In critical care and resuscitation, teams use interventions involving pressure within the chest to support a patient’s breathing and circulation. Two of these are the impedance threshold device (ITD) and positive end-expiratory pressure (PEEP). While both relate to respiratory and circulatory support, they are based on different principles and used for different primary goals. Understanding their distinct functions and applications is important for grasping their roles in medical emergencies.
The Role of an Impededance Threshold Device
An impedance threshold device (ITD) is a disposable valve used during cardiopulmonary resuscitation (CPR). It is a circulatory enhancer, designed to improve blood flow during a cardiac arrest. The ITD is attached to a patient’s airway, such as a face mask or an advanced airway like an endotracheal tube.
The ITD’s mechanism is to prevent air from entering the lungs as the chest wall recoils after a compression. During standard CPR, chest recoil naturally draws some air in. The ITD blocks this passive influx of air during the recoil phase, creating a vacuum, or negative intrathoracic pressure, inside the chest.
This negative pressure is central to the device’s function. The vacuum generated enhances the return of venous blood to the heart, a factor known as preload. By increasing the amount of blood that refills the heart’s chambers between compressions, the subsequent compression can pump more blood to the body. This process improves cardiac output and helps deliver oxygenated blood to the brain and heart.
The use of an ITD is specific to the active phase of CPR. It works in synergy with chest compressions to make them more effective at circulating blood. The intrathoracic pressure in a patient receiving CPR with an ITD can drop to between -3 and -8 mmHg, compared to the 0 to -2 mmHg seen in standard CPR. This enhanced negative pressure also helps to lower intracranial pressure, which can further protect the brain during resuscitation efforts.
Understanding Positive End-Expiratory Pressure
Positive end-expiratory pressure (PEEP) is not a device but a ventilator setting used to support patients with respiratory failure. PEEP applies a constant, low level of positive pressure to the lungs at the end of each exhalation. This pressure is maintained continuously as long as the patient is connected to the ventilator.
The primary function of PEEP is to prevent the tiny air sacs in the lungs (alveoli) from collapsing completely when a patient breathes out. In many respiratory conditions, such as Acute Respiratory Distress Syndrome (ARDS), damaged alveoli are prone to collapse. By keeping them partially inflated with a steady stream of pressure, PEEP helps maintain a greater surface area for gas exchange.
This mechanism improves oxygenation. When more alveoli remain open, more oxygen can pass from the lungs into the bloodstream, and more carbon dioxide can be removed. PEEP also helps to improve lung compliance, making the lungs easier to inflate during the next breath. This can reduce the work of breathing for the patient and minimize further lung injury.
PEEP is a standard setting for nearly all patients on mechanical ventilators. The level of PEEP is carefully adjusted by the clinical team based on the patient’s specific condition, ranging from low levels of around 5 cmH2O to much higher levels in severe respiratory failure. Its application is continuous and geared towards optimizing lung function.
Key Physiological Distinctions
The primary difference between an ITD and PEEP is their effect on intrathoracic pressure. An ITD is engineered to generate intermittent negative pressure by limiting air entry during the chest wall recoil phase of CPR. This negative pressure is directly tied to the rhythm of chest compressions.
In contrast, PEEP maintains a constant state of positive pressure within the lungs and chest. It is applied at the end of exhalation and sustained until the next breath begins. This continuous positive pressure is intended to keep the airways and alveoli open, creating an opposite pressure environment.
These opposing pressure mechanics lead to different primary goals. The ITD’s objective is hemodynamic and circulatory. By generating negative intrathoracic pressure, it aims to increase venous return, cardiac preload, and the amount of blood circulated with each chest compression. Its focus is on improving perfusion to the brain and heart during a cardiac arrest.
PEEP’s primary goal is respiratory and focused on oxygenation. By preventing alveolar collapse, it increases the functional residual capacity of the lungs, which is the volume of air remaining after a normal exhalation. This enlarges the surface area available for gas exchange between the lungs and the blood, leading to improved oxygen levels.
Consequently, their effects on venous return are also opposite. The ITD is designed to increase the flow of blood back to the heart, as its negative pressure draws blood from the venous system into the chest. PEEP, on the other hand, can have the opposite effect. Because it maintains positive pressure inside the chest, it can compress the large veins, which may decrease venous return, particularly at higher PEEP levels.
Clinical Applications and Combined Use
The clinical scenarios for using an ITD and PEEP are distinct. An ITD is an intervention for cardiac arrest to enhance the quality of CPR and improve blood flow. It is often used in out-of-hospital cardiac arrest situations and can be applied to patients with a basic or an advanced airway. Its application is temporary and confined to the duration of the resuscitation attempt.
PEEP is a method of managing patients on mechanical ventilators, particularly those with severe respiratory distress or failure. It is a long-term supportive measure used in the intensive care unit to treat conditions that impair oxygenation. Its use is not for cardiac arrest but for ongoing respiratory support in a controlled clinical environment.
A complex situation arises when a patient who is already on a ventilator with PEEP goes into cardiac arrest. In this scenario, the continuous positive pressure from PEEP can directly counteract the negative pressure an ITD is trying to create. The PEEP works to keep the chest pressurized, while the ITD works to create a vacuum. This conflict can reduce the effectiveness of the ITD and the quality of CPR.
To manage this, clinical protocols often recommend adjustments to the ventilator settings during CPR. Rescuers might temporarily disconnect the patient from the ventilator circuit to eliminate the influence of PEEP, allowing the ITD to function optimally. Alternatively, the PEEP setting on the ventilator may be reduced to its lowest possible level.