Peak and Plateau Pressures in Mechanical Ventilation

Mechanical ventilation relies on monitoring specific pressures to understand a patient's underlying physiology and ensure safe lung management.
The two primary pressures discussed are peak pressure and plateau pressure.
Management of these pressures is critical to prevent barotrauma, a condition where the lungs or alveoli become over-distended and damaged by excessive pressure.

Respiratory Anatomy Path: The breath starts at the ventilator, passes through the ventilator tubing, enters the endotracheal tube (placed in the trachea), moves through the main stem bronchi, branches into bronchioles, and terminates in the balloon-shaped alveoli.
The Alveoli: These are the functional units where gas exchange occurs. They act like small balloons that inflate and deflate.
Endotracheal Tube (ET Tube): A plastic tube inserted into the trachea. It features a balloon cuff that is inflated to create a seal, ensuring the ventilator system is a closed circuit.
Breath Delivery Process: * Inspiration: The ventilator pushes the breath through the ET tube and into the airways to expand the lungs. * Expiration: Once the ventilator stops delivery, the lungs contract naturally, pushing air back out through the bronchi, trachea, and ET tube toward the ventilator.

Definition: The maximum pressure the ventilator experiences while actively trying to push a breath into the lungs.
Detection: It is measured with every single breath and is typically displayed in the top right corner of the ventilator screen (depending on the brand).
Physiologic Representation: Peak pressure is a manifestation of the resistance encountered in both the airways and the alveoli. This includes: * Airway Resistance: The force required to move air through the ventilator circuit and the patient's bronchial tree. * Alveolar Resistance/Lung Compliance: The force required to inflate the lung tissue itself.
Analogy: If using a pump and pipe to blow up a balloon, the peak pressure is the total force required to push air through the pipe and expand the balloon.
Ventilation Mode: Discussions regarding peak pressure usually occur within the context of Volume Control ventilation, the most common modality.
Acceptable Peak Pressure: \text{Acceptable } P_{peak} < 40 \, \text{cmH}_2\text{O}. * "Acceptable" is used instead of "normal" because a healthy person’s pressure would be much lower; this is simply the threshold used to avoid immediate ventilator adjustments.

Definition: The pressure experienced within the lungs when the breath has been delivered but before it is allowed to be exhaled, while flow is briefly paused.
Detection (Inspiratory Hold Maneuver): Unlike peak pressure, the plateau pressure is not automatic. A clinician must perform an "inspiratory hold." The ventilator pushes a breath in and then pauses, preventing air from entering or leaving the lungs (zero flow condition).
Physiologic Representation: Because air is not moving, airway resistance drops to zero. Therefore, the plateau pressure represents the static recoil pressure of the lungs, effectively measuring lung compliance (how easily the lungs distend and collapse).
Acceptable Plateau Pressure: \text{Acceptable } P_{plat} < 30 \, \text{cmH}_2\text{O}. * Staying below this limit is vital to prevent over-distention and barotrauma.

The Relationship: Peak pressure can be conceptually viewed as the sum of airway resistance and plateau pressure: $P_{peak} \propto \text{Airway Resistance} + P_{plat}$ .
Case 1: $P_{peak} = P_{plat}$ (or within $5 \, \text{cmH}_2\text{O}$ ): * This indicates that the majority of the pressure is coming from the lungs themselves rather than the airways. * Finding: This represents a lung compliance problem.
Case 2: P_{peak} > P_{plat}: * If the peak pressure is significantly higher than the plateau (e.g., $P_{peak} = 50$ and $P_{plat} = 20$ ), the difference ( $30 \, \text{cmH}_2\text{O}$ ) is attributed to airway resistance. * Finding: This represents an airway resistance problem.
Case 3: Both are Elevated: * If $P_{peak} = 60$ and $P_{plat} = 40$ , both values are above their respective thresholds ( $40$ and $30$ ). This indicates a combined problem involving both lung compliance and airway resistance.

When both peak and plateau pressures are high, the clinician should focus on issues affecting the lung parenchyma or available lung volume. A mnemonic for this is "Blood, Water, Pus, Inflammation, and Fibrosis."
Blood: Hemoptysis or arterial bleeding into the lung parenchyma which clogs alveoli.
Water: Pulmonary edema (fluid in the lungs).
Pus: Infections, most notably pneumonia.
Inflammation: Conditions like pneumonitis (caused by aspiration or chemotherapy agents).
Fibrosis: Interstitial lung disease that stiffens the lung tissue.
Volume/Ventilation Issues: * Main Stem Intubation: If the ET tube is pushed too far into one bronchus, the entire tidal volume (e.g., $500 \, \text{mL}$ ) is forced into one lung instead of two, causing high pressure. * Atelectasis: Lung collapse in specific areas reduces the total functional lung volume. * Pneumothorax: A collapsed lung prevents ventilation on one side, forcing the entire tidal volume into the remaining functional lung.

When the plateau pressure is normal but the peak is high, the clinician should "trace the circuit" from the ventilator to the patient.
Endotracheal Tube (ET) Pathology: * Kinks in the ventilator tubing or the ET tube itself. * Patient biting down on the ET tube, narrowing the diameter. * The ET tube is fundamentally too small for the patient. * Mucus buildup within the tube (narrowing the lumen).
True Bronchial Airway Pathology: * Mucus Plugging: Secretions blocking the bronchi. * Bronchospasm: Sudden constriction of the bronchi, often seen in COPD or Asthma patients. * Foreign Body: An object obstructing the airway. * Ventilator Desynchrony: The patient "fighting" the ventilator or pushing back against the delivered breath.

The pressure-time scalar displays pressure on the y-axis and time on the x-axis during Volume Control ventilation.
The Peak: The highest point reached during the inspiratory phase of the breath.
The Plateau on the Graph: During an inspiratory hold maneuver, the graph rises to the peak, slightly drops, and then levels off into a flat horizontal line (the plateau) while the ventilator holds the breath. After the hold button is released, the pressure drops back toward zero (or the PEEP level).
Clinical Practice: Respiratory therapists (RTs) typically perform the inspiratory hold maneuvers. Junior trainees are encouraged to work with RTs rather than attempting these adjustments independently.