Ventilator Waveforms: Interpretation, Lecture notes of Mechanics

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Ventilator Waveforms:

Interpretation

Albert L. Rafanan, MD, FPCCP

Pulmonary, Critical Care and Sleep Medicine Chong Hua Hospital, Cebu City

Types of Waveforms

• Scalars are waveform representations of pressure,

flow or volume on the y axis vs time on the x axis

• Loops are representations of pressure vs volume or flow vs volume

Loop

Common problems that can be diagnosed by analyzing Ventilator waveforms Abnormal ventilatory Parameters/ lung mechanics E.g.. Overdistension, Auto PEEP COPD Patient-ventilator Interactions E.g. flow starvation, Double triggering, Wasted efforts Active expiration Ventilatory circuit related problems E.g. auto cycling and Secretion build up in the Ventilatory circuit

Understanding the basic ventilator circuit diagram ventilator Diaphragm Essentially the circuit diagram of a mechanically ventilated patient can be broken down into two parts….. The ventilator makes up the first part of the circuit. Its pump like action is depicted simplistically as a piston that moves in a reciprocating fashion during the respiratory cycle. The patient’s own respiratory system makes up the 2nd^ part of the circuit. The diaphragm is also shown as a 2 nd piston; causing air to be drawn into the lungs during contraction. These two systems are connected by an endotracheal tube which we can consider as an extension of the patient’s airways. ET Tube airways Chest wall

Understanding airway pressures The respiratory system can be thought of as a mechanical system consisting of resistive (airways +ET tube) and elastic (lungs and chest wall) elements in series Diaphragm ET Tube airways Chest wall

P

PL Pleural pressure

Paw

Airway pressure

P

alv Alveolar pressure ET tube + Airways (resistive element) Resistive pressure varies with airflow and the diameter of ETT and airways. Flow resistance The elastic pressure varies with volume and stiffness of lungs and chest wall. Pel = Volume x 1/Compliance

P

aw

THUS = Flow X Resistance + Volume x 1/Compliance

Lungs + Chest wall (elastic element) Airways + ET tube (resistive element) Lungs + Chest wall (elastic element)

ventilator Diaphragm Ppeak Pres RET tube Rairways Pres Pplat

Understanding the pressure-time waveform

using a ‘square wave’ flow pattern

time pressure The pressure-time waveform is a reflection of the pressures generated within the airways during each phase of the ventilatory cycle. At the beginning of the inspiratory cycle, the ventilator has to generate a pressure Pres to overcome the airway resistance. Note: No volume is delivered at this time. After this, the pressure rises in a linear fashion to finally reach Ppeak. Again at end inspiration, air flow is zero and the pressure drops by an amount equal to Pres to reach the plateau pressure Pplat. The pressure returns to baseline during passive expiration. Pres

Pressure-time waveforms using a ‘square wave’ flow pattern This is a normal pressure-time waveform With normal peak pressures ( Ppeak) ; plateau pressures ( Pplat )and airway resistance pressures (Pres) time pressure Pres Pplat Pres

Scenario # 1

P

aw(peak)

= Flow x Resistance + Volume x 1/ Compliance

time flow ‘Square wave’ flow pattern Paw(peak)

Waveform showing increased airways resistance

Ppeak Pplat Pres ‘Square wave’ flow pattern

Waveform showing high inspiratory flow rates

This is an abnormal pressure-time waveform time pressure Paw(peak) Pres Pplat Pres

Scenario # 3

The increase in the peak airway pressure is caused by high inspiratory flow rate and airways resistance. Note the shortened inspiratory time and high flow e.g. high flow rates

P

aw(peak) = Flow x Resistance + Volume x 1/compliance + PEEP time flow ‘Square wave’ flow pattern Normal Normal (low) flow rate

Common problems that can be diagnosed by analyzing Ventilator waveforms Abnormal ventilatory Parameters/ lung mechanics E.g.. Overdistension, Auto PEEP COPD Patient-ventilator Interactions E.g. flow starvation, Double triggering, Wasted efforts Active expiration Ventilatory circuit related problems E.g. auto cycling and Secretion build up in the Ventilatory circuit

Recognizing Lung Overdistension

Recognizing lung overdistension There are high peak and plateau Pressures… Suspect this when: PEARL: Think of low lung compliance (e.g. ARDS), excessive tidal volumes, right mainstem intubation etc Accompanied by high expiratory flow rates

The Stress Index

• In AC volume ventilation using a constant flow waveform observe the pressure time scalar.
• Normal, linear change in airway pressure Stress index =
• Upward concavity indicates decreased compliance and lung overdistension Stress index > 1
• Downward concavity indicates increased compliance and potential alveolar recruitment Stress index < 1 flow time Paw Note: Patient effort must be absent