Here We Go Loopty Loop

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Reading Time: 4 minutes
Danelle Howard
Danelle Howard
Registered Respiratory Therapist, cross-trained in the Pulmonary Lab, caring for critically ill patients one breath at a time. Professional interests: mechanical ventilation, capnography, and waveforms.
Sam Epstein
Sam Epstein

Aspiring Medical Student and current Critical Care RN. Enjoys everything outdoors but can also be found inside nerding out on her medical education artwork.

The Pre-brief

Understanding ventilator loops is essential and beneficial in evaluating your patient’s respiratory mechanics and interactions with the ventilator. Understanding loops can help the clinician evaluate respiratory mechanics while diagnosing airway resistance, changes in lung compliance, leaks, auto-PEEP, trigger issues, and other processes of the lung.  

Loops

Loop variables are either pressure or flow plotted against the volume of each breath. Each loop shows the inspiratory and expiratory phases of each breath allowing for interpretation of respiratory mechanics. There are two basic types of loops: flow/volume and pressure/volume.  We are going to dive into pressure/volume loops.  In pressure/volume loops the loop begins at PEEP, inspiration is on the right limb while exhalation is on the left limb returning back to the point of initiation.  Yes, the loop is counter-clockwise with the exception of a spontaneous mode. Pressure is plotted horizontally while volume is plotted vertically.   The highest point of the pressure/volume loop on the y-axis represents tidal volume while the same point on the x-axis represents Ppeak. As the pressure delivered to the lung increases, there is an increase in the volume of gas delivered. 

Volume Control

In Volume Control the volume is mandatory, the pressure is variable, and the flow is constant.  The lungs fill with a constant flow of gas which gradually increases in pressure during inspiration.  With each breath the lung increases to the same volume and at end inspiration the pressure equals that of the breathing system.   During expiration, the exhalation valve opens wide enough to maintain the level of set PEEP.  

* It is important to note that patient triggered breaths look different then time triggered or machine triggered breaths.  A patient triggered breath generates a negative pressure at the beginning of inspiration represented by a fish tail.

Pressure support/CPAP

As stated earlier, in a spontaneous mode, the loop runs clockwise. Inspiration creates a negative pressure, which gradually decreases to zero as the lungs fill to the full capacity of the tidal volume. At expiration, the elastic recoil of the chest wall and lung tissue creates a positive pressure, which decreases towards zero as the volume is exhaled.   The patient’s inspiratory effort creates a negative pressure in the lung, which then has an effect in the breathing system where the pressure is measured by the ventilator. The ventilator supplies the patient with enough flow to ensure that the set CPAP pressure is maintained although a slight negative deviation is inevitable. The area to the left of an imaginary vertical axis (A) at the set CPAP pressure is thus a measure of the patient’s efforts to combat the inspiratory resistances of the ventilator. 

*It is important to note, If PEEP is applied the entire loop shifts right as the baseline pressure becomes the PEEP

Compliance 

Lung compliance will change with body position and age along with changing respiratory mechanics.  Compliance is mathematically determined by the change in volume divided by the change in pressure and is graphically displayed in the loop screen.  On the pressure/volume loop, normal compliance is upright or in the shape of a football.  If an imaginary line is drawn down the middle of the loop to connect the origin of the loop with the PIP, it can estimate the dynamic compliance of the lung. Decreased compliance in volume control, the volume will stay the same  while the pressure increases. In pressure control with decreased compliance  the pressure will stay the same while the volume decreases. The area to the right of the imaginary line represents inspiratory resistance while the area to the left of the imaginary line represents expiratory resistance. As airway resistance increases the loop will become wider. The steepness of the inspiratory phase is proportional to the change in lung compliance.  As compliance decreases, the lung becomes less elastic and the loop flattens like it’s lying on its side.

Over-distention 

The area of normal compliance represents the ideal pressure, the linear compliance, and the alveoli opening gradually as the pressure rises representing optimal compliance. When the ventilator delivers a volume that exceeds lung capacity the loop takes on a beak-like look and becomes flatter in the upper part of the inspiratory phase.  This beaking indicates over-distention of certain areas of the lungs where the delivery of excess pressure results in little or no further delivery of volume.   This causes an abrupt decrease in lung compliance near the end of inspiration.

Leak

When the expiratory limb of the loop does not return to baseline, this signifies the presence of a leak. Leaks prevent the normal closure of the pressure volume loop. The amount of leak is the difference in the measured inspiratory and expiratory tidal volumes.

PEEP

The pressure/volume loop has in the past been used to set optimal PEEP..  The lower inflection point of the loop was thought to be the minimum baseline PEEP needed for optimal alveolar recruitment. The lower inflection point is determined early in the inspiratory limb where there is a change in the slope that shows a more rapid increase in volume per unit of pressure. The initial rapid rise in pressure is a reflection of alveolar recruitment.

The Debrief

  • Can be a useful tool in evaluating respiratory mechanics
  • Graphical representation between pressure and volume with each breath

References

  1. Yartsev, Alex.  Interpreting the Shape of the Pressure-Volume Loop.  Deranged Physiology.com, 17 June 2015, derangedphysiology.com/main/cicm-primary-exam/required-reading/respiratory-system/Chapter%20554/interpreting-shape-pressure-volume-loop.
  2. Ritter, F., Doring, M. (n.d.). Curves and Loops in Mechanical Ventilation. https://wee.draeger.com/Library/Content/9097421-fibel-curves-loops-en.pdf

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