Pulmonary Graphics Monitoring

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Pulmonary Graphics Monitoring

Pulmonary graphics monitoring has emerged as a valuable tool in the Pediatric Intensive Care Unit (PICU).
The flow sensor collects the air flow data to help derive multiple respiratory mechanics parameters.
The basic parameters measured include the pressure ( $P$ ) necessary to cause a flow ( $Q$ ) of gas to enter the airway and increase the volume ( $V$ ) of the lungs.

Derived Parameters

Compliance ( $C$ ): Calculated as $$C = \frac{dV}{dP}$$.
Resistance ( $R$ ): Derived from the pressure and flow parameters.
Time constant ( $T_{c}$ ): Calculated as $$T_c = C \times R$$
Elastic Work of Breathing (WoB): Derived from the monitored variables.

Classification of Graphics

Category	Description	Examples
Scalars	Control variables (Y-axis) plotted against time (X-axis)	Pressure waveform, Flow waveform, Volume waveform.
Loops	One control variable plotted against another	Pressure-volume (P-V) loop, Flow-volume loop.
Calculated Values	Numerical data derived from the waveforms	Compliance, Resistance.

Scalar Waveforms

Waveform Type	Characteristics & Clinical Interpretations
Pressure-Time	Represents the breath type.
	In volume-controlled, constant flow ventilation, the shape is triangular.
	In pressure-controlled ventilation, the shape is square and shows no changes when pressure is preset and regulated.
	Patient triggering is indicated by the presence of a negative deflection before inspiration.
	Airway obstruction is indicated by a disproportionate rise in peak airway pressure relative to the plateau pressure, and a decrease in Peak Inspiratory Pressure (PIP) following a bronchodilator.
	The area under the pressure-time curve represents the Mean Airway Pressure (MAP).
Volume-Time	In constant flow volume-controlled ventilation, the upstroke represents inspiration, and the downstroke represents expiration.
	The corresponding peak volume on the Y-axis denotes the tidal volume ( $V_{T}$ ).
	Endotracheal tube (ETT) leaks, bronchopleural fistulas, or other air leaks are identified by the expiratory limb failing to return to the baseline.
Flow-Time	The shape of the flow waveform is a descending ramp in pressure ventilation and square in volume ventilation.
	Failure of the expiratory flow to return to zero indicates gas trapping, auto-PEEP, or an air leak.
	Auto-PEEP can cause pneumothorax or pulmonary interstitial emphysema; remedial measures include decreasing the set respiratory rate, decreasing the inspiratory time to give more expiratory time, or considering hypoventilation if the patient is triggering the rate.
	Airway obstruction is indicated if the expiratory flow limb is deeply curved and takes a longer time to return to the baseline.
	In emphysema, the peak expiratory flow has a more relaxed, rounded contour.

Graphic Loops

Loop Type	Characteristics & Clinical Interpretations
Pressure-Volume (P-V)	Displays compliance and indicates the resistance of the respiratory system.
	Identification of patient-triggered breaths is indicated by a negative deflection at the beginning of the loop.
	Choosing appropriate inspiratory pressure: An over-distended lung (decreased compliance) is seen as "beaking" of the P-V loop, where the compliance of the last 20% of the curve is lower than the compliance of the entire loop; this requires lowering either PIP or $V_{T}$ .
	Inadequate flow is indicated by inadequate hysteresis, with little separation between the inspiratory and expiratory limbs; air hunger creates a "figure-of-eight" appearance at the end of inspiration.
	An air leak is identified by the expiratory limb not reaching the baseline.
	Changes in the shape of the P-V loop can indicate increased elastic work (when compliance decreases) and increased resistive work (such as in bronchospasm).
Flow-Volume	A normal flow-volume loop should be circular or oval in appearance, with the upper and lower limits (representing peak inspiratory and expiratory flows, respectively) being nearly equivalent.
	Increased airway resistance is indicated when both the flow and $V_{T}$ are reduced.
	The presence of an air leak is identified when the expiratory limb fails to reach the baseline.
	Airway secretions cause undulations in the shape of the curve.