Ventilator Waveforms and Scalars Lecture

Ventilator graphics are visual representations of mechanical breaths, often preloaded on the display screen. These curves are technically referred to as scalars.
A scalar plots a specific value (Pressure, Flow, or Volume) against time ( $\text{X-axis}$ ).
There are three primary scalars commonly monitored:
- Pressure Scalar: Plots pressure ( $\text{cmH}_2\text{O}$ ) over time ( $\text{seconds}$ ). It illustrates how the ventilator circuit is pressurized during the delivery of a breath.
- Flow Scalar: Plots flow against time. This is usually a biphasic curve. The inspiratory phase is represented as a positive deflection, and the expiratory phase as a negative deflection.
- Volume Scalar: Plots the patient's tidal volume over time, tracking both inhalation and exhalation of the breath.
Loops, which will be discussed in further sessions, plot one value against another (e.g., Pressure-Volume and Flow-Volume loops), excluding time as a direct axis.

The pressure-time scalar provides exhaustive information visible even from "across the room."
Breath Type Identification: Clinicians can immediately distinguish between Pressure Control, Volume Control, Assist Control, IMV, and Spontaneous breaths.
Work of Breathing (WOB): Assesses the effort required by the patient to trigger a breath.
Mean Airway Pressure ( $\bar{P}_{aw}$ ): Vital for oxygenation; a higher mean airway pressure corresponds to higher Functional Residual Capacity (FRC), driving lung recruitment and improving oxygenation.
Compliance and Resistance: Patterns in the waveform reflect changes in lung elasticity and airway friction.
Plateau Pressure: Allows for the assessment of static pressure in the lungs.
Patient-Ventilator Synchrony: Identifies if the patient is "fighting" the ventilator. Synchronization ensures respiratory muscles rest for recovery; asynchrony delays weaning.

Axes: The Y-axis measures pressure (typically in $\text{cmH}_2\text{O}$ ); the X-axis measures time (typically in $\text{seconds}$ , e.g., $0$ , $2$ , $4$ , $6$ , $8$ ).
Baseline (PEEP): The pressure point before inspiration. An elevated baseline above zero is known as Positive End-Expiratory Pressure (PEEP). A common starting value is $5\text{ cmH}_2\text{O}$ .
Trigger: The signal that initiates inspiration. It can be time-triggered (ventilator-initiated) or patient-triggered.
Inspiratory Phase: The portion of the graph above the baseline until the peak. It terminates at the Peak Inspiratory Pressure (PIP).
Cycle: The transition point where inspiration ends and exhalation begins.
Expiratory Phase: Defined as any time the patient is not in the inspiratory phase, even if there is no detectable air movement.

Pressure Waveform Shape: In Volume Control with a square flow waveform, the pressure curve resembles a gradual rise, often compared to a "shark fin." The PIP occurs at the very end of inspiration.
Square (Constant) Flow Waveform:
- Flow starts and stays at a fixed rate throughout the inspiratory phase.
- Disadvantage: It can be uncomfortable for awake patients because the flow is fixed.
- Distribution Issue: Air follows the path of least resistance, primarily ventilating the lung apices and central regions rather than peripheral or dependent zones. This results in the gradual rise seen on the pressure scalar.
Descending/Decelerating Ramp Flow:
- Found on ventilators like the Galileo or AVEA.
- Flow starts at a peak (e.g., $50\text{ L/min}$ ) and immediately begins to decrease, approaching zero.
- This promotes better distribution to peripheral lung zones and lower regions.
- Impact on Pressure: Causes an abrupt rise to pressure that is maintained throughout inspiration, changing the shape of the pressure-time scalar.

Pressure Waveform Shape: Characterized by an immediate, abrupt rise to the set pressure limit, which is then held as a plateau until the cycle ends.
Decelerating Ramp Flow: This is the default in Pressure Control. Flow slows down as the lungs fill and back-pressure increases.
Factors Determining Initial Flow in PC:
1. Compliance.
2. Resistance.
3. Patient Effort (high effort results in higher initial flow).
The Role of the Time Constant: A math-driven factor representing how long it takes to fill the lungs. If the Inspiratory Time ( $T_I$ ) is set too short, the decelerating flow may not reach zero, indicating incomplete inflation.
Clinical Preference: PC is often preferred for ARDS (Acute Respiratory Distress Syndrome) and restrictive lung diseases. The immediate pressurization increases mean airway pressure, recruits collapsed alveoli, and helps treat shunt conditions.

Assist Control (AC): All breaths (whether triggered by time or the patient) are mandatory breaths. If every breath on the screen looks identical in shape and volume/pressure delivery, it is AC. Patients can trigger breaths, but the ventilator provides full support for every one.
Intersittent Mandatory Ventilation (IMV): A combination of mandatory breaths and spontaneous breaths.
- Mandatory breaths are usually machine-timed.
- Spontaneous breaths occur between mandatory ones; they appear different, showing negative pressure deflections during inspiration if unsupported.
Spontaneous/CPAP (Continuous Positive Airway Pressure): No mandatory breaths are delivered. Every breath is patient-initiated.
- Waveform shows negative deflection for inspiration and positive for exhalation, all occurring above an elevated baseline.
- Historical Note: References the recent passing of the last individual living in an iron lung (negative pressure ventilation).

Definition: PS is a "boost" given to spontaneous breaths in modes like IMV or CPAP to overcome the work of breathing caused by the endotracheal tube (ETT).
Waveform Characteristics: Starts with a negative patient-trigger dip, followed by the ventilator delivering pressure up to the set PS limit. The breath is cycled based on flow.
Determining PS Levels:
- Formula: A quick assessment method is $PIP - \text{Plateau Pressure} = \text{Airway Resistance}$ .
- Standard Protocol: Initially set between $5$ and $6\text{ cmH}_2\text{O}$ .
- Small Diameter Adjustment: If the patient has an ETT size less than $7.5\text{ mm}$ , the initial PS should be set at $8\text{ cmH}_2\text{O}$ due to higher resistance.
Monitoring: PS should be titrated based on the patient's respiratory rate, tidal volume, and observed work of breathing.

Q: How can you distinguish flow from pressure on a screen?
A: Two ways. First, units (flow is usually in L/min). Second, flow is biphasic; it has a positive inspiratory phase and a negative expiratory phase.
Q: Is the I:E ratio important when adjusting inspiratory time in Pressure Control?
A: Yes. Avoid a $1:1$ ratio to prevent air trapping. A ratio like $1:3.7$ allows room to increase inspiratory time, whereas $1:1.5$ offers little "wiggle room," especially in obstructuve diseases like asthma or COPD where long expiratory times are necessary.
Q: What indicates a patient-triggered breath on the pressure scalar?
A: A small negative deflection (dip) below the baseline occurring just before the rise in pressure.