What is ventilation?

What is Ventilation?

Ventilation is the process of moving gas (air) into and out of the lungs to maintain arterial blood gas regulation and acid-base balance, achieved either through spontaneous breathing or mechanical assistance when respiratory function is inadequate. 1, 2

Physiological Mechanism

Normal ventilation involves the coordinated movement of air through the respiratory system to deliver oxygen and remove carbon dioxide from the body. 1

Gas Exchange Process

• Oxygen passes from inspired air in the lungs into the bloodstream and is delivered to tissues throughout the body 1
• Carbon dioxide, a product of metabolism, is transferred from the bloodstream into the alveoli and then exhaled from the lungs 1
• The lung diverts blood flow away from poorly ventilated areas through "hypoxic vasoconstriction," ensuring efficient oxygen replenishment and carbon dioxide clearance 1

Ventilatory Control

• When partial pressure of oxygen (PaO2) falls in the blood, receptors in the carotid body sense this and stimulate increased ventilation 1
• This increases the amount of oxygen entering the lungs and subsequently the blood 1

Quantifying Ventilation

Minute Ventilation

Minute ventilation (V̇E) is the volume of expired air exhaled from the lungs in one minute, calculated as tidal volume × respiratory rate. 3

• Normal minute ventilation at rest in healthy adults is approximately 5-7 liters per minute (70-100 mL/kg/min) 3
• Normal tidal volume is 6-8 mL/kg predicted body weight (approximately 500-600 mL per breath in average adults) 3
• Normal respiratory rate at rest is 10-12 breaths per minute 3

Breathing Pattern During Exercise

During exercise, ventilation increases through both depth (tidal volume) and frequency (respiratory rate) of breathing. 1

• At low exercise levels, increases in tidal volume are primarily responsible for increased ventilation 1
• As exercise progresses, both tidal volume and respiratory frequency increase until 70-80% of peak exercise, after which respiratory frequency predominates 1
• Tidal volume usually plateaus at 50-60% of vital capacity 1

Mechanical Ventilation

Mechanical ventilation is a life-support technique where gas is moved toward and from the lungs through an external device connected directly to the patient, used when spontaneous breathing is inadequate. 1, 2

Types of Mechanical Ventilatory Support

Mechanical ventilation encompasses several modalities depending on the degree of support needed: 1

• Invasive mechanical ventilation: Full respiratory support through an endotracheal tube or tracheostomy 1
• CPAP (Continuous Positive Airway Pressure): Applies mild air pressure continuously to keep airways open in patients who can breathe spontaneously 1
• BiPAP (Bilevel Positive Airway Pressure): Noninvasive therapy with higher pressure during inspiration and lower during expiration 1
• Adaptive servo-ventilation: Positive airway pressure adjusted according to breathing patterns (note: causes harm in NYHA class II-IV heart failure with reduced ejection fraction and central sleep apnea) 1

Physiological Effects

Positive-pressure mechanical ventilation differs considerably from normal physiologic breathing and can lead to several consequences: 4

• Hemodynamic changes affecting cardiovascular performance, cerebral perfusion, and renal drainage 4
• Potential compression stress on the alveolar-capillary membrane causing local and systemic inflammation 4
• Risk of ventilator-induced lung injury (VILI) if parameters are not optimized 4

Critical Ventilation Parameters

During mechanical ventilation, multiple parameters must be adjusted to minimize injury: 4

• Target tidal volumes of 6-7 mL/kg ideal body weight to prevent ventilator-induced lung injury 3
• Driving pressure (difference between inspiratory plateau pressure and PEEP) 4
• Mechanical power (amount of mechanical energy imparted as a function of respiratory rate) 4

Clinical Applications

Resuscitation Settings

During CPR with an advanced airway, provide ventilation at 8-10 breaths/minute (1 breath every 6-8 seconds) without pausing chest compressions. 3

For patients with spontaneous circulation requiring rescue breathing, provide 10-12 breaths/minute (1 breath every 5-6 seconds). 3

Clinical Monitoring

Tachypnea (respiratory rate >29 breaths/minute) is an extremely sensitive marker of clinical deterioration in critically ill patients, though nonspecific. 3

Common Pitfalls

• Do not use actual body weight for tidal volume calculations in obese patients—use ideal body weight to avoid excessive volumes and potential barotrauma 3
• Do not hyperventilate during resuscitation—rates exceeding 12 breaths/minute during CPR decrease cardiac output and worsen outcomes 3
• Recognize that breath-to-breath variability is substantial; base measurements on adequate sampling periods (approximately 250 breaths for reproducible results) 3