Peak vs Plateau Pressure in Mechanical Ventilation Explained

High pressure alarms on ventilators are caused by two main categories: increased airway resistance (secretions blocking the tube, tube kinking, biting on the tube, worsening bronchospasm) or decreased lung compliance (worsening underlying lung disease, pneumothorax). Pneumothorax is a critical cause that must be ruled out. The first troubleshooting step is to press the inspiratory hold button and wait 2-3 seconds. A large difference between peak and plateau pressure indicates increased resistance, while a normal difference indicates decreased compliance. This systematic approach prevents unnecessary suctioning and ensures appropriate intervention based on the actual problem.

When ventilator problems occur, disconnect and revert to BVM. Systematically assess patient (ABCs, airway placement via chest rise, ETCO2, lung sounds) then ventilator (oxygen source, settings, connections). Use D.O.P.E. acronym: Dislodged tube, Obstruction (mucus plug/biting), Pneumothorax, Equipment failure. High pressure alarms during CPR are normal; for patients with pulse, check for pneumothorax, mainstem intubation, pulmonary edema, ARDS, rigid chest, or obstruction. Low pressure indicates leaks, deflated cuffs, or disconnections. Document airway type/size, ETCO2 readings and waveforms, multiple vital sign sets, initial and adjusted ventilator settings, and all troubleshooting performed. This comprehensive documentation supports quality improvement and legal protection.

Three-step troubleshooting approach: consult respiratory therapist, check alarms, and address patient ABCs. High peak pressure with normal plateau indicates airway resistance from mucus plugs, inadequate sedation, kinked tubes, or bronchospasm. High plateau pressure indicates lung stiffness from pulmonary edema, ARDS, or tension pneumothorax. Low pressure alarms typically indicate disconnected circuits. Auto-PEEP occurs in obstructive lung disease when patients cannot fully exhale, causing volume stacking. Management includes disconnecting the ventilator, adjusting sedation, using bronchodilators, and lowering respiratory rate.

This section details high airway pressure alarm causes and management. When this alarm sounds, the machine automatically stops inflating and vents the patient to prevent lung injury. Causes include airway obstructions (mucus, kinked tubing, biting), decreased lung compliance (ARDS, pneumonia), increased airway resistance (bronchospasm), tube insertion too deep (right mainstem intubation), and pneumothorax. To determine if the problem is patient or machine-related, disconnect the patient and manually squeeze the resuscitation bag. If difficult to squeeze, the problem is in the patient's airway; if easy, the problem is in the ventilator circuit.

Ventilators feature two critical alarm systems: low pressure (low tidal volume) and high pressure (high peak pressure) alarms. Low pressure alarms signal air leaks from cuff leaks, ET tube displacement, or tube disconnection. High pressure alarms indicate airflow blockages from biting the tube, kinked tubing, mucus plugs, coughing, patient agitation, pulmonary edema, or pneumothorax. When responding to high pressure alarms, nurses must assess for blockages first, checking for biting, secretions, kinks, and patient-ventilator dyssynchrony. Common NCLEX questions test understanding that high pressure equals high blockage, distinguishing it from low pressure causes like leaks.

Double triggering is diagnosed using ventilator scalars: pressure-time, flow-time, and volume-time. The pressure-time scalar shows two negative deflections (patient's inspiratory efforts) followed by two pressure increases (ventilator breaths). The flow-time scalar shows two inspiratory flow waves without the normal expiratory negative flow deflection. The volume-time scalar shows two tidal volume deliveries without proper expiratory return to baseline.

Ventilator graphics provide real-time information about patient-ventilator interaction and respiratory mechanics. They are classified as scalars (plotting pressure, volume, or flow against time) and loops (plotting pressure and flow against volume). In pressure controlled ventilation, the pressure time scalar shows a square wave format with constant pressure throughout inspiration. The waveform does not start from baseline due to PEEP. Mandatory breaths show no negative deflection, while patient-triggered breaths show negative deflection as the patient attempts to pull pressure.

