Episode 3 - Basic Mechanical Ventilation I: Fundamentals
Summary
In order to understand why a patient may be dysynchronous with the ventilator or what factors may be contributing to a sudden increase in peak inspiratory pressures, it’s necessary to understand what variables go into determining how a breath is delivered on the ventilator. Using the respiratory equation of motion, we’ll examine the key anatomic and physiologic concepts that determine pulmonary mechanics. In future episodes we’ll apply these concepts in our clinical approach to patient.
Learning Objectives
#1 – Discuss indications and objectives for invasive positive pressure mechanical ventilation
#2 – Understand core physiologic concepts comprising the respiratory equation of motion aka force balance equation
#3 – Describe the 4 phases of a breath on the ventilator
#4 – Understand what we mean by a mode of ventilation
Take Home Points
In order for a positive pressure mechanical ventilator breath to be delivered, the resistive and elastic pressures of the respiratory system must be balanced or overcome.
Pressure, volume, and flow are the key variables that can be adjusted on the ventilator as a function of time. BUT only one of these variables can be controlled at a time.
Whereas compliance is a measure of the stiffness of the lungs, resistance depends on both airflow and tissue resistance through the airways including the ETT
Mean airway pressure is a key determinant of oxygenation
Time Stamps
00:12 Introduction
02:29 Objectives
03:54 Indications for mechanical ventilation
06:36 Goals of mechanical ventilation
09:00 Systemic effects of positive pressure ventilation
10:48 Respiratory equation of motion
14:55 Pressures (transairway, transthoracic, transpulmonary, transrespiratory)
16:34 Resistance & compliance
18:34 Dynamic & static compliance
19:23 Plateau pressure
20:56 Peak airway pressure
21:44 Mean airway pressure
24:11 4 phases of a breath
28:32 Basic description of a MODE of ventilation
31:25 Outro
Key Respiratory Mechanics Equations
Respiratory equation of motion
Pmus + Pvent = F x R + TV x E
where Pmus = negative pressure generated by muscles of inspiration; Pvent = positive pressure generated by vent; F = flow; R = resistance; TV = tidal volume; E = elastane = 1 / compliance
Transairway pressure = Pressure at mouth opening - alveolar pressure
Transthoracic pressure = Alveolar pressure - body surface pressure
Transpulmonary pressure = alveolar pressure - pleural pressure
Transrespiratory pressure = Pressure at airway opening - body surface pressure
Compliance = change in volume / change in pressure (normal: >=100ml/cmH20)
Dynamic compliance = tidal volume / PIP - PEEP
Static compliance = tidal volume / Pplateau - PEEP
Resistance = pressure / flow = PIP - Pplateau / inspiratory flow
Work = pressure x volume
Ventilation pressure = resistive pressure x elastic pressure
Time constant = compliance x resistance
Recommended Reading
David T. Neilipovitz. Acute Resuscitation and Crisis Management: Acute Critical Events Simulation (ACES). University of Ottawa Press, 2005.
