Fundamentals of mechanical ventilation

Key points

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New technology as illustrated in the next chapter provides several modes by which a patient may be ventilated based on the fundamentals of mechanical ventilation (MV), with the goal of improved gas exchange, better patient comfort, and rapid liberation from the ventilator.
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Noninvasive ventilation (NIV) may avoid intubation or reintubation for awake, cooperative patients with marginal oxygenation or ventilation. With COVID-19 therapy, a shift to high-flow oxygen therapy to attempt intubation has been utilized.
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Basic modes of assisted ventilation include assist-control ventilation (ACV), intermittent mandatory ventilation (IMV), and pressure support ventilation (PSV).
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Ventilation of patients with acute lung injury (ALI) end stage lung disease and acute respiratory distress syndrome (ARDS) using a low tidal volume (6 mL/kg) decreases morbidity and mortality from mechanical ventilation (MV).
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Sedation should be used judiciously and withdrawn transiently each day during a “sedation holiday” to assess readiness for liberation from MV.
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Adherence to a “ventilator bundle” (e.g., prophylaxis for stress-related gastric mucosal hemorrhage and venous thromboembolic disease, head-of-bed up 30 degrees at all times unless contraindicated medically, daily sedation holiday, oral care with topical chlorhexidine solution) decreases the length of MV and the incidence of ventilator-associated pneumonia (VAP).

MV is often required to manage emergency conditions or critical illness, whether for airway protection, administration of general anesthesia, or management of acute respiratory failure (ARF) ( Table 1 ). New technology incorporating established fundamentals provides several modes by which a patient may be ventilated, with the goal of improved gas exchange, better patient comfort, and rapid liberation from the ventilator. Moreover, noninvasive positive-pressure ventilation permits some cases of ARF to be managed without insertion of an artificial endotracheal airway and some patients who are extubated with marginal reserves to avoid reintubation. All of these patients should be closely monitored with continuous pulse oximetry and as indicated arterial blood gases (ABG). Nearly all ventilators can be set to allow full support of the patient on the one hand and periods of either total or partial liberation from mechanical ventilation on the other. Thus, the choice of ventilator settings for the majority of patients is a matter of physician preference ( Table 2 ). Controlled ventilation with suppression of spontaneous breathing leads rapidly to respiratory muscle deconditioning and atrophy, and therefore, modes of assisted ventilation are preferred wherein machine-delivered breaths are triggered and sustained by the patient’s own inspiratory efforts. Basic modes of assisted ventilation include ACV, synchronized intermittent mandatory ventilation (SIMV), and PSV.

TABLE 1

General Indications for Mechanical Ventilation

Airway maintenance or protection

Airway obstruction

General inhalational anesthesia

Hemodynamic instability

Hypoxemia

Metabolic acidosis

Pulmonary respiratory physiotherapy (excessive secretions)

TABLE 2

A Glossary of Basic Terminology of Mechanical Ventilation

Control: regulation of gas flow:

Volume-controlled— Airway pressure is variable (volume limited, volume targeted)
Pressure-controlled— Volume delivered is variable (pressure limited, pressure targeted)
Dual-controlled— Seldom used in practice (volume targeted [guaranteed], pressure limited)

Cycling: ventilator switching:

Time-cycled— Example: pressure-controlled ventilation
Flow-cycled— Example: pressure support ventilation
Volume-cycled— Example: volume-controlled ventilation

Triggering: causes the ventilator to cycle to inhalation:

Time-triggered— The ventilator cycles at a set frequency as determined by the controlled rate.
Pressure-triggered— The ventilator senses the patient’s inspiratory effort from a decrease in airway pressure.
Flow-triggered— A constant flow of gas is delivered throughout the respiratory cycle (flow-by). Altered flow caused by patient inhalation is detected by the ventilator, which delivers a breath. Less work is done by the patient than with pressure-triggering.

