4 Ventilation Principles and Support

Ventilation and oxygenation

Ventilation moves air into and out of the lungs and supports carbon-dioxide removal. Oxygenation transfers oxygen into the blood. These processes are related but not interchangeable: oxygen saturation may remain acceptable while ventilation is failing and carbon dioxide is rising.

The principles here focus on adults and are educational. Airway and ventilator care should be performed by trained clinicians under local protocols.

Providing bag-mask ventilation

Bag-mask ventilation (BVM) provides temporary positive-pressure breaths when a patient is apneic or breathing inadequately. Effective ventilation depends on an open airway, a well-fitting mask seal, and breaths delivered slowly enough to allow exhalation.

1. Position the airway. Use a head-tilt/chin-lift when appropriate, or a jaw thrust if cervical-spine injury is suspected. Suction visible secretions and use an appropriate airway adjunct when indicated.

2. Fit the mask. Choose a mask that covers the mouth and nose without covering the eyes. Whenever possible, use two trained rescuers: one uses both hands to lift the jaw and seal the mask; the other squeezes the bag.

3. Deliver each breath smoothly. Give each breath over about $1$ second, using only enough volume to produce visible chest rise. Let the chest fall fully before the next breath. Avoid forceful or rapid squeezing, which can cause gastric inflation, regurgitation, lung injury, or reduced venous return.

4. Add oxygen and reassess. Add supplemental oxygen and monitor chest movement, oxygen saturation, breath sounds, and, when available, a waveform capnograph. A persistent leak, absent chest rise, or absent or abnormal waveform should prompt reassessment of the mask seal, airway position, and airway patency.

During adult cardiac arrest, follow the current resuscitation algorithm and avoid excessive ventilation. With an advanced airway, the AHA recommends one breath every $6$ seconds while compressions continue. For patients with a pulse, set the rate to the clinical situation and reassess the response rather than applying a CPR rate. Pediatric patients require age-specific guidance.

Mechanical ventilation modes and settings

Mechanical ventilation supports breathing through an invasive airway or a noninvasive interface. Common modes differ in what they set and what may vary:

• Volume control: Delivers a set tidal volume; airway pressure varies with lung and chest-wall mechanics.

• Pressure control: Delivers a set inspiratory pressure for a specified time; tidal volume varies with mechanics and patient effort.

• Assist-control: Provides a minimum set rate and delivers supported breaths when the patient triggers additional breaths.

• Pressure support: Assists patient-triggered breaths and generally does not guarantee a minimum rate by itself.

Key settings include tidal volume ($V_T$) or inspiratory pressure, respiratory rate, oxygen concentration ($F_{IO_2}$), positive end-expiratory pressure ($PEEP$), inspiratory flow or time, and trigger sensitivity.

In volume-targeted ventilation, minute ventilation is approximately tidal volume multiplied by respiratory rate:

Here, $\dot V_E$ is minute ventilation, $V_T$ is tidal volume, and $f$ is respiratory rate. Alveolar ventilation is lower because some of each breath remains in anatomical and physiologic dead space.

Lung-protective settings and pressure

Use predicted body weight—not actual weight—to select tidal volume in lung-protective ventilation. For adults with ARDS, guidelines recommend a tidal volume of $4\text{–}8\,\mathrm{mL/kg}$ predicted body weight and plateau pressure below $30\,\mathrm{cm\ H_2O}$. Many protocols begin near $6\,\mathrm{mL/kg}$ and adjust according to pressures and clinical response.

Plateau pressure is measured during an inspiratory pause when the patient is passive. A higher peak pressure with a relatively unchanged plateau pressure often suggests increased airway resistance. Elevations in both peak and plateau pressure may indicate reduced compliance or overdistention.

Adjusting ventilatory support

Make changes in response to the patient, blood gases, trends, waveforms, and the treatment goal—not a single monitor value.

• Carbon dioxide and pH: PaCO₂ is affected mainly by alveolar ventilation. If carbon dioxide is too high, first check for obstruction, disconnection, inadequate delivered volume, or increased dead space. Clinicians may adjust respiratory rate or tidal volume while protecting the lungs. If carbon dioxide is too low, reduce excessive ventilation when appropriate. Reassess blood gases or capnography after changes.

• Oxygenation: FiO₂ and PEEP are the principal adjustable supports. Use the lowest combination of FiO₂ and PEEP that achieves the prescribed oxygenation target, while considering hemodynamics and lung mechanics. Increasing PEEP can improve oxygenation but may also reduce blood pressure or overdistend lung units.

• Obstructive disease: In asthma or COPD, prolonged exhalation is important. A high rate or short expiratory time can cause air trapping and auto-PEEP. Clinicians may allow more time to exhale by lowering the rate or increasing inspiratory flow, while monitoring for worsening acidosis or fatigue.

• Pressure-targeted breaths: Check exhaled tidal volume regularly. Changes in airway resistance or compliance can change the delivered volume even when the pressure setting is unchanged.

Recognizing ventilation problems

Assess the patient first: mental status, chest movement, work of breathing, skin color, oxygen saturation, circulation, and breath sounds. Then inspect the airway, circuit, ventilator settings, alarms, and waveforms. A sudden change can be life-threatening.

A practical rapid check is DOPE: Displacement of the airway; Obstruction, such as secretions, a kink, or bronchospasm; Pneumothorax; and Equipment failure or disconnection. Also consider worsening lung disease, aspiration, pulmonary edema, auto-PEEP, and patient–ventilator dyssynchrony. If the patient is deteriorating, call for help, provide appropriate oxygenation and ventilation, and follow emergency protocols. Do not silence or ignore an alarm without identifying its cause.

High-pressure alarms can result from secretions, biting, a kinked tube, bronchospasm, reduced lung compliance, or pneumothorax. Low-pressure or low-volume alarms commonly indicate a leak, disconnection, cuff problem, or displaced airway.

An unexpectedly rising PaCO₂ or falling pH suggests inadequate effective ventilation. A sudden fall in end-tidal CO₂ can reflect reduced pulmonary blood flow, disconnection, or loss of airway integrity. Trends and context matter: end-tidal CO₂ is not identical to arterial CO₂, and the difference can widen when dead space or ventilation–perfusion mismatch increases.

Principles for safe support

Effective support requires an open airway, a good mask or circuit seal, appropriate breath size and timing, and continuous reassessment. BVM breaths should produce visible chest rise without overventilation. On a ventilator, tidal volume and rate chiefly affect carbon-dioxide removal, while FiO₂ and PEEP chiefly support oxygenation. Use lung-protective settings, allow adequate exhalation in obstructive disease, and respond promptly to patient deterioration, abnormal waveforms, or alarms.