What is the name of the volume of air moved in or out of the lungs during a normal respiratory cycle?

Peak Inflation Pressures and Tidal Volumes

Tidal volumes are not usually measured in the delivery room; therefore, clinicians must use peak pressure and clinical judgment of chest rise as a proxy for volume delivery. Neither of these provide reliable guidance regarding volume delivery.56,68 However, they remain the most practical methods of targeting delivered volume given the equipment currently available. The actual delivered volume depends on multiple factors—the infant's spontaneous breathing effort, lung compliance, mask leak, obstruction at the mouth and nose, and the resuscitation device used. This means that tidal volumes delivered during PPV are variable and usually much higher than those generated during spontaneous breathing.30 Animal studies have shown that high tidal volumes given soon after birth result in reduced oxygenation, increased need for ventilatory support, and instability of cerebral blood flow that increases the risk of brain injury.55 The ideal target volumes for ventilation of term and preterm infants remain uncertain. Avoidance of tidal volumes greater than 8 mL/kg appears reasonable.78 Development and testing of inexpensive, reliable, and accurate techniques to measure and display tidal volume is an important priority in neonatal resuscitation.

In the absence of tidal volume monitoring devices, clinicians rely on set peak pressure and clinical signs to deliver safe and effective ventilation. Typically, pressures in the range of 20-30 cm H2O are suggested as initial settings51,94 and have been shown to produce reasonable tidal volumes. Infants who make no respiratory effort may require higher pressures initially. As the lungs aerate and become more compliant, pressure may need to be reduced.35 A rising heart rate is a good sign that effective ventilation is being delivered.51

Tidal Volume

Tidal volume is a measure of the amount of air a person inhales during a normal breath. Traditional preset tidal volumes higher than 10 ml/kg have been proved to be associated with increased risk of pulmonary barotrauma and should be avoided. High tidal volumes also decrease venous return and reduce cardiac output. For patients with no significant lung disease, such as patients who have experienced drug overdose or trauma, a maximum tidal volume of 10 ml/kg should be used for mechanical ventilation. Recent studies have shown decreased mortality with the use of lower tidal volume in patients with acute lung disease. Therefore, patients with an acute lung disease such as pneumonia, ARDS, fibrotic lung disease, or COPD should be ventilated with tidal volumes of 6–8 ml/kg.

Tidal Volume

There will be an optimal combination of tidal volume, respiratory rate, I:E ratio, and pressure- or volume-control ventilation for each individual patient during OLV. However, to try to assess each of these parameters while still providing anesthesia with the available anesthetic ventilators is not practical, and the clinician must initially rely on a simplified strategy (Table 53.9). The results of alterations in tidal volume are unpredictable. This may be caused partly by the interaction of tidal volume with auto-PEEP. The use of 5 to 6 mL/kg ideal body weight tidal volumes plus 5 cm H2O PEEP initially for most patients (except those with COPD) seems a logical starting point during OLV.170 Tidal volume should be managed so that peak airway pressures do not exceed 35 cm H2O. This will correspond to a plateau airway pressure of approximately 25 cm H2O.171 Peak airway pressures exceeding 40 cm H2O may contribute to hyperinflation injury of the ventilated lung during OLV.172

Turning the patient to the lateral position will increase respiratory dead space and the arterial to end-tidal CO2 tension gradient (Pa-ETCO2). This will usually require a 20% increase in minute ventilation to maintain the same PaCO2. Individual variations in Pa-ETCO2 gradient become much larger, and PETCO2 is less reliable as a monitor of PaCO2 during OLV. This effect is possibly because there are individual differences in the excretion of CO2 between the dependent and nondependent lungs.

