• The principal function of respiration is to exchange oxygen and carbon dioxide with circulating blood at the level of the alveoli (takes place during inspiration and expiration).
• Tidal volume (TV) refers to the amount of air that is involved in one half of respiration, either the volume of expiration or the volume of inspiration. In addition to being used to calculate minute ventilation (respiratory rate times TV), it serves as a major component of several lung capacities:
  • Total lung capacity (TLC)
  • Inspiratory capacity (IC)
  • forced vital capacity (FVC)
• The concept of the TV is utilized when discussing:
  • Spontaneous ventilation: An average measure of inspiration or expiration volumes that occurs for an individual, typically during normal breathing. Direct measurement of the TV occurs using a spirometer or through a variety of lesser-utilized dilutional techniques.
  • Mechanical ventilation: The amount of air delivered or returned into the breathing circuit. Volume modes allow for the TV to be set; they are often pressure limited to avoid barotrauma. Conversely, pressure modes have variable tidal volumes and depend on the lung's compliance.

• Normal values
  • Spontaneous ventilation: ~6–7 cc/kg; for an average 70 kg adult, this equates to roughly 500 cc of volume.
  • Mechanical ventilation: ~7–15 cc/kg are programmed for delivery secondary to compliance of the circuit, chest wall resistance, and increased apparatus dead space.
• Components of TV. Not all of the TV is involved in air exchange. Dead space describes the portion of the volume not involved in actual gas exchange.
  • Anatomic dead space: Involves passageways such as the trachea and upper bronchioles. Typically around 30%.
  • Alveolar dead space: Alveoli that are not being perfused (contributes a negligible amount in healthy patients).
  • Physiologic dead space: The combined volume of the anatomic and pathologic dead space. This is roughly equivalent to 2 cc/kg or 150 cc in an adult.
  • Apparatus, mechanical, or equipment dead space describes the additional TV that does not participate in air exchange (endotracheal tube, laryngeal mask airway, corrugated extendors, humidifiers, etc.). Clinically, this is often minor in adults, but can become significant in pediatrics, particularly neonates.
• Movement of air
  • Spontaneous ventilation at rest. Inspiration, at rest, results primarily from contraction of the diaphragm and the external intercostal muscles. This causes an increase in the thoracic cavity volume and the development of negative pressure compared to the atmosphere. The pressure difference that is created allows the flow of air into the pulmonary system. Expiration, at rest, is normally passive. As the muscles of inspiration relax, the thoracic cavity decreases in volume and the elastic recoil of the lungs act in concert to create a positive pressure compared with the atmosphere; air flows out of the pulmonary system. Not all of the air is expelled from the lungs; the volume left behind is referred to as the functional residual capacity (comprises the expiratory reserve volume and residual volume).
  • Spontaneous ventilation during disease states or distress. Adequate oxygenation requires the use of accessory muscles. Inspiration can involve the scalene, sternocleidomastoid, or pectoralis muscles. Expiration becomes an active process and involves the abdominal muscles or internal intercostals.
  • Mechanical ventilation. Inspiration is dependent on the generation of positive pressure. Two general modes for controlled ventilation are commonly used: Volume control and pressure control. Volume control will attempt to deliver a preset volume of gas to the patient using a variable, but limited, amount of pressure. Pressure-dependant ventilation utilizes a set pressure and, therefore, results in variable TVs to the patient. Volumes and pressures then can be adjusted based on clinical and laboratory values to optimize ventilation. Expiration remains a passive process; occurs after the removal of positive pressure ventilation due to chest wall recoil and peritoneal cavity contents.
• Measurements of TV
  • Spirometry: A device that measures the volume of air for both inspiration and expiration breathed by a patient. The rate of volume movement is also recorded. Overall, it provides a means to identify possible pulmonary pathology (e.g., COPD, pulmonary fibrosis).
  • Ventilator. TV can be measured directly using a respirometer, usually found in the expiratory limb of the ventilator circuit. A unidirectional microvane or a bidirectional volumeter is physically spun by the gas flow as it leaves the patient; the number of rotations correlate (are calibrated) to a specified TV. A second indirect measure of TV is a gas flow rate meter found in newer equipment. Ultrasound techniques measure the volume of air moving past the transducers and the TV is calculated based on this relationship.
• Position
  • Spontaneous ventilation. In the supine position, the relaxed diaphragm is located higher in the thoracic cavity compared to when the patient is in a standing or upright position. Because of this height difference, the diaphragm will have more space to contract and results in a larger volume being moved for a given amount of energy (work).
  • Mechanical ventilation. Often performed in the supine position. Neuromuscular blockade and/or general anesthesia result in the diaphragm rising high into the thoracic cavity from abdominal contents pushing against a "weakened" diaphragm. This will result in decreased lung volumes, and therefore, larger pressures are necessary to expand the alveoli when ventilating a patient.
  • Prone and, to a lesser degree, lateral positioning can improve respiratory mechanics secondary to the effects of gravity (shifting of abdominal contents and improved diaphragm excursion). Similarly, a patient in reverse Trendelenburg has improved respiratory mechanics compared to the supine and Trendelenburg position.