Breaths: cause the ventilator to cycle from inhalation to exhalation:

Mandatory (controlled)—Determined by the respiratory rate
Assisted— Examples: assist control, synchronized intermittent mandatory ventilation, pressure support
Spontaneous— No additional assistance, as in continuous positive airway pressure (CPAP)

Flow pattern: constant, decelerating, or sinusoidal:

Sinusoidal— Examples: spontaneous breathing and CPAP (PEEP-PS)
Constant— Flow continues at a constant rate until the set tidal volume is delivered; seldom used in practice
Decelerating— Inhalation slows from a high initial flow rate as alveolar pressure increases; example: pressure-targeted ventilation, often used in volume-targeted ventilation, as it causes lower peak airway pressures than constant flow

Mode (breath pattern):

Controlled medical ventilation— Controlled ventilation, without allowances for spontaneous breathing, typical of anesthesia ventilators with heavy sedation and possible neuromuscular blockade
Assist-control— Assisted breaths simulate controlled breaths
Intermittent mandatory ventilation— Admixes controlled and spontaneous breaths, which may also be synchronized to prevent “stacking”
Pressure support— The patient controls all aspects of the breath except the pressure limit

Most patients are started on MV for management of ARF, during which the work necessary to initiate a breath is increased by a factor of 4 to 6. The most common reason to initiate MV is to decrease the work of breathing by the patient. Additional potential benefits of MV include improved gas exchange, enhanced coordination between support and the patients’ own efforts, resting of respiratory muscles, prevention of deconditioning, and prevention of iatrogenic lung injury while promoting healing. However, unless settings are chosen carefully to synchronize with the patients’ own central respiratory drive, MV can cause an increase in work. Regardless of the mode chosen, all MV is a modification of the manner in which positive pressure is applied to the airway and the interplay of the mechanical support and the patients’ own efforts.

Noninvasive ventilation

Ventilatory support delivered without establishing an endotracheal airway is NIV. NIV was administered previously with intermittent negative pressure such as the ‘Iron Lung’ used during the polio epidemic; the current technique uses positive-pressure ventilation delivered through a nasal or face mask, and usage is expanding in the management of acute and chronic respiratory failure, for some patients with heart failure, and for respiratory distress related to COVID-19 interstitial lung disease.

Putative benefits of NIV are numerous, owing to avoidance of the complications of endotracheal intubation. NIV preserves swallowing, feeding, speech, cough, and physiologic air warming and humidification by the naso-oropharynx. Nonintubated patients generally communicate more effectively, require less sedation, and are more comfortable. In addition, patients are often able to continue with standard enteral nutrition. NIV eliminates complications such as the trauma with tube insertion, mucosal ulceration, aspiration, infection (e.g., pneumonia, sinusitis), and dysphagia and hoarseness after extubation.

In a randomized, prospective trial following pulmonary resection of 48 patients with acute hypoxemic respiratory insufficiency, Auriant et al compared standard invasive MV with nasal mask NIV. The need for postoperative reintubation and the mortality rate were clearly reduced in patients receiving NIV as a part of their respiratory support. Similarly, Squadrone et al randomized 209 patients with respiratory failure in the postanesthesia care unit after major abdominal surgery to oxygen alone, or with continuous positive airway pressure (CPAP) by mask. Patients who received oxygen via CPAP had a significantly lower intubation rate and also lower rates of pneumonia, infection, and sepsis.

Contraindications to noninvasive ventilation

Crucial to successful NIV is an awake, cooperative patient who is breathing spontaneously. Airway, electrocardiographic, or hemodynamic instability argues against the use of NIV. An additional requirement is an intact cough reflex and ability to clear secretions, the absence of which is a common reason for failure of NIV. Relative contraindications include the inability to fit and seal the mask adequately, inability to cough with prompting, or inability to remove the mask in the event of emesis. A hypothetical contraindication is recent gastrointestinal surgery with aerophagia and bowel and abdominal distention. If pressures used to ventilate the patient are kept below 30 mm Hg, the closing pressure of the lower esophageal sphincter should not be exceeded, and aerophagia should be avoided. Morbid obesity is also a relative contraindication, secondary to increased ventilatory pressure requirements arising from body habitus, upward displacement of the diaphragms and the weight of the chest wall and abdominal viscera while the patient is supine.

Complications of noninvasive ventilation

The most common complication of NIV is focal skin necrosis, which is most common over the bridge of the nose but may also occur over the zygoma. The incidence is 7% to 10% among patients receiving full-face mask NIV. Other complications (incidence, 1%–2% each) include gastric distention, aspiration, and pneumothorax. Conceptual concerns with gastric distention are subsequent vomiting, aspiration, and pneumonia. Conjunctivitis may develop secondary to air leaking near the eyes in about 2% of patients.

