Chapter Outline
PHYSIOLOGY AND MECHANICAL CONCEPTS
LUNG FUNCTION DURING ANESTHESIA AND MECHANICAL VENTILATION
CLASSIFICATION, SPECIAL FEATURES, AND MODES OF VENTILATION
CAPABILITIES AND LIMITATIONS OF ANESTHESIA VENTILATORS
Anesthesia Versus Critical Care Ventilators
System Compliance and Compression of Gas
Atmospheric Pressure Variations
Spontaneous Breathing Without Assistance or Support
CURRENT DESIGNS OF ANESTHESIA VENTILATORS
VENTILATOR CONCERNS WITH UTILIZATION
Overview
Since the 1960s, the use of intermittent positive-pressure ventilation (IPPV) has become widespread. Today’s observer might wrongly conclude that the research, development, and methods for mechanical ventilation in the operating room (OR) had occurred only recently. However, much of the necessary experimentation and design took place much earlier.
Anesthesia ventilators are commonly compared with mechanical ventilators used in the intensive care unit (ICU); however, anesthesia ventilators are unique; they not only deliver oxygen and remove carbon dioxide, they also facilitate the delivery of inhalational agents used to render patients unconscious and maintain surgical anesthesia. Modern delivery systems typically are semiclosed systems, which require removal of carbon dioxide and conservation of potent inhalational agents. Low gas flows are commonly used to aid in agent sparing and reduce patient exposure to unheated gases. In contrast, the ICU ventilator typically has an open system because no gases are recirculated through the system; as such, a carbon dioxide absorber is not used. High gas flows can be used because elaborate gas-warming and humidification techniques are available and have proven cost effective in the ICU.
In the past, it was not uncommon to bring an ICU ventilator into the OR for oxygenation and ventilation of patients with extremes of pulmonary pathophysiology because these machines had more options. Recent advances in ventilator technology have made the differences between ICU ventilators and anesthesia ventilators negligible. Outcome data continue to be lacking in the scientific literature regarding differences in modes used in the OR: synchronized intermittent mechanical ventilation (SIMV) or pressure-support ventilation (PSV). However, this has not stopped manufacturers from providing these modes of ventilation, nor has it stopped clinicians from using them while delivering anesthetic during surgery.
In this chapter, the sections on pulmonary mechanics, physiology, and basic principles of mechanical ventilation delineate the technology and contemporary strategies for lung ventilation. Major features of commonly used anesthesia ventilators in the United States are described.
Of note, no chapter can substitute for the detailed information provided by each ventilator manufacturer, which is found in the educational materials and the operator’s and service manuals for each piece of equipment. The reader should refer to these documents for the most detailed pneumatic and electrical schematics.
History
Early recorded attempts to artificially ventilate the lungs of a person date from the 1400s. Baker found records of mouth-to-mouth resuscitation of a newborn in 1472 and of an asphyxiated miner in 1744. Paracelsus is credited with the first use of a bellows in 1530 to artificially inflate the lungs. Open-chest ventilation of a dog via an endotracheal reed was described by Andreas Vesalius in 1555, using mouth-to-tube pressurization, but it was later replaced with bellows ventilation by Robert Hooke in 1667. By the late 1700s, Denmark had initiated a formal campaign and monetary reward for those using a bellows to resuscitate victims of near-drowning. The metal endoral tube, with a conical adapter for the glottic opening, was introduced in 1887 by O’Dwyer for the treatment of patients with diphtheria. This tube was combined with George Fell’s manual ventilating bellows and valve device and was used to treat opium overdose in 1891. The resulting Fell-O’Dwyer apparatus ( Fig. 6-1 ) was simplified by removing the valve and placing a hole in the circuit; the hole could be occluded by the thumb during inspiration, thus providing positive-pressure ventilation and passive exhalation. In France in 1896, Tuffier and Hallion were able to partially resect the lung of a patient whose trachea they had intubated blindly with a cuffed tracheal tube and whose lungs they ventilated during the surgical procedure. Finally, in 1898, the first rudimentary anesthesia machine was developed by Rudolph Matas of New Orleans, who added an anesthetic vapor delivery system to the Fell-O’Dwyer apparatus, thus allowing the resection of a chest wall lesion under positive-pressure ventilation with anesthesia.
Attempting to circumvent the difficulties of tracheal intubation, in 1904, Sauerbruch developed a negative-pressure operating chamber that required the patient’s head to be sealed outside the chamber. Further development resulted in the electrically powered “iron lung” by Drinker and Shaw in 1928, which was widely used to treat patients with respiratory failure during the polio epidemics. In an alternative approach, in 1905, Brauer provided positive-pressure ventilation via the head, sealed within a chamber and thus eliminating the need for intubation or operating within a chamber ( Fig. 6-2 ).
