Ventilator Waveforms Interpretation

Understanding the Basic Ventilator Circuit Diagram

The circuit diagram for a mechanically ventilated patient can be broken down into two main parts:

1. The Ventilator: This forms the first part of the circuit. Its function is depicted as a piston that moves reciprocally during the respiratory cycle, simulating a pump-like action.

2. The Patient’s Respiratory System: This constitutes the second part of the circuit. It includes the diaphragm, which acts as a second piston, drawing air into the lungs during contraction.

These two systems are connected by an endotracheal tube, which serves as an extension of the patient’s airways.

Understanding the Basic Ventilator Circuit Diagram

The circuit diagram for a mechanically ventilated patient can be broken down into two main parts:

1. The Ventilator: This forms the first part of the circuit. Its pump-like action is depicted simplistically as a piston that moves reciprocally during the respiratory cycle.

2. The Patient’s Respiratory System: This constitutes the second part of the circuit. It includes the diaphragm, which acts as a second piston, drawing air into the lungs during contraction.

These two systems are connected by an endotracheal (ET) tube, which can be considered an extension of the patient’s airways.

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Understanding Airway Pressures

The respiratory system can be viewed as a mechanical system with resistive (airways + ET tube) and elastic (lungs and chest wall) elements arranged in series:

• Resistive Elements:
  • ET Tube and Airways (resistive elements)
  • Resistive Pressure varies with airflow and the diameter of the ET tube and airways.
• Elastic Elements:
  • Lungs and Chest Wall (elastic elements)
  • Elastic Pressure varies with the volume and stiffness of the lungs and chest wall.
  • Pel = Volume × (1/Compliance)
• Flow Resistance Equation:
  • Paw = Flow × Resistance + Volume × (1/Compliance)

Pressures:

• Ppl: Pleural Pressure
• Paw: Airway Pressure
• Palv: Alveolar Pressure

The total airway resistance (Raw) in a mechanically ventilated patient is the sum of the resistances from the endotracheal tube (R_ET tube) and the patient’s airways (R_airways).

The total elastic resistance (Ers) offered by the respiratory system is the sum of elastic resistances from the lungs (E_lungs) and the chest wall (E_chest wall).

To move air into the lungs at any given time (t), the ventilator must generate sufficient pressure (Paw(t)) to overcome the combined elastic (Pel(t)) and resistive (Pres(t)) properties of the respiratory system.

Equation of Motion:

• Paw(t) = Pres(t) + Pel(t)

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Understanding Basic Respiratory Mechanics

1. Pressure-time Waveform: The pressure-time waveform reflects the pressures generated within the airways during each phase of the ventilatory cycle.

   • Inspiratory Cycle: Initially, the ventilator generates a pressure (Pres) to overcome airway resistance. No volume is delivered at this stage.
   • As inspiration progresses, pressure rises linearly to reach Ppeak.
   • At the end of inspiration, airflow is zero, and pressure drops by an amount equal to Pres to reach the plateau pressure (Pplat).
   • Pressure returns to baseline during passive expiration.

2. Pressure-time Waveform with 'Square Wave' Flow Pattern:

   • Normal Waveform: Shows normal peak pressures (Ppeak), plateau pressures (Pplat), and airway resistance pressures (Pres).

Scenario #1:

• Paw(peak) = Flow × Resistance + Volume × (1/Compliance)