The most serious complication is failure to recognize when NIV is not providing a patient with adequate ventilation, oxygenation, or airway patency. Delayed placement of an artificial airway, or inability to provide an airway, may cause continued deterioration or the death of a patient. When not able to be placed nasally or orally, an emergency cricothyroidotomy with a needle for access or an incision or a tracheostomy may be lifesaving,

Pressure support ventilation

Pressure support is a method of assisting spontaneous breathing in a ventilated patient, either partially or fully. The patient controls all parts of the breath except the inspiratory pressure limit. The patient triggers the ventilator, which delivers a flow of gas in response up to a preset pressure limit (for example, 10 cm H ₂ O) depending on the desired minute ventilation. Gas flow cycles off when a certain percentage of peak inspiratory flow (usually 25%) has been reached. Tidal volumes (V t ) may vary, just as they do with spontaneous breathing.

Positive end-expiratory pressure (PEEP) is added to restore functional residual capacity (FRC) to normal for the patient. When lung volumes are low, the work of breathing during early inhalation is reduced. Noncompliant lungs require higher airway pressures to inflate to a normal V t , even with pressure support. The addition of pressure support assists the patient to move upward the pressure-volume curve (larger changes in volume for a given applied pressure, i.e., increased lung compliance). The term “pressure support ventilation” describes the combination of pressure support and PEEP, and many have interpreted this to be CPAP, which technically applies to only to inspiration, not including PEEP, which occurs at the end of exhalation and provides an increased baseline of positive pressure from which inspiration is initiated. Although useful in the patient breathing spontaneously, pressure support may be used to assist spontaneous breaths in SIMV. Weaning may be facilitated using this combination, as the backup (SIMV) rate is weaned initially, and then the pressure support generally to a baseline of 10 mm of Hg.

Heliox

Helium has a lower density than air or nitrogen. Substituting helium for nitrogen reduces the density of the gas in direct proportion to the amount of helium admixed. Breathing heliox (the concentration in clinical use ranges from 80:20 to 60:40 ratio of helium to oxygen) results in more laminar flow, reduced airway resistance, and reduced work of breathing. Heliox may be useful in clinical situations when resistance to airflow is high, including asthma, acute exacerbations of chronic bronchitis or chronic obstructive pulmonary disease (COPD), other causes of bronchospasm, and upper airway obstruction with stridor. Breathing heliox is well tolerated, without appreciable adverse effects. Disadvantages include high cost and limited use when a high fraction of inspired oxygen (Fio ₂ ) is required. Ventilators must also be recalibrated when heliox is delivered by ventilator rather than nebulizer (the usual route), to ensure that the flow of gas is measured correctly.

Modes of mechanical ventilation

Assist-control ventilation

The ACV mode is the most commonly used mode in medical/surgical critical care units. Set parameters in ACV mode are inspiratory flow rate, frequency ( f ), and V t . The ventilator delivers a set number of equal breaths per minute each of a given V t . Tidal volume and flow determine inspiratory (I) and expiratory (E) time and the I:E ratio. Plateau or alveolar pressure is related to V t and respiratory system compliance. The patient has the ability to trigger extra breaths by exerting an inspiratory effort exceeding a preset trigger level. Typically, each patient will display a preferred rate for a given V t and will trigger all breaths when f is set a few breaths per minute below the patient’s rate. In this mode, the control rate serves as adequate support should the patient stop initiating breaths. When high inspiratory effort continues during a ventilator-delivered breath, the patient may trigger a second superimposed breath. Patient effort can be increased, if desired, by increasing triggering threshold or lowering V t .

Synchronized intermittent mandatory ventilation

In a passive patient, SIMV cannot be distinguished from ACV. Ventilation is determined by f and V t . However, if the patient is not truly passive, respiratory work may be performed during mandatory breaths. In addition, the patient may trigger additional breaths by spontaneous effort. If the triggering effort comes in a brief, defined interval before the next mandatory breath, the ventilator will deliver the mandatory breath ahead of schedule to synchronize with patient inspiratory effort. If a breath is initiated outside the synchronization window, V t , flow, and I:E ratio are determined by patient effort and respiratory system mechanics, not by ventilator settings. These spontaneous breaths tend to be of low V t and are variable from breath to breath. The SIMV mode is often used to augment patient work of breathing gradually by lowering the mandatory breath frequency or V t , compelling the patient to breathe more rapidly in order to maintain adequate minute volume of ventilation (MVV). Some ventilators allow combinations of modes. A useful combination is SIMV plus PSV and PEEP as a means to add adequate volumes of “sigh” breaths and decrease atelectasis. Because SIMV plus PSV guarantees some backup for adequate MVV that PSV alone does not, this combination may be particularly useful for patients at high risk for a deteriorating central respiratory drive, and it is also popular as an adjunct to weaning from the ventilator.

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