Modern ventilators display graphical interfaces including scalars (graphs) and loops. Scalars plot pressure, flow, and volume against time. Flow is volume divided by time. The flow-time scalar shows positive deflection during inspiration and negative during passive exhalation. Mechanical and spontaneous breaths have different inspiratory patterns. The trigger marks when the machine detects patient effort. The limit determines maximum flow (pressure or volume-limited). Cycling determines when inspiration transitions to expiration. Airway obstruction causes prolonged expiration that doesn't touch baseline. Air trapping (auto-PEEP) occurs when incomplete exhalation leads to progressive air accumulation. Circuit leaks show premature inspiration beginning in the flow-time scalar. Spontaneous breathing is negative pressure breathing, while mechanical ventilation is positive pressure breathing. In volume control ventilation, the pressure-time scalar shows a square wave pattern during inspiration (constant flow) followed by a plateau. Peak Inspiratory Pressure (PIP) is the highest pressure at end of inspiration. Plateau Pressure (Pplat) is measured during an inspiratory pause. PIP reflects both resistive and elastic pressures, while Pplat reflects only elastic pressures. The difference between PIP and Pplat represents transairway resistance. When PIP increases but Pplat remains normal, this indicates high airway resistance (bronchospasm, tube obstruction, secretions). When Pplat increases with normal PIP-Pplat difference, this indicates low lung compliance (ARDS, pneumothorax, pulmonary edema). Three scenarios exist in mechanical ventilation: (1) Normal patient - normal compliance and resistance; (2) Obstructive airway disease - normal compliance but increased resistance; (3) Stiff lungs (ARDS) - decreased compliance. Modern ICU ventilators do not have true control mode - all are assist control modes. Assist control modes have trigger sensitivity that coordinates with patient effort. Control mode (true control) has no triggering and delivers breaths at fixed intervals. Ventilators measure flow and calculate volume by multiplying flow by time. The volume-time scalar shows volume on the positive side. An air leak is indicated when the exhalation curve does not touch baseline and drops vertically before the next breath cycle begins.

Ventilator-patient dyssynchrony occurs when mechanical ventilation conflicts with the patient's respiratory drive. Signs include: negative deflection in pressure-time graph with positive deflection in flow-time graph (indicating diaphragmatic contraction). Types include: trigger reversal (patient initiates breath but ventilator doesn't respond), ineffective effort, and double triggering. Dyssynchrony increases mortality and can cause barotrauma or volutrauma. Treatment requires adequate sedation and potentially neuromuscular blockade.

Pressure-time and flow-time graphs are essential tools for analyzing mechanical ventilation. In pressure-time graphs, when the machine delivers air, pressure increases in the patient's airways. When the patient makes an effort to trigger the machine, a small negative deflection appears in the pressure curve. In flow-time graphs, the flow curve has both positive and negative sides, with the positive side representing inspiratory flow and the negative side representing expiratory flow. The shape and characteristics of these curves help clinicians assess patient-ventilator synchrony and identify potential issues.

Lung-protective ventilation uses low tidal volumes (6-8 cc/kg ideal body weight) with high respiratory rates (16-18 breaths/min) to prevent ventilator-induced lung injury; oxygenation is managed by titrating FiO2 and PEEP together using a standardized scale to maintain SpO2 between 88-95%, while plateau pressure must remain below 30 cm H2O to prevent alveolar overdistension, with inspiratory flow rate adjusted for patient comfort.

Protective ventilation requires three pressure limits: peak inspiratory pressure (PIP) less than 35 cm H2O, plateau pressure less than 30 cm H2O, and driving pressure (plateau minus PEEP) less than 15 cm H2O. These limits prevent ventilator-induced lung injury. The driving pressure is particularly important as it reflects the actual stress on the lung tissue during ventilation.

Lung protective ventilation reduces ventilator-induced lung injury (VILI), which increases fibrotic ARDS risk. Volume control is preferred for fixed tidal volume delivery. Target tidal volume ≤6 ml/kg predicted body weight (reduced to 4 ml/kg if needed). Plateau pressure must be ≤30 cm H2O. Drive pressure (plateau minus PEEP) should be ≤15 cm H2O. Respiratory rates of 35-45 breaths/min are common to maintain CO2 elimination.

Driving pressures below 14 cm H2O are generally safe, while pressures above this threshold increase mortality risk exponentially. When plateau pressure is maintained below 30 cm H2O, driving pressures exceeding 14 cm H2O should be considered potentially harmful. Lung protective ventilation aims to minimize driving pressure and prevent ventilator-induced lung injury through individualized tidal volume based on compliance, maintaining plateau pressures below 30 cm H2O. To maintain plateau pressure while reducing driving pressure, tidal volume can be increased while simultaneously decreasing PEEP.