What is Mechanical Ventilation and Why it is being used for COVID-19

Mechanical ventilation, or assisted ventilation, is the medical term for artificial ventilation where mechanical means are used to assist or replace spontaneous breathing. This may involve a machine called a ventilator, or the breathing may be assisted manually by a suitably qualified professional, such as an anesthesiologist, Registered Nurse, respiratory therapist, or paramedic, by compressing a bag valve mask device.

Mechanical ventilation can be

Noninvasive, involving various types of face masks

Invasive, involving endotracheal intubation

Selection and use of appropriate techniques require an understanding of respiratory mechanics.

Indications

There are numerous indications for endotracheal intubation and mechanical ventilation (see table Situations Requiring Airway Control), but, in general, mechanical ventilation should be considered when there are clinical or laboratory signs that the patient cannot maintain an airway or adequate oxygenation or ventilation.

Concerning findings include

Respiratory rate > 30/minute

Inability to maintain arterial oxygen saturation  >  90% with fractional inspired oxygen (FIO2) > 0.60

pH < 7.25

PaCO2 > 50 mm Hg (unless chronic and stable)

The decision to initiate mechanical ventilation should be based on clinical judgment that considers the entire clinical situation and not simple numeric criteria. However, mechanical ventilation should not be delayed until the patient is in extremis.

Respiratory Mechanics

Normal inspiration generates negative intrapleural pressure, which creates a pressure gradient between the atmosphere and the alveoli, resulting in air inflow. In mechanical ventilation, the pressure gradient results from increased (positive) pressure of the air source.

Peak airway pressure is measured at the airway opening (Pao) and is routinely displayed by mechanical ventilators. It represents the total pressure needed to push a volume of gas into the lung and is composed of pressures resulting from inspiratory flow resistance (resistive pressure), the elastic recoil of the lung and chest wall (elastic pressure), and the alveolar pressure present at the beginning of the breath (positive end-expiratory pressure [PEEP]

Resistive pressure is the product of circuit resistance and airflow. In the mechanically ventilated patient, resistance to airflow occurs in the ventilator circuit, the endotracheal tube, and, most importantly, the patient’s airways. (NOTE: Even when these factors are constant, an increase in airflow increases resistive pressure.)

Components of airway pressure during mechanical ventilation, illustrated by an inspiratory-hold maneuver

PEEP = positive end-expiratory pressure.

Elastic pressure is the product of the elastic recoil of the lungs and chest wall (elastance) and the volume of gas delivered. For a given volume, elastic pressure is increased by increased lung stiffness (as in pulmonary fibrosis) or restricted excursion of the chest wall or diaphragm (eg, in tense ascites or massive obesity). Because elastance is the inverse of compliance, high elastance is the same as low compliance.

End-expiratory pressure in the alveoli is normally the same as atmospheric pressure. However, when the alveoli fail to empty completely because of airway obstruction, airflow limitation, or shortened expiratory time, end-expiratory pressure may be positive relative to the atmosphere. This pressure is called intrinsic PEEP or auto PEEP to differentiate it from externally applied (therapeutic) PEEP, which is created by adjusting the mechanical ventilator or by placing a tight-fitting mask that applies positive pressure throughout the respiratory cycle.

Any elevation in peak airway pressure (eg, > 25 cm H2O) should prompt measurement of the end-inspiratory pressure (plateau pressure) by an end-inspiratory hold maneuver to determine the relative contributions of resistive and elastic pressures. The maneuver keeps the exhalation valve closed for an additional 0.3 to 0.5 second after inspiration, delaying exhalation. During this time, airway pressure falls from its peak value as airflow ceases. The resulting end-inspiratory pressure represents the elastic pressure once PEEP is subtracted (assuming the patient is not making active inspiratory or expiratory muscle contractions at the time of measurement). The difference between peak and plateau pressure is the resistive pressure.

Elevated resistive pressure (eg, > 10 cm H2O) suggests that the endotracheal tube has been kinked or plugged with secretions or that an intraluminal mass or bronchospasm is present.

Increased elastic pressure (eg, > 10 cm H2O) suggests decreased lung compliance due to

Edema, fibrosis, or lobar atelectasis

Large pleural effusions, pneumothorax, or fibrothorax

Extrapulmonary restriction as may result from circumferential burns or other chest wall deformity, ascites, pregnancy, or massive obesity

A tidal volume too large for the amount of lung being ventilated (eg, a normal tidal volume being delivered to a single lung because the endotracheal tube is malpositioned)

Intrinsic PEEP (auto PEEP) can be measured in the passive patient through an end-expiratory hold maneuver. Immediately before a breath, the expiratory port is closed for 2 seconds. Flow ceases, eliminating resistive pressure; the resulting pressure reflects alveolar pressure at the end of expiration (intrinsic PEEP). Although accurate measurement depends on the patient being completely passive on the ventilator, it is unwarranted to use neuromuscular blockade solely for the purpose of measuring intrinsic PEEP. A nonquantitative method of identifying intrinsic PEEP is to inspect the expiratory flow tracing. If expiratory flow continues until the next breath or the patient’s chest fails to come to rest before the next breath, intrinsic PEEP is present. The consequences of elevated intrinsic PEEP include increased inspiratory work of breathing and decreased venous return, which may result in decreased cardiac output and hypotension.

