Virtual simulation offers insights to graphical analysis of ventilator alarms

Neil MacIntyre, MD, FCCP
Neil MacIntyre, MD, FCCP

Mechanical ventilation has long been an important part of routine care in pulmonary medicine, but complex displays with multiple waveforms and information windows can be confusing. A CHEST 2021 virtual simulation session, Ventilator Graphics: ‘Fix the Vent,’ featured seven cases from intensive care units (ICUs) around the country, with panelists sharing key tips to help clinicians improve ventilator outcomes and avoid common pitfalls.

“You can utilize interpretations of ventilator graphics and waveforms to solve various clinical problems and understand the various types of ventilator problems, including asynchrony,” said Neil MacIntyre, MD, FCCP, Professor of Medicine at Duke University School of Medicine.

Ventilator displays vary by model, he noted, but screens commonly display the ventilator mode, volume control, or flow control, with waveforms showing pressure, flow, and volume. Ventilator measurements and limits are usually shown in one area and patient parameters in another section.

Inhalation is usually shown as a square wave and exhalation by a triangular wave. Depending on the ventilator, he explained, controlled, machine-triggered breaths and patient-triggered breaths may be shown in different colors to help interpret the waveforms.

A familiar scenario in many ICUs is a combination of alarms showing loss of tidal volume and excess peak pressure. The most common problem is a peak pressure alarm that prevents the ventilator from delivering gas, which reduces the amount of gas delivered and limits tidal volume. The most direct solution is to reset settings for pressure, volume, and breathing rate. The key concept, Dr. MacIntyre said, is driving pressure, a measure of mechanical pressures during tidal volume delivery.

“Driving pressure is a fine-tuning mechanism to help ensure lung-protective ventilation,” he explained. “The questions to always ask are if the patient is safe, if the settings are effective, and if the patient is synchronous with the ventilator.”

Another common alarm is a peak flow alarm, which could be the result of either an airway obstruction or stiffening of the respiratory system. The peak flow waveform alone cannot differentiate between the two problems, but the combination of peak flow and plateau graphics can.

When both peak flow and plateau are increased, the problem is likely decreased compliance, or stiffening, in the respiratory system. But if only the peak flow is up and the plateau is unchanged, the problem is more likely an obstruction such as a mucus plug.

“You may also see changes in expiratory flow from a triangular, almost right-angle form with a sharp decrease to a slow, gentle expiration that is typical of an airway obstruction,” Dr. MacIntyre said.

There are many potential mechanisms for decreased compliance. More common problems include pneumothorax or large effusion, right mainstem intubation, congestive heart failure/pulmonary edema/ARDS, obesity, and intraabdominal hypertension. Common problems leading to airflow obstruction include bronchospasm, patient biting the tube, kinking in the tube, mucus plug, or some other blockage.

Another common alarm is autoPEEP, excessive pressure trapped from gas that has been trapped in the lung.

“You will see the classic exhalation signal, a slow expiration that does not return to zero before the next breath,” Dr. MacIntyre said. “You can resolve the pressure by increasing the flow volume or decreasing the respiration rate to increase the expiratory time, which allows the trapped gas to escape from the lung.”

John Davies, MA, RRT
John Davies, MA, RRT

John Davies, MA, RRT, Clinical Research Coordinator, Respiratory Care Services, Duke University Medical Center, presented a case of flow asymmetry in a patient with COVID-19 who was paralyzed for 3 days but has been taken off paralysis and is interacting with the ventilator on volume control.

“This is a classic example of flow asynchrony, or flow starvation,” Davies explained. “We can fix it by increasing the flow, but whenever we change settings, there can be unintended consequences, in this case decreasing the inspiratory time. Or you can switch to pressure control, which could increase the tidal volume. The real message is that whatever changes you make, you need to watch the patient for a few minutes in case there are unintended consequences.”

Another common occurrence with patients on controlled ventilation is different amounts of air going into the system than are coming out. The difference can be in either direction, either more gas going in than coming out or more gas coming out than the ventilator is putting in.

“A skilled clinician can recognize the problem and fix it,” he said. “When you see more inhaled volume than exhaled volume, you have a leak in the circuit. Some of the tidal volume is going somewhere else.”

Cuff leaks are the most common, he continued. Connectors and adaptors are other common sources of leaks, as well as damaged tubing or valves. The leak may also be due to a large bronchopleural fistula.

More gas coming out of the circuit than the ventilator means an additional source of volume. One of the most common sources is an external nebulizer, particularly in patients with asthma who may be on inhaled steroids.


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