Understanding the physiology of the normal lung, the pathophysiology of the diseased lung, and how various forms of respiratory support affect lung physiology and function are key to achieving the goals of neonatal respiratory support—optimizing pulmonary gas exchange while minimizing lung injury. Modern conventional ventilators are always used along with monitors that display real-time breath-to-breath pulmonary function measurements at the bedside and also store the data, enabling a better assessment of the patient’s overall pulmonary mechanics. The accuracy of these measurements has improved dramatically so that one can assess pulmonary function even in extremely low birth-weight patients ( Tables 12-1 and 12-2 ). Unfortunately this rich source of information is often ignored by clinicians when adjusting ventilator settings. This chapter discusses the use and limitations of pulmonary function assessment and specifically graphics in the management of ventilated neonates, so that clinicians may learn to use these bedside displays along with the results of other bedside monitoring devices, laboratory tests, and, most importantly, physical examination findings, in adjusting ventilator settings. Despite the wealth of electronic data now available at the bedside, appropriate “hands-on” evaluation is still the most important aspect of care, allowing clinicians to interpret and use these data wisely in decision making, particularly if the various data are conflicting: “If all else fails, try examining the patient.”
|Weight Range (g) Percentiles||PEAK INSPIRATORY FLOW (L/MIN)||PEAK EXPIRATORY FLOW (L/MIN)||TIDAL VOLUME (ML/KG)||MINUTE VENTILATION (ML/MIN)|
|Birth Weight (g)||Gestational Age (weeks)||Pulmonary Compliance (mL/cm H 2 O/kg)||Pulmonary Resistance (cm H 2 O/L/s)||Likelihood Ratio for BPD||Percentage Predicted Probability|
|500-750||26 ± 0.4||0.3 ± 0.03||102 ± 16||537 ± 171||93% ± 3%|
|751-1000||28 ± 0.3||0.5 ± 0.05||176 ± 24||76 ± 35||73% ± 5%|
|1001-1250||29 ± 0.3||1.0 ± 0.2||96 ± 1.1||5.5 ± 1.8||42% ± 7%|
|1251-1500||31 ± 0.3||1.5 ± 0.2||69 ± 8||0.8 ± 0.3||15% ± 5%|
|1501-2000||32 ± 0.3||1.8 ± 0.3||69 ± 11||0.3 ± 0.1||8% ± 3%|
∗ Predicted probability and likelihood ratio (LR) of BPD evaluated on the previously reported predictive model based on gestational age (GA) and pulmonary mechanics: LR = exp(33.6 − 1.13 GA − 0.93 Cl/kg − 0.001 Rt), where Cl is compliance and Rt is resistance.
Using this multimodal approach, clinicians can determine an individual infant’s pathophysiology, select the appropriate ventilator mode and settings, and assess the infant’s responses to changes in ventilator settings.
Bedside pulmonary function assessment and graphics as of this writing are available only with conventional ventilators employing flow sensors and not with high-frequency ventilation or with less invasive forms of support such as continuous positive airway pressure (CPAP), noninvasive positive pressure ventilation (NIPPV), high-flow nasal ventilation (HFNV), and bilevel positive airway pressure (BIPAP).
There are multiple flow measuring technologies that are effective for use in bedside pulmonary function assessment and graphics in term and preterm neonates.
These are resistive-type devices utilizing either a fine mesh screen or a group of small capillaries (Fleisch type). Gas flowing through a fixed resistance creates a pressure differential, which is measured with a differential pressure transducer. This is linear for a specific range of flow. Conventional pneumotachometers for flow measurement, when appropriately calibrated, have linear input and output characteristics. They generally have a dead space that is excessive for neonates, especially preterm neonates. One can use this device for single measurement pulmonary function tests but not for continuous monitoring.
Alternatives to pneumotachometers include the following: (1) nonlinear flow resistive sensors, which measure a pressure difference produced by gas flowing through a tube but which are not linear; (2) flow sensors that use a piezoelectric film for detecting flow, in which vibration of the film results in an electrical output proportional to the flow; and (3) hot wire anemometers, which measure the electrical current needed to maintain a specific temperature in a heated wire placed across the airflow. The relationship is nonlinear and not inherently direction sensitive (but can be modified to sense direction).
These alternative devices all have the limitations of being nonlinear, nondirectional, or potentially both. Because of these characteristics, they are more difficult to calibrate. These devices have the advantages of being much smaller, having significantly less dead space, and being light enough to be placed in the ventilator circuit near the endotracheal tube (ETT). These features make them suitable for continuous monitoring and their accuracy is sufficient for clinical use. The sensors typically have a heated wire to negate the condensation of water, which will create inaccurate measurements. Volumes should be measured only near or at the ETT, as this avoids inaccurate patient tidal volumes due to circuit tubing distention.
Although these sensors are typically used in intubated patients, they can be used with a mask as well. The mask must fit snugly without a leak. Whereas most newborns breathe primarily through their nose, some mouth breathing does take place. Because of this, the use of tight nasal prongs or a nasal mask may result in inaccurate measurements.
