lung volume and compliance
• Decreased functional residual capacity (FRC)
• Altered ventilation-perfusion relationships
• Redistribution of blood flow

Initial Ventilator Settings and Modifications

• Initial settings: based on patient data, such as height, weight, and lung disease
• Modifications: based on patient response, such as arterial blood gases, oxygen saturation, and ventilator waveforms

Capnography

• Phases of a normal capnogram: I (inspiratory), II (expiratory), and III (alveolar)
• ETCO2/PaCO2ETCO_2/PaCO_2ETCO2​/PaCO2​ gradient: normally 2-5 mmHg, indicating alveolar ventilation and perfusion
• Abnormal capnograms: indicating respiratory or cardiac disease, rebreathing, or exhausted CO2 absorbers
• Clinical interventions: based on capnogram morphology and gradient, such as adjusting ventilation or perfusion
• Normal ventilation-perfusion relationships: dependent on regional lung volume and perfusion
• Abnormal ventilation-perfusion relationships: indicating respiratory disease, such as COPD or ARDS
• Mainstream vs. side-stream CO2CO_2CO2​ technology: mainstream more accurate, but side-stream more convenient and easier to use

Continuous Positive Airway Pressure (CPAP)

• CPAP applies positive baseline pressure above ambient pressure during continuous spontaneous ventilation
• Increases functional residual capacity (FRC) by recruiting previously unventilated or underventilated alveoli, improving oxygenation
• Patient must be capable of sustaining eucapnic ventilation before CPAP initiation, confirmed by an appropriate PaCO2P_{a}CO_{2}Pa​CO2​ and pH from an arterial blood gas
• Patient interface includes a mask, artificial airway, or nasal prongs in neonates, and must seal to allow pressure to build above ambient pressure
• Leaks in the interface can result in pressure loss, decreased FRC, oxygen saturation (SpO2SpO_{2}SpO2​), and partial pressure of oxygen (PaO2P_{a}O_{2}Pa​O2​)

Bilevel Positive Airway Pressure (BiPAP)

• BiPAP applies positive airway pressure during inspiration and exhalation (baseline pressure) during spontaneous ventilation
• Inspiratory positive airway pressure (IPAP) and expiratory positive airway pressure (EPAP) are set independently
• IPAP must always be set higher than EPAP
• IPAP improves ventilation (PaCO2P_{a}CO_{2}Pa​CO2​) and oxygenation in conditions due to hypoventilation
• EPAP functions like CPAP, recruiting underventilated alveoli during exhalation, increasing FRC and oxygenation
• Drive pressure (IPAP-EPAP) measures the pressure assistance required to maintain adequate ventilation (PaCO2P_{a}CO_{2}Pa​CO2​ and pH), and increasing it improves ventilation (PaCO2P_{a}CO_{2}Pa​CO2​)

Pressure Support

• Breath is triggered by meeting either a pressure or flow threshold, making it a spontaneous ventilation type
• Tidal volume delivered varies based on patient's effort
• Inspiratory time lasts only as long as the patient is actively inhaling
• Breath cycle ends when patient's inspiratory flow reaches a preset value

Augmentation

• Augmenting spontaneous breath with pressure improves alveolar ventilation
• Improved alveolar ventilation reduces hypercapnia and improves oxygenation
• Degree of improved alveolar ventilation is proportional to pressure support setting used

Inspiration

• Ventilator targets a set pressure during inspiration
• Flow increases to build pressure in the circuit during inspiratory phase
• Flow decays once pressure target is reached
• Cycle ends when inspiratory flow drops to a preset threshold (usually a percentage of peak flow)

Pressure

• Graph illustrates pressure changes over time, exhibiting a repeating pattern
• Pressure increases, plateaus, and then drops back down, with each subsequent increase reaching a slightly higher pressure
• Pressure rise is highlighted on the graph with an arrow pointing upwards from the highest pressure point

Flow

• Graph depicts flow changes over time, displaying a repeating pattern
• Flow is zero when pressure is being maintained
• When pressure drops, flow begins and increases before decreasing again as pressure returns to baseline
• Flow is absent during inspiration, as indicated on the graph
• The relationships between pressure and flow likely represent the breathing process

Pressure and Flow Graph

• The graph displays pressure and flow over time, with pressure shown on the top graph and flow on the bottom graph.

Pressure

• A dotted line represents the baseline pressure on the top graph.
• Pressure spikes above the baseline at certain times before returning to the baseline.
• One such spike is labeled as "Flow starvation", indicating a specific instance of pressure increase.

Flow

• The bottom graph shows that the flow is typically high and constant.
• There are two distinct periods where the flow drops rapidly before returning to its initial state.
• Time is measured in seconds and displayed on the horizontal axis.

Modes of Ventilation

Control / Assist Control Mode

• Targets volume and uses volume control, indicated for Acute Respiratory Failure (ARF)
• Targets pressure and uses pressure control, indicated for Acute Respiratory Distress Syndrome (ARDS)
• Targets volume with pressure-regulated flow and uses PRVC / VW+, indicated for ARF with dyssynchrony

SIMV Mode

• Targets volume and spontaneity, uses SIMV-Vol Cont, indicated for ARF
• Targets pressure and spontaneity, uses SIMV-Press Cont, indicated for infrequent use
• Targets volume or pressure with support, uses VC or PC with PS, indicated for ARF

Support Mode

• Targets volume and uses volume support, indicated for weaning
• Targets pressure and uses pressure support, indicated for weaning

ABG Analysis

pH Levels and Associated Conditions

• Low pH: Indicates Acidemia
• High pH: Indicates Alkalemia
• Normal pH: No Abnormality or Mixed Acid-Base Disorder

Identifying Acidemia

• High PCO2: Respiratory Acidosis
• Low HCO3: Metabolic Acidosis

Identifying Alkalemia

• Low PCO2: Respiratory Alkalosis
• High HCO3: Metabolic Alkalosis