Michael J. Frazer and Paul N. Lanken

Types of Ventilator Alarms

The types and number of alarms that are functional on a ventilator vary depending on which ventilatory mode is being used. In addition, identical alarms may have different meanings and response times in different modes. Finally, alarms can signify different levels of clinical urgency (Table 47.1).

In face of this complexity, the ability to categorize ventilator alarms is essential. All alarms can be categorized into three types based on whether they indicate (1) a ventilator failure, (2) a patient-ventilator interface problem, or (3) a patient-related event.

Because most modern ventilators function by combining a microprocessor-based electromechanical system with a pneumatic system, ventilator failure can result from a malfunction in its electronic or pneumatic systems. Ventilator manufacturers incorporate preset alarms to ensure the safety of these systems. In the case of a true ventilator failure and not user error—for example, a power cord being disconnected—there is little choice but to recognize the problem quickly and replace the ventilator if necessary.

Patient-ventilator interface problems refer to the interaction of the ventilated patient with the ventilator. Thresholds for alarms arising from problems at this interface are usually set by a respiratory care practitioner and relate closely to the ventilator’s monitoring capabilities. Significantly, these alarms may alert the ICU clinician to problems in how ventilatory support is being provided to the patient. They can signify a range of situations, for example, from a patient becoming disconnected from the ventilator to the peak inspiratory flow failing to meet a patient’s inspiratory demand. Practitioner settings that are specifically designed to assist in alleviating patient-interface issues include peak inspiratory flow, inspiratory time (“I time”) or rise time, and expiratory sensitivity.

Patient-related event alarms are synonymous with “monitoring alarms.” They are designed to detect changes in the patient’s underlying physiologic condition. When a simple physiologic monitor alarm (e.g., an electrocardiographic [ECG] monitor) activates, the reason for the alarm arises from a patient-related event and not the functioning of the monitor itself. In contrast, a ventilator with its alarms serves not only as a physiologic monitor but also as a therapeutic intervention whose effects are continuously self-monitored. ICU clinicians often have difficulty recognizing when a problem arises from the patient or from the ventilator and distinguishing patient-ventilator interface problems from those solely attributable to the patient (patient-related events).

Each commonly used ventilator alarm is designed to detect a certain acute abnormality, as indicated by its name. Many alarms, however, overlap in the underlying conditions that they detect, as well as in the clinical consequences that they aim to prevent (Table 47.2). For example, if the patient becomes disconnected from the ventilator, multiple alarms activate. Early-response alarms monitor low inspiratory pressure, low exhaled tidal volume, and low positive end-expiratory pressure (PEEP) (if PEEP had been applied before the disconnection). Later alarms include low expired minute ventilation and apnea. Not only do these ventilator-associated alarms form a redundant safety net, but they also are complemented by nonventilator alarms, such as a pulse oximeter, or ECG lead monitoring. Tables 47.3 to 47.7 list conditions in which alarming is used frequently, with their differential diagnoses, categorized according to site of the problem, and matched with appropriate corrections.