Modern techniques of endotracheal ventilation during general anesthesia were initiated by Magill in 1928 for head and neck surgery. The beginnings of modern mechanical ventilation are attributed to Engström and his ventilator during the polio epidemics in Denmark circa 1952. This ventilator was later modified for use during general anesthesia, which stimulated the development of a huge number of anesthesia ventilators with a wide diversity of characteristic behaviors, mechanisms, and power sources. Gradually, designs were modified and eventually replaced by pneumatically controlled and fluidically time-cycled systems that were optional accessories for the anesthesia machine. These stand-alone ventilators substituted for the reservoir bag at the connection to the breathing circuit and took on the appearance of modern “bag in a bottle” double-circuit systems.
Contemporary anesthesia machines have replaced these freestanding ventilators by integrating the fresh gas delivery system, scavenging system, and ventilator into one unit. Modern ventilators have electronically controlled circuits and, in some cases, closed feedback loops with microprocessor-regulated flow control valves. Modern ventilators allow digital and graphic displays to aid in ventilator management.
Physiology and Mechanical Concepts
Gas Exchange
The two major functions of the lung are taken into consideration during mechanical ventilation: ventilation, the elimination of carbon dioxide (CO 2 ), and oxygenation, the intake of oxygen (O 2 ). A clear distinction should be made between the elimination of carbon dioxide and the intake of oxygen, even though these two processes are mechanically coupled during natural, spontaneous breathing and are interrelated at the metabolic level. Each is capable of stimulating ventilation.
Carbon dioxide elimination depends on ventilation, which means the lungs are inflated with non–CO 2 -containing gases. The carbon dioxide gas of metabolism enters the alveoli of the lungs, and the CO 2 -containing gas is expelled from the lungs on exhalation. As such, the achieved ventilation determines the partial pressure of CO 2 in the arterialized blood (PaCO 2 ).
Oxygenation is best represented by the partial pressure of oxygen in the arterialized blood (P A O 2 ). Predictable improvements in oxygenation can be facilitated by enriching the inspired gas as dictated by the alveolar gas equation:
PaO 2 = Fi O 2 ( P B − P H 2 O ) − ( Pa CO 2 / R )
Carbon Dioxide Equilibrium
The quantity of carbon dioxide produced normally dictates the minute ventilation. With the exception of using cardiopulmonary bypass or an extracorporeal membrane oxygenator (ECMO), no alternative method has proved satisfactory for eliminating carbon dioxide. Breathing is essential. Normally, in the absence of disease, high altitude, and pharmacologic intervention, spontaneous ventilation results in a PaCO 2 of approximately 40 mm Hg. However, the quantitative relationship between CO 2 production and minute ventilation often is poorly understood.
Carbon Dioxide Production
A resting adult weighing 70 kg produces approximately 0.008 gram molecules (moles) of carbon dioxide per minute. At standard temperature (0° C) and pressure (760 mm Hg), one mole of any gas occupies 22.4 L. Therefore, 0.008 moles of carbon dioxide occupy approximately 180 mL. At body temperature (37° C), this is approximately 200 mL.
Carbon Dioxide Elimination
Expressing carbon dioxide production either in moles or in milliliters at atmospheric pressure provides no insight into the volume of ventilation required to maintain homeostasis. More helpful information is provided when the same quantity of carbon dioxide is expressed at different partial pressures, using Boyle’s law ( Table 6-1 ).
| Partial Pressure (mm Hg) | Volume Occupied (mL) | |
|---|---|---|
| Normal atmospheric pressure | 760 | 200 |
| One-tenth atmospheric or double alveolar partial pressure | 76 | 2000 |
| Normal alveolar partial pressure | 38 | 4000 |
| Half alveolar partial pressure | 19 | 8000 |
The volume of carbon dioxide shown at each partial pressure in Table 6-1 is the volume occupied by the metabolic production for 1 minute (0.008 moles). At each pressure, the volume shown is the volume of carbon dioxide produced and therefore is the least alveolar ventilation per minute that is capable of eliminating the carbon dioxide produced. Any lesser alveolar ventilation is insufficient to allow the carbon dioxide to escape at that partial pressure.
The patient’s minute volume is made up of both the alveolar and the total dead space ventilation. Normally, the dead space ventilation is one third of the minute volume. For an alveolar minute ventilation of 4000 mL, the required total minute ventilation therefore is approximately 6000 mL. The mixed expired carbon dioxide has a partial pressure of approximately 27 mm Hg.
During IPPV under anesthesia, the total dead space typically increases to approximately 45% of the tidal volume. The same alveolar ventilation of 4000 mL/min thus requires a total minute ventilation of approximately 7275 mL, which will eliminate a mixed expired partial pressure for carbon dioxide of approximately 22 mm Hg.
Oxygen Uptake
At rest, the 70-kg adult human has an oxygen consumption of approximately 250 mL/min. Strictly speaking, ventilation is not essential for oxygenation. When the patient is breathing oxygen, the pulmonary reservoir represents approximately 12 minutes worth of metabolic consumption. Furthermore, if a denitrogenated apneic patient is connected to an oxygen supply (apneic oxygenation), oxygenation theoretically is unlimited. In that case, survival is limited by carbon dioxide accumulation, not by hypoxia.
