CCRN · Respiratory · 9 min read
Mechanical Ventilation Modes and Troubleshooting
- Differentiate the operational characteristics, pressure profiles, and dyssynchrony risks among common mechanical ventilation modes.
- Analyze respiratory mechanics, including peak inspiratory pressure, plateau pressure, driving pressure, and auto-PEEP, to detect acute pulmonary deterioration.
- Execute systematic troubleshooting for ventilator alarms, acute hemodynamic compromise, and liberation failure within the scope of critical care nursing practice.
Ventilator Modes and Breath Delivery Mechanics
Mechanical ventilation relies on specific breath delivery patterns tailored to patient stability. In Assist-Control (AC) mode, the ventilator delivers a preset tidal volume or inspiratory pressure for every breath, whether the breath is machine-initiated by a timer or patient-triggered by spontaneous effort. In volume Assist-Control, each successfully triggered breath targets the preset tidal volume, but a pressure limit or leak may prevent full delivery; assess delivered volume and total respiratory rate. This unvarying volume delivery can quickly drive down carbon dioxide levels, predisposing the patient to respiratory alkalosis. Pressure Control Ventilation (PCV) takes a different approach by delivering a preset inspiratory pressure over a set inspiratory time. Because pressure is fixed, the delivered tidal volume is variable, changing in response to dynamic shifts in lung compliance and airway resistance. While PCV limits the set inspiratory airway pressure, it does not eliminate the risk of barotrauma or volutrauma. The critical care nurse must closely monitor delivered tidal volume and respiratory mechanics, especially when compliance fluctuates.
Synchronized Intermittent Mandatory Ventilation (SIMV) provides a bridge between full mandatory support and spontaneous breathing. SIMV delivers a set number of mandatory breaths synchronized with the patient's inspiratory effort while allowing spontaneous breaths between cycles at the patient's own tidal volume. To ease the muscular effort of drawing air through artificial airways, spontaneous breaths during SIMV can be augmented with Pressure Support (PS). Pressure support can offset some resistance from the endotracheal tube and reduce inspiratory work, but its effect depends on the support level, respiratory mechanics, and patient, ventilator synchrony.
Pressure Support Ventilation (PSV) functions as a purely patient-triggered, flow-cycled spontaneous mode where the ventilator delivers a set inspiratory pressure during each patient-initiated breath. In PSV, the patient initiates breaths and influences the respiratory rate, inspiratory time, and tidal volume. However, the patient does not act alone; the set pressure-support level and the ventilator's internal cycling settings also directly affect inspiratory time and delivered tidal volume. Patient-ventilator dyssynchrony can emerge in any assisted mode, increasing patient work of breathing, myocardial oxygen consumption, and the risk of ventilator-induced lung injury. Ineffective triggering occurs when patient inspiratory efforts fail to generate enough flow or negative pressure to trigger a ventilator breath, a complication frequently worsened by gas trapping. Premature or delayed flow cycling can occur in Pressure Support and other flow-cycled modes when ventilator termination of inspiration does not match the patient's neural inspiratory time.
Common mistakes
- Using actual total body weight instead of predicted body weight calculated from height and biological sex when setting lung-protective tidal volumes.
- Assuming an elevated peak inspiratory pressure always represents worsening lung compliance without checking plateau pressure to rule out airway resistance.
- Relying solely on an RSBI below 105 breaths/min/L to decide on extubation without evaluating airway protection, secretion management, or spontaneous breathing trial tolerance.
- Waiting for classic tracheal deviation to appear before suspecting and acting on a life-threatening tension pneumothorax.
Key points
- Volume Assist-Control targets the set tidal volume for delivered breaths, although a pressure limit or leak can prevent full delivery; frequent triggered breaths can increase minute ventilation and cause respiratory alkalosis, whereas Pressure Control targets inspiratory pressure and yields variable tidal volumes.
- In volume-controlled ventilation with comparable settings, an increased peak pressure with stable plateau pressure suggests increased resistance; an increase in both pressures suggests reduced compliance, increased total PEEP, or both.
- Lung-protective ventilation uses tidal volumes of approximately 4 to 8 mL/kg of predicted body weight and, in ARDS, generally targets plateau pressure at or below 30 cm H2O to reduce, not eliminate, the risk of ventilator-induced lung injury.
- High PEEP and auto-PEEP raise intrathoracic pressure, which can impair venous return, increase right ventricular afterload, worsen V/Q mismatch, and cause severe hypotension.
- When sudden deterioration occurs, apply the DOPE mnemonic, and if ventilation cannot be promptly restored, manually ventilate with a resuscitation bag and 100% oxygen while investigating the cause.
- Endotracheal tube cuff pressure is generally maintained at 20 to 30 cm H2O; audible vocalization with low exhaled volume should prompt assessment for a cuff leak or tube displacement.
- Spontaneous breathing trial failure is signaled by tachypnea over 35 breaths per minute, oxygen saturation under 90%, sustained tachycardia, hypotension, diaphoresis, or paradoxical breathing.
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