Ventilator Graphics

Neonatal Ventilator GraphicsA Clinical GuideBY:DR. AHMED HEGAZIM.SC, MRCPCHNICU-ADAN HOSPITAL

Evolution of ventilator graphics

Principles of Pulmonary Graphics

Principles of Pulmonary Graphics

WaveformsWaveforms depict the relationship between respiratory parameters and time on a breath-to-breath basis.

The three most commonly used signals are pressure (cm H2O), volume (mL), and flow (mL/s), and these three signals describe the respiratory cycle.

When displayed in aggregate, the cyclic phases of respiration can be appreciated.

Waveforms

Volume WaveformThe volume waveform displays the changes in delivered volume over time.

It is determined by integrating the inspiratory and expiratory flow signals.

expired volume is usually a bit less than inspired volume because of air leak around the uncuffed neonatal endotracheal tube.

8

Volume Waveform

Pressure Waveform The pressure waveform represents the airway pressure throughout the respiratory cycle.Virtually every newborn requiring conventional mechanical ventilation receives some degree of PEEP. Thus, the waveform at end inspiration or the initiation of inspiration is above the baseline (zero) value.

Pressure WaveformPressure rises during inspiration, reaching its maximum value, or peak inspiratory pressure (PIP), then declines during expiration to the PEEP level.

The area under a single cycle represents the mean airway pressure (mean Paw).

The difference between the PIP and the PEEP is referred to as the amplitude or delta P.

Techniques to Alter Mean Airway Pressure

Pressure Waveform

Changes in PIP and PEEP

Change in Inspiratory Time

Pressure Overshoot Pressure control and pressure support ventilation utilize an accelerating-decelerating inspiratory flow waveform.

If set too high, it may deliver pressure too rapidly for the patients need. This creates a condition known as pressure overshoot.

Pressure OvershootThe pressure waveform exhibits a notch and double peak at PIP. Most ventilators have an adjustable rise time function to respond to this.

Flow Waveform The flow waveform is the most complex because its inspiratory and expiratory phases each have two components.

the baseline represents a zero flow state, meaning that no gas is entering or leaving the airway.

anything above the baseline (positive value) represents inspiratory flow (gas flow into the patient), and conversely, anything below the baseline (negative value) represents expiratory flow (gas flow from the patient).

Flow Waveform

Flow Waveform Two major ways in which inspiratory flow can be delivered to the patient: Variable (sinusoidal wave): pressure control pressure support ventilation.Constant (square wave): volume targeted ventilation

Flow Waveform Flow wave form during volume control ventilation. Inspiratory flow is continuous, rather than variable, and produces a characteristic square waveform

Flow Waveform Increased Expiratory Resistance

shallow accelerating expiratory flow and decreased peak expiratory flow rate.a longer time to return to baseline during decelerating expiratory flow

Flow Waveform Gas Trapping

the decelerating expiratory component never reaches the baseline (zero flow state) before the subsequent breath is initiated.

Flow Waveform Gas TrappingIt occurs when the expiratory flow is less than the inspiratory flow, resulting in more gas entering than leaving the lung.

This is a potentially dangerous situation that can lead to alveolar rupture and air leak.

Now, careful observation of the flow waveform can detect this condition, allowing time to avoid its consequences.

Flow Waveform Gas Trapping (how to intervene?)

decreasing the ventilator rate.decreasing the flow rate.shortening the inspiratory time.increasing the PEEP.

depending upon the clinical condition, ventilator modality, and underlying pathophysiology.

Cycling MechanismsCycling refers to the mechanism that transitions inspiration to expiration and expiration to inspiration. Time cycling mechanism:

Cycling MechanismsFlow-cycling As a breath is delivered, the ventilator notes the peak inspiratory flow rate. The inspiratory flow rate then decelerates, by 5-25%, the exhalation valve will open, discharging the remainder of inspiratory flow

Cycling MechanismsFlow-cycling Flow-cycling takes advantage of the natural pattern of breathing by focusing on the babys inspiratory flow.It prevents gas trapping and the inversion of the inspiratory:expiratory ratio during patient-triggered ventilation.It can be used in conjunction with time-cycling, in that a breath will be terminated by whichever condition occurs first.

Endotracheal Tube Leaks Because cuffed ETTs are not used in newborns, there will almost always be some degree of leak around the ETT.Most of this occurs during inspiration when pressure is higher. A significant leak may divert gas flow, such that the decelerating inspiratory flow may never reach the termination point. The breath will then be time-cycled, but often with inadequate pressure or volume delivered to the baby.