The demonstration of intrinsic PEEP should prompt a search for causes of airflow obstruction (eg, airway secretions, decreased elastic recoil, bronchospasm); however, a high minute ventilation (> 20 L/minute) alone can result in intrinsic PEEP in a patient with no airflow obstruction. If the cause is airflow limitation, intrinsic PEEP can be reduced by shortening inspiratory time (ie, increasing inspiratory flow) or reducing the respiratory rate, thereby allowing a greater fraction of the respiratory cycle to be spent in exhalation.

Means and Modes of Mechanical Ventilation

Mechanical ventilators are

Volume cycled: Delivering a constant volume with each breath (pressures may vary)

Pressure cycled: Delivering constant pressure during each breath (volume delivered may vary)

A combination of volume and pressure cycled

Assist-control (A/C) modes of ventilation are modes that maintain a minimum respiratory rate regardless of whether or not the patient initiates a spontaneous breath. Because pressures and volumes are directly linked by the pressure-volume curve, any given volume will correspond to a specific pressure, and vice versa, regardless of whether the ventilator is pressure cycled or volume cycled.

Adjustable ventilator settings differ with mode but include

Respiratory rate

Tidal volume

Trigger sensitivity

Flow rate

Waveform

Inspiratory/expiratory (I/E) ratio

Volume-cycled ventilation

Volume-cycled ventilation delivers a set tidal volume. This mode includes

Volume-control (V/C)

Synchronized intermittent mandatory ventilation (SIMV)

The resultant airway pressure is not fixed but varies with the resistance and elastance of the respiratory system and with the flow rate selected.

V/C ventilation is the simplest and most effective means of providing full mechanical ventilation. In this mode, each inspiratory effort beyond the set sensitivity threshold triggers delivery of the fixed tidal volume. If the patient does not trigger the ventilator frequently enough, the ventilator initiates a breath, ensuring the desired minimum respiratory rate.

SIMV also delivers breaths at a set rate and volume that is synchronized to the patient’s efforts. In contrast to V/C, patient efforts above the set respiratory rate are unassisted, although the intake valve opens to allow the breath. This mode remains popular, despite not providing full ventilator support as does V/C, not facilitating liberation of the patient from mechanical ventilation, and not improving patient comfort.

Pressure-cycled ventilation

Pressure-cycled ventilation delivers a set inspiratory pressure. This mode includes

Pressure control ventilation (PCV)

Pressure support ventilation (PSV)

Noninvasive modalities applied via a tight-fitting face mask (several types available)

Hence, tidal volume varies depending on the resistance and elastance of the respiratory system. In this mode, changes in respiratory system mechanics can result in unrecognized changes in minute ventilation. Because it limits the distending pressure of the lungs, this mode can theoretically benefit patients with acute respiratory distress syndrome (ARDS); however, no clear clinical advantage over V/C has been shown, and, if the volume delivered by PCV is the same as that delivered by V/C, the distending pressures will be the same.

Pressure control ventilation is a pressure-cycled form of A/C. Each inspiratory effort beyond the set sensitivity threshold delivers full pressure support maintained for a fixed inspiratory time. A minimum respiratory rate is maintained.

In pressure support ventilation, a minimum rate is not set; all breaths are triggered by the patient. The ventilator assists the patient by delivering a pressure that continues at a constant level until the patient's inspiratory flow falls below a preset level determined by an algorithm. Thus, a longer or deeper inspiratory effort by the patient results in a larger tidal volume. This mode is commonly used to liberate patients from mechanical ventilation by letting them assume more of the work of breathing. However, no studies indicate that this approach is more successful than others in discontinuing mechanical ventilation.

Noninvasive positive pressure ventilation (NIPPV)

NIPPV is the delivery of positive pressure ventilation via a tight-fitting mask that covers the nose or both the nose and mouth. Helmets that deliver NIPPV are being studied as an alternative for patients who cannot tolerate the standard tight-fitting face masks. Because of its use in spontaneously breathing patients, it is primarily applied as a form of PSV or to deliver end-expiratory pressure, although volume control can be used.

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