A Tidal Volume

Tidal volume is the volume of air delivered to the lungs with each breath by the mechanical ventilator. Historically, initial tidal volumes were set at 10 to 15 mL/kg of actual body weight for patients with neuromuscular diseases. Over the past 2 decades, VILI has been associated with excessive tidal volume leading to alveolar distention.6,7 The mechanism of lung injury includes regional overinflation,8 stress of repeated opening and closing of lung units,9,10 and sheer stress between adjacent structures with differing mechanical properties.11

The low-tidal-volume strategy, which uses 6 mL/kg of predicted body weight, has become the standard of care for patients with ARDS, following the Acute Respiratory Distress Syndrome Network (ARDS Network) publication in 2000.12 The ARDS Network prospectively studied intubated patients with acute lung injury (ALI) or ARDS to determine whether a low-tidal-volume strategy, compared with a traditional-tidal-volume strategy, could improve mortality and decrease the total number of ventilator days. The final analysis showed a 23% reduction in all-cause mortality and a 9% absolute decrease in mortality with the use of a tidal volume of 6 mL/kg of predicted body weight and plateau pressures of 30 cm H2O or less, compared with the usual practice of 12 mL/kg of predicted body weight and plateau pressures of 50 cm H2O or less. Low tidal volume or so-called lung protective ventilation is recommended for all patients with ARDS. In patients without ARDS, a retrospective review demonstrated the relationship between ALI and the use of tidal volumes greater than 10 mL/kg of predicted body weight.13 Considering the current evidence, tidal volumes greater than 10 mL/kg of predicted body weight should not be routinely used in the care of the mechanically ventilated patient.12,13

Exhaled Tidal Volume

Exhaled tidal volume (Vte) is measured by a pneumotachometer in the ventilator circuit during exhalation. In VCV, part of the machine-delivered volume may leak out during inspiration and therefore never reach the patient. Measurement of Vte more accurately describes the Vt that is contributing to the patient's alveolar ventilation. In PCV the Vte depends on the patient's respiratory system compliance and resistance and therefore offers valuable diagnostic clues. A decrease in Vte during PCV is indicative of either decrease in compliance or increase in resistance and is helpful in directing the clinician to appropriate investigation and management. An increase in Vte is indicative of improvement and may require weaning of inflation pressures to adjust the Vte.

Pulmonary Graphic Representation of Tidal Volume

Tidal volume is measured separately for inspiration and expiration. Many modern ventilators have a feature that allows the patient’s weight to be entered through the ventilator interface, and many have a default weight included. This weight is then used to calculate parameters such as tidal volume per kilogram body weight. If the entered weight is inaccurate or only the default weight is being used, the displayed “per-kilogram” values will be inaccurate and misleading. Therefore clinicians should look at the total values for such parameters (e.g., total tidal volume) and also examine the infant for inconsistencies between the values displayed on the pulmonary graphics and the clinical examination. For example, a baby with a poor chest expansion in response to a ventilator inflation but a high tidal volume per kilogram displayed on the ventilator may have an inaccurately low weight entered into the ventilator.

The presence of a leak around the ETT makes the exhaled tidal volume a more accurate reflection of true tidal volume, because the leak is always greater during a mechanical inflation. Normal volumes in healthy spontaneously breathing neonates have been shown to be 5 to 8 mL/kg.13-17 Table 12-1 shows parameters of tidal volumes at the 10th, 50th, and 90th percentiles.

Tidal volumes delivered are dependent on the ventilator settings and the pathophysiology of the lung. The use of 4- to 6-mL/kg tidal volume breaths has been espoused as avoiding volutrauma. Volumes greater than 8.5 mL/kg are considered to cause overdistention. However, if the lung is at the upper end of the PV curve because of excessive PEEP, the 4- to 6-mL/kg tidal volume will excessively distend the lung, resulting in volutrauma. Thus, what is thought to be lung protective will instead be injurious. Conversely if the lung resides at the lower end of the PV curve because of inadequate PEEP a 4- to 6-mL/kg tidal volume will allow portions of the lung to remain atelectatic, causing atelectrauma. The nonhomogeneous lung is more complicated. To recruit atelectatic areas the inflated portion of the lung must become overinflated to a point at which its compliance is less than the atelectatic areas, subsequently allowing for these areas to inflate. Thus a combination of atelectrauma and volutrauma occurs. Maintaining an optimal lung volume by monitoring pulmonary graphics can help avoid these issues.