Endotracheal Tube Leaks

Endotracheal Tube LeaksThe flow waveform, has virtually no expiratory component.the actual end of the expiratory volume waveform is shown by (the arrows, and by the short blue line) which is followed by a reset artifact (yellow coloured line dropping to the zero baseline).The volume waveform, in the lower panel, shows almost no expired volume.This also results in auto-cycling, with a rate of 75/m.

Auto-cycling (Auto-triggering)When it occurs, there may be rapid delivery of mechanical breaths, inducing hypocapnia as well as the risk of lung injury.

Note the relative uniformity of the breaths, which helps to distinguish this from just rapid breathing, where there will be some variability

Auto-cycling (Auto-triggering)It may occur during flow-triggered ventilation if the ventilator interprets an aberrant flow signal as patient effort.This can happen if there is a leak that exceeds the trigger threshold, and it may occur anywhere in the path of gas flow. It may also occur from excessive condensation in the ventilator circuit (rainout).

Pulmonary Mechanics and Loopspressure, flow, and volume to time, may be presented relative to each other commonly referred to as loops.The two most frequently used in clinical practice are the pressure-volume (P-V) loop and the flow-volume (F-V) loop. The interpretation of which can provide valuable information about the mechanical properties of the lung, how it is performing on a breath-to-breath basis, and how it responds to changes in pathophysiology, mechanical ventilation

The P-V Loop

The P-V Loop

The P-V LoopIt displays the relationship of pressure and volume during a single breath. the origin of the loop does not start at the origin of the graph because of the application of positive end-expiratory pressure (PEEP). The P-V loop provides valuable information about lung mechanics. The dotted line is the compliance axis , a measure of the stiffness or elasticity of the lung.

The P-V Loop ComplianceCompliance is defined as the change in volume divided by the change in pressure. Thus, if a 1 cm H2O increase in pressure results in a 1 mL increase in lung volume, the axis will be 45.As compliance decreases, the axis will shift downward and to the right. Conversely, as compliance improves, the axis will shift upward and to the left.

The P-V Loop work of breathing The work of breathing can be qualitatively estimated by the P-V loop. It is the area bounded by the inflation limb and a horizontal line connecting the PIP with the y-axis. As the compliance decreases and the loop shifts downward and to the right, this area increases and more pressure must be applied to achieve the same lung volume.

The P-V Loop resistanceA line drawn from the midpoint of the inflation limb to the compliance axis is a measure of inspiratory resistance.a line drawn from the midpoint of the deflation limb to the compliance axis is a measure of expiratory resistance.

The P-V Loop HysteresisHysteresis is a term that is used to describe the difference between the inflation and the deflation limbs and is determined by the elastic properties of the lung.Under normal circumstances, the shape of the P-V loop is oval, resembling a football. Hysteresis, thus represents the resistive work of breathing.

The F-V Loop

The F-V Loop

The F-V Loop describes the pattern of airflow during tidal breathing. Volume is shown on the x-axis and flow is shown on the y-axis.Like the flow waveform, inspiratory and expiratory flows are in opposite directions.

The F-V Loop Inspiration begins at the origin of the graph (zero flow) and increases until it reaches the peak inspiratory flow rate (PIFR).Flow then decelerates, and reaches the zero flow state at the point where it crosses the volume axis, representing the delivered V

The F-V Loop Expiratory flow begins with the accelerating phase, reaches the peak expiratory flow rate (PEFR). Then, it decelerates until it returns to the origin and another zero flow state at end expiration.The general shape of the loop should be round or ovoid and the two halves (inspiratory and expiratory) should be close to mirror images.

Decreased Compliance

Lung Compliance

Lung Inflation

Lung Inflation HyperinflationAs the lung approaches maximum filling and tissue distensibility becomes more limited, the compliance will decrease, resulting in less volume gain per unit of incremental pressure, and the slope of the compliance axis will shift downward.This creates an upper inflection point on the P-V inflation limb and graphically creates a penguin beak or duck bill appearance to the loop.

Lung Inflation HyperinflationHyperinflation can be quantified by using a metric known as the C20/C ratio (Fisher, 1988).The C20/C ratio examines the slope of the last 20 % of the inflation limb and compares it with the linear portion of the curve.If the curve remained linear to the peak pressure, the ratio would remain at 1.0; if the loop begins to bend to the right, the slope will decrease and the ratio will decrease to