• Impaired oxygenation or ventilation in the spontaneously ventilating patient often results in the recruitment of accessory muscles during inspiration and expiration to increase the TV and compensate for increases in dead space or shunting. However, not all pathological processes are limited to the pulmonary system, and may include cardiac and metabolic disease.
• Dead space increases in either mechanical ventilation or spontaneous breathing in the following situations:
  • Rapid, shallow breathing as may be seen from abdominal or thoracic incisional pain. The smaller TVs result in an increased fraction of dead space (can exceed 50%).
  • Pulmonary embolism prevents perfusion to alveolar units being ventilated, thus increasing pathological dead space. The location of the embolism will determine the extent of the increase in dead space.
  • Cardiac arrest/severe hypotension results in alveolar units that continue to be ventilated but now have decreased or absent perfusion.
• Obesity. Pulmonary compliance may decrease by 35% in morbidly obese patients secondary to reductions in both pulmonary and chest wall compliance. This results in an increased respiratory rate, but smaller TVs. This breathing pattern creates a preferential ventilation of poorly perfused lung tissue and leads to hypoxemia and/or hypercapnia particularly while under anesthesia (1) unless TVs are augmented.
• Geriatrics. Lung volumes decrease; however, the volume solely attributable to age is difficult to predict because smoking history, occupational illnesses, environmental conditions, and exposure to lung pollutants also factor into the decrease. The loss of height associated with spinal osteoporosis can affect thoracic volumes (2). In addition, the internal structure of the lungs is affected; TV begins to decrease in the mid-40's secondary to an increasing closing volume. The encroachment of closing volume on TV increases with age such that by age 65 they are equal in the sitting position. In general, respiratory rates remain unchanged and there is a diminished response to correct hypercapnia or hypoxia.

Pregnancy Considerations

Pediatric Considerations

• Obesity. Initial interventions include increasing TV and/or adding PEEP. However, the dangers of a greatly increased TV include hyperventilation, hypocapnia, high airway pressures, and increased risk of parenchyma damage due to the combination of volume and pressure. Frequently, while normalizing oxygen levels, hypocapnia results. The respiratory alkalosis can cause a shift in the oxyhemoglobin curve that increases affinity of the heme molecule for oxygen and results in worsening of tissue oxygenation (5). The strategy for maintaining oxygenation and normocapnia for obese patients is complex, particularly during laparoscopic surgery. Despite multiple studies, no single method has been demonstrated to improve ventilation.
• Acute lung injury. Recent studies have indicated that patients who have acute lung injury such as ARDS may benefit from low TVs (6 cc/kg) when utilizing mechanical ventilation. These lower volumes have been associated with lower serum concentrations of inflammatory mediators. Overall, this is correlated with a decrease in mortality in patients who have ARDS and other acute lung injury (6).
• The geriatric patient is at increased risk of hypoxia, particularly when sedated and the use of supplementary oxygen is warranted even for simple procedures (7).

• IC = TV + IRV, where IC is inspiratory capacity, TV is tidal volume, and IRV is inspiratory reserve volume.
• FVC = IRV + TV + ERV, where FVC is forced vital capacity and ERV is expiratory reserve volume.
• TLC = IRV + TV + ERV + RV, where TLC is total lung capacity and RV is residual volume.

• Functional Residual Capacity
• Dead Space Ventilation

Quinn L. Johnson , MD