The requirement to maintain accuracy of measurements entails regular assessment of the flow sensor. Cleaning and calibration must be regularly accomplished, and one must recognize that temperature, humidity, and gas consumption may significantly affect the accuracy of the flow sensor.
The sensors should be adequately calibrated and recalibrated periodically. Calibration should be performed under both static and dynamic conditions. Typically this is accomplished by using the following appropriate devices: calibrated syringes, precise ball-in-tube flowmeters, water column manometers, and reference transducers. To maintain accuracy of measurements, the devices must be operated in their linear calibration range.
Respiratory Physiology and Pathophysiology
A detailed description of respiratory physiology is provided in Chapter 2 . Here, a brief recap of aspects relevant to bedside pulmonary monitoring is provided.
Figure 12-1 demonstrates typical spirometry findings. It shows the various measurements of tidal volume, inspiratory and expiratory reserve, functional residual capacity (FRC), residual volume (RV), vital capacity, and total lung capacity.
Normal spontaneous respiration takes place by fixing the diaphragm and expanding the thorax with the respiratory muscles. This creates flow into the lung by decreasing the intrapulmonary pressure compared to the pressure at the mouth. Expiration occurs passively, driven by the elastic recoil of the lungs, which increases intrathoracic pressures to a level greater than the pressure at the mouth ( Fig. 12-2 ). The inspiratory driving pressure, either by the ventilator or by the respiratory muscles, must be great enough to overcome the elastic, resistive, and inertial properties of the respiratory system.
Rohrer described this relationship with the equation P = P e + P r + P i , where P e is the elastic, P r the resistive, and P i the inertial pressure. In this relationship, the elastic pressure is assumed to be proportional to volume change by a constant ( E ) representing the elastance (or elastic resistance) of the system. The resistive pressure component is assumed proportional to airflow <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='(V˙)’>(V˙)(V˙)
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by a constant ( R ) representing inelastic airway and tissue resistance. The inertial component of pressure is assumed to be proportional to gas and tissue acceleration ( V ) by an inertial constant ( I ) and is usually negligible during conventional ventilation; thus P = EV + R <SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='(V˙)’>(V˙)(V˙)
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. This model is based on a single component and assumes linearity between pressure and volume and pressure and flow with the coefficients E , R , and I remaining constant during the ventilatory cycle. However, sick lungs, especially those mechanically ventilated, do not fit this model.
Measurements Displayed onPulmonary Graphics
The appropriate pressure needed is that pressure necessary to overcome the elastic, resistive, and inertial properties of the respiratory system and delivering an adequate volume to the exchange areas of the lung. Figure 12-2 shows the pressure generated with spontaneous breathing. For mechanical breaths the driving pressure is the difference between the peak inspiratory pressure (PIP) and the positive end-expiratory pressure (PEEP)—that is, Δ P = PIP − PEEP).
The mean airway pressure is a function of inspiratory time, flow, PIP, PEEP, and respiratory rate. In general, mean airway pressure reflects oxygenation and driving pressure reflects ventilation. However, as noted they do have an interrelationship, especially in nonhomogeneous lung disease.
Instrumentation for Pressure Measurement
Pressure is measured by pulmonary graphics devices at the bedside at the attachment of the circuit to the ETT or CPAP device. This measures PIP and PEEP/CPAP. This provides adequate monitoring for the pressure–volume (PV) loops displayed in the ventilator graphics. True pulmonary function testing, however, requires more sophisticated pressure measurements. This would be the differential pressure between the ETT and the pleural space. The pleural space pressure is typically estimated using an esophageal catheter. Such a catheter can be used if a chest tube is in place, is patent, and has no ongoing air leak. Esophageal pressure measurement may be used for pleural pressure in the larger preterm and term infant. In the extremely and very low birth-weight preterm infant, the esophageal pressure has been shown to correlate poorly with pleural pressure. Using this technique to determine pulmonary function tests for these infants would yield suspect results.
Volume is the area under the curve of the flow signal. Inspiratory and expiratory volumes will differ somewhat because of the change in temperature, water vapor, viscosity, and gas consumption (O 2 + CO 2 ). However, a difference of more than 10% is likely to be due to a faulty flow sensor or a leak, either around the ETT or out of a chest tube, and should be investigated.
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. 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.
Flow Measurement (Inspiratory and Expiratory Airflow)
Figure 12-2 demonstrates visually how airflow occurs during inspiration and expiration. Airflow is measured at its peak because of its dependence on airway resistance. Peak flow ranges for various weight infants are shown in Table 12-1 . Ventilator circuit airflow is different from the flow that traverses the ETT. Airflow during normal respiration is determined by the tidal volumes and peak inspiratory and expiratory flow. The expiratory flow pattern will demonstrate the natural expiratory airflow limitations in normal newborns, but this may be exaggerated by airway disease ( Figs. 12-3 and 12-4 ). Typically inspiratory airflow peaks at midinspiration and peak expiratory flow values precede midexpiration ( Fig. 12-5 ).