Net Effect of Respiratory Quotient
Respiratory quotient (RQ) is the ratio between carbon dioxide production and oxygen consumption. The production of carbon dioxide, such as 200 mL/min, normally is slightly less than the oxygen consumption, at 250 mL/min (RQ: 200/250 = 0.8). This discrepancy has interesting implications. With an RQ of 0.8, the sum of the arterial partial pressures for carbon dioxide and oxygen (PaO 2 [100] + PaCO 2 [40] = 140 mm Hg) will always be slightly less than the humidified inspired oxygen tension (PiO 2 = 0.21 × [760 − 47] = 149 mm Hg) instead of being exactly equal to it. Because the minute volume inspired is slightly greater than that exhaled, a continuing, small, net inward movement of gas into the lungs is observed. At equilibrium, the partial pressure of nitrogen, or nitrous oxide (N 2 O), is slightly greater in the lung than in the inspired gas and, of necessity, this causes an equal reduction in the space that would have been available for the respiratory gases, carbon dioxide, and oxygen.
Physics of Gas Flow
Each ventilator discussed in this chapter has a selection of primary variables that the user may set. These may include any of the following:
| Inspired pressure: | P I |
| Peak inspiratory pressure: | P Imax |
| Tidal volume: | V T |
| Minute volume: | V M |
| Inspiratory flow: | Q I |
| Frequency: | f |
| Respiratory cycle time: | T c |
| Inspiratory pause time: | T plat |
| Inspiratory time: | T I |
| Expiratory time: | T E |
| Inspiratory/expiratory ratio: | I:E |
As spontaneous breathing occurs, work is done to move gas into and out of the lung. This work has been designated the work of breathing (WOB). Another way to view this is the energy expended to move the gas into and out of the lungs. It is well known that the total work of breathing (WOB T ) is the sum of the work related to overcoming the elastic properties of the lung and chest wall (WOB E ) and the work related to overcoming the resistance aspects of the circuit, endotracheal tube, and large and small airways (WOB R ). Thus,
WOB T = WOB E + WOB R
Under normal circumstances, the work related to overcome the elastance of the lung and chest wall is nearly 70% of the total WOB; the work related to overcome the resistance of the airways is nearly 25%, and approximately 5% is related to inertial properties of the tissues and gases. Elastance (E) of the chest wall is defined as the change of airway pressure (ΔP) divided by the change in volume (ΔV):
E = Δ P / Δ V
C = Δ V / Δ P
Experimentally derived, formulated, and published in the 1840s by Jean Louis Marie Poiseuille (1797–1869), Poiseuille’s law identifies the relationship of gas flow (Q) directly to the pressure gradient (ΔP) and identifies an inverse relationship to the resistance (R) of the system:
Q = Δ P / R
This equation can be manipulated to show that:
R = Δ P / Q and Δ P = Q × R
Because the purpose of the ventilator is to perform the work of breathing, it becomes advantageous to examine these physical relationships. In so doing, a method for classifying and understanding ventilator function using the “equation of motion” has emerged.
The force exerted by a ventilator is measured as pressure. This pressure must overcome two distinct impedances to motion during inspiration: compliance and resistance . Exhalation is passive when mechanical ventilation is used and usually remains unaccounted. The pressure required to overcome the compliance properties of the lung and chest wall can be expressed as:
P C = V T / C
P R = Q I × R
P T = P I = P C + P R
P I = ( [ V T / C ] + [ Q I × R ] )
Compliance and resistance may be regarded as the “load” facing the inspiratory pressure that results in the two fundamental variables: tidal volume and inspiratory flow . Changes in inspiratory pressure result in changes in both tidal volume and inspiratory flow. Changes in a desired tidal volume can be achieved by changes in inspiratory pressure and/or flow. Changes in a desired inspiratory flow can be achieved as the result of changes in inspiratory pressure, tidal volume, or both. While using specific ventilator modes, a ventilator attempts to control the inspiratory flow rate to provide a “set” tidal volume or inspiratory pressure ( Fig. 6-3 ). When the inspiratory flow is matched to a desired tidal volume, the inspiratory pressure varies to the given load ( Fig. 6-4 ). When the inspiratory flow is matched to a desired inspiratory pressure, the tidal volume varies to the given load ( Fig. 6-5 ).
Interdependence of Ventilator Settings
Many of these respiratory variables are interdependent, such as minute volume, tidal volume, and respiratory rate (V M = V T × f). In some ventilators, the tidal volume is determined by dividing the minute volume by the rate:
V T = V M / f
V T = Q I × T I
V T = Q I × ( T I − T plat )
Clinicians are also concerned about the relationship between inspiratory and expiratory time because the I:E ratio and the absolute times T I and T E affect ventilation and oxygenation. This ratio and the absolute time of inspiration are related to frequency and may be expressed mathematically.
Frequency determines time of the respiratory cycle (T c ), usually expressed in seconds, by the following relationship:
T c = 60 / f
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