22.13e Negative Pressure versus Positive Pressure

Two types of mechanical ventilators are negative pressure and positive pressure.

Negative-pressure ventilators suck air into the lungs by making the chest expand.

An iron lung is an example.

Negative-pressure ventilators are rarely used in the current era.

Positive-pressure ventilators push air into the lungs.

They were developed in the 1950s to treat patients with polio who developed respiratory paralysis.

Almost all mechanical ventilators used today are positive-pressure ventilators of which there are two types: invasive and noninvasive.

Invasive ventilators deliver airflow through an endotracheal tube or tracheostomy tube.

Noninvasive ventilators deliver airflow through a mask (nasal/oral or nasal).

Noninvasive ventilators include CPAP devices that deliver constant pressure and BiPAP devices that deliver air with different pressures for inhalation and exhalation, and auto-titrating positive airway pressure (APAP) devices that change the air pressure according to the patient’s breathing pattern.

EMS practitioners often use both noninvasive and invasive ventilation measures.

As already noted, positive-pressure ventilation is the most common form of mechanical ventilation today.

These devices push the air into the patient’s airway and eventually to the alveoli.

Expiration is by chest recoil.

The ventilator continually blows and stops in regular, preset cycles that enable the lungs to receive oxygenated air and expel CO2.

The cycles can be driven by time, pressures, or a combination of these.

Volume-controlled ventilators, also called volume-cycled ventilators, deliver a preset volume of air into the patient even if a high airway pressure is necessary.

When the flow is stopped after delivering the preset volume of air, the chest recoils and expels the air.

Pressure-controlled ventilators deliver air until the airway pressure reaches a predefined limit and the valve opens to allow air to exit.

The volume of air delivered can vary depending on both the airway resistance and lung capacity.

With this ventilator, the volume of air delivered continues until a preset pressure is reached, and then the chest recoils and expels the air.

Dual-control ventilators, also called pressure-regulated volume-control ventilators, combine the advantages of volume control and pressure control.

They deliver airflow based on the patient’s requirement and response.

Assist control (AC) is one of the most commonly used modes for mechanical ventilation and is often used for patients who require the most ventilator support.

The AC mode has several basic settings including the respiratory rate (RR), tidal volume (TV), inspired oxygen level, positive-end expiratory pressure, and others.

Essentially, the ventilator delivers the tidal volume at the preset rate the practitioner determines regardless of the airway pressure needed to achieve that goal.

Pressure regulated volume controlled (PRVC) is a newer mode of ventilation that is very similar to AC mode.

The PRVC ventilator delivers a volume-controlled breath (like AC mode), but then, using the airway pressure generated on the previous breath, it delivers the next breath with the lowest delivery pressure possible to reach the target tidal volume.

The Synchronized intermittent mandatory ventilation (SIMV) mode works on the same basic principles as AC mode.

That is, it delivers a set number of breaths per minute.

However, the patient can breathe as many times a minute as needed.

The ventilations can be patient- or ventilator-initiated.

There is, however, a difference in how the TV is delivered.

Generally speaking, all ventilator-initiated breaths have a full TV delivery.

However, for patient-initiated breaths, the entire TV cannot be delivered.

The rationale behind using SIMV instead of AC is to increase the work of the patient’s respiratory muscles by providing periods of decreased support.

If the respiratory rate is set high or if the patient is not breathing spontaneously, SIMV functions identically to AC mode.

Airway pressure release ventilation (APRV) is a complicated mode used for patients who are having problems with lung compliance or difficulty with oxygenation and often used for patients who have acute respiratory distress syndrome (ARDS) or COVID-19.

It uses an inverse inhalation/exhalation (I/E) ratio to achieve high levels of pressure (the expiratory phase is longer than the inspiratory phase).

Pressure support (PS) is a ventilator-weaning mode used to determine the patient’s readiness for extubation.

It does not use a set respiratory rate and is a pressure-driven mode rather than a time-triggered one.

PS requires the patient to initiate each breath and then that breath is assisted through the ET tube with a set amount of pressure.

This helps to overcome the resistance of the ET tube.

Most modern ventilators allow the EMS practitioner to adjust the mode of ventilation to meet the patient’s specific requirements (Table 22-18).

Many new ventilators can also be set to specialty modes such as high-frequency oscillatory ventilation (HFOV).