21 How and where is tidal volume measured? Why are different measures frequently not equal?

Tidal volume is measured using several techniques and at several sites in the breathing circuit. Common measures include the setting on the ventilator control panel, bellows excursion, and flow through the inspiratory or expiratory limbs of the circuit. These measures frequently differ because they may or may not include the contribution of the inspiratory flow, are measured at different pressures, and compensate differently for flow rates. Since each measure can in theory be an accurate measure of a different parameter, it is more important to record a consistent measure of tidal volume than to debate which measure is correct.

Targeted Tidal Volume

Targeted tidal volume (TTV) is a modality on the SLE 4000 and SLE 5000 neonatal ventilators (Specialised Laboratory Equipment Ltd., Croydon, UK). This device is not available in the United States but is widely used outside of North America. The standard TTV is in essence a simple volume limit function. The device increases the rise time of the pressure waveform to improve the chance of effectively limiting VT to the desired target. To avoid the risk of excessive PIP when the TTV function is turned off, the PIP automatically drops to 5 mbar above the PEEP, and the user must then actively adjust the PIP. Reliance on inspiratory VT measurement may lead to inadequate VT delivery with significant leak around the ETT. A recent enhancement referred to as TTVplus makes the modality function more like VG by using exhaled VT measurement and actively modulating inflation pressure to target the desired VT. A leak compensation feature has also been added.

Respiratory Rate, Tidal Volume, and Monitoring

Tidal volume, respiratory rate, and minute volume in the unanesthetized rhesus macaque is similar to that reported for other primate species [48]. There are some differences in pulmonary mechanics between males and females. Mean tidal volume for males is 38.9 mL and mean tidal volume for females is 46 mL. The respiratory rate is similar: 38 breaths per minute for males and 40 for females. Additional values, including minute volumes, are displayed in Table 6.10. Mild differences in pH, pCO2, pO2, and base excess are also reported between males and females and between animals evaluated at different time points [48]. The effects of anesthesia on blood gas values are represented in Table 6.11. Naturally occurring pulmonary disease is uncommon. Systemic inflammatory response syndrome and acute lung injury has been reported in both toxicology studies and viral research studies [31,49].

Table 6.10. Measurement of Lung Mechanics in Adult Rhesus Macaques

Adapted from Binns et al. [48]

Table 6.11. Arterial Blood Gas Values for Rhesus Macaques, by Anesthetic Agent

Adapted from Hom et al. [22]

Intubation is typically done in dorsal recumbency. Infants can be intubated using a small, straight laryngoscope blade (the 00 Miller). A curved Macintosh of appropriate size is generally used for juveniles and adults. Bronchoscopy of the trachea and mainstem bronchi of adults can be achieved using a pediatric scope with 3.8-mm outer diameter. Total and differential cell counts from bronchoalveolar lavage fluid are presented in Table 6.12 [50].

Table 6.12. Total and Differential Cell Analysis of Bronchoalveolar Lavage Fluid from Rhesus Macaques

Adapted from TateRico and Roy [50]

Open Lung Approach

The low tidal volume ventilation strategy endorsed by the NHLBI study results in a modest decrement in gas exchange over the first several days of treatment, as compared with the higher tidal volume ventilation strategy. A contrasting “open lung” strategy is to adjust tidal volume and PEEP based on gas exchange and airway pressure measurements (see Chapter 25). Although use of the open lung strategy is effective in animal models of ALI,92,93 the effect was short-lived in patients with ALI/ARDS who were treated with a low tidal volume strategy and 30-second applications of continuous positive airway pressure of 35 to 40 cm H2O.94,95