Inspired gases

Anoxia Anoxia is the absence of oxygen in inspired gases or in arterial blood and/or in the tissues. This is closely related to hypoxia, which is a severe oxygen deficiency in the tissues. One can think of anoxia as the most extreme case of hypoxia. 

Desflurane does not have a marked bronchodilator effect and in cigarette smokers it is associated with significant bronchoconstriction. In clinical practice, both humidification of inspired gases and opioids are thought to reduce airway irritability but even at moderate concentrations (2 MAC), desflurane is more likely to cause coughing than sevoflurane. In common with other volatile agents, desflurane causes dose-related respiratory depression. Tidal volume is reduced and respiratory rate increases, initially. As inspired concentrations of desflurane increase, the trend is to hypoventilation and hypercardia and apnoea is to be expected at concentrations of 1.5 MAC or greater. 

Effects of Varying Maternal Arterial Oxygen Tension. Oxygen, air, and various N2-air mixtures were administered randomly to four ewes for 3-10-min intervals (47). The inspired gases achieved maternal arterial 02 tensions ranging from 30-350 mm Hg. During this interval the isolated... 

FIG. 27-33 Inspirator (gas-jet) mixer feeding a large port premix nozzle of the flame retention type. High-velocity gas emerging from the spud entrains and mixes with air induced in proportion to the gas flow. The mixture velocity is reduced and pressure is recovered in the ventnii section. (F om North Ameiioan Comhnstion Handbook, 3d ed., Notih American Manufacturing Company Cleveland 1996. )... 

An accurate nasal model must also account for the airflow rate and the concentration of the inspired gas. Aharonson et al, conclusively demonstrated that the uptake coefficient," or average mass-transfer coefficient, over the entire nose for acetone, ozone, sulfur dioxide, and ether increased with increasing airflow rate. 

When the pulmonary response is activated by irritant receptors in the nose, response for different flows and concentrations would not be expected to correlate with the volume of inspired gas, but rather with regional dosage (e.g., nasal) or the local dosage of gas to irritant receptors lining the airway. ... 

Various anesthetic agents require widely different partial pressures to produce the same depth of anesthesia (Table 25.2). Methoxyflurane, for example, with a MAC of 0.16%, is the most potent agent listed in the table. Only 0.16% of the molecules of inspired gas need be methoxyflurane. N2O is the least potent agent, with a MAC that exceeds 100%. Thus, a level of unconsciousness needed to eliminate movement is seldom achieved with N2O. 

One of the most important factors influencing the transfer of an anesthetic from the lungs to the arterial blood is its solubility characteristics (Table 25-2). The blood gas partition coefficient is a useful index of solubility and defines the relative affinity of an anesthetic for the blood compared with that of inspired gas. The partition coefficients for desflurane and nitrous oxide, which are relatively insoluble in blood, are extremely low. When an anesthetic with low blood solubility... 

The concentration of an inhaled anesthetic in the inspired gas mixture has direct effects on both the maximum tension that can be achieved in the alveoli and the rate of increase in its tension in arterial blood. Increases in the inspired anesthetic concentration increase the rate of induction of anesthesia by increasing the rate of transfer into the blood according to Fick s law (see Chapter 1). Advantage is taken of this effect in anesthetic practice with inhaled anesthetics that possess moderate blood solubility (eg, enflurane, isoflurane, and halothane). For example, a 1.5% concentration of isoflurane may be administered initially to increase the rate of rise in the brain concentration the inspired concentration is subsequently reduced to 0.75-1% when an adequate depth of anesthesia is achieved. In addition, these moderately soluble anesthetics are often administered in combination with a less soluble agent (eg, nitrous oxide) to reduce the time required for loss of consciousness and achievement of a surgical depth of anesthesia. 

Inhaled anesthetics change heart rate either directly by altering the rate of sinus node depolarization or indirectly by shifting the balance of autonomic nervous system activity. Bradycardia can be seen with halothane, probably because of direct vagal stimulation. In contrast, enflurane, and sevoflurane have little effect, and both desflurane and isoflurane increase heart rate. In the case of desflurane, transient sympathetic activation with elevations in catecholamine levels can lead to marked increases in heart rate and blood pressure when high inspired gas concentrations are administered. 

The theory that volatile anesthetics may act by specific binding to lipoprotein components of nerve tissue membranes or of the membrane itself has some experimental evidence. The use of 19F nuclear magnetic resonance spectroscopy with halothane indicated that saturable anesthetic sites for halothane exist in living rats at 2.5% inspired gas. The authors consider this to support the idea that volatile anesthetics do act specifically even stereospecifically (Moody etal., 1994). 

In clinical practice, one can monitor the equilibration of a patient with anesthetic gas. Equilibrium is achieved when the partial pressure in inspired gas is equal to the partial pressure in end-tidal (alveolar) gas. This defines equilibrium because it is the point at which there is no net uptake of anesthetic from the alveoh into the blood. For inhalational agents that are not very soluble in blood or any other tissue, equilibrium is achieved quickly (e.g., nitrous oxide, Figure 13-4). If an agent is more soluble in a tissue such as fat, equilibrium may take many hours to reach. This occurs because fat represents a huge anesthetic reservoir that will be filled slowly because of the modest blood flow to fat (e.g., halothane, Figure 13 ). 

Many studies have explored the effects of inhaled NO in humans. A fundamental basis for these studies is the demonstration of an active L-arginine-NO pathway in humans. Kobzik et al. (1993) demonstrated that a variety of cells in the human respiratory system, including endothelial cells, contain NO synthase. Blockade of this pathway by an arginine analog (N°-monomethyl-L-arginine) causes an increased PVR, as demonstrated in healthy adults by Stamler et al. (1994) and in children studied during heart catheterization by Celermajer et al. (1994). Human HPV can be reversed by the addition of NO to inspired gas (Frostell et al 1993). 

NO has been added to inhaled gas by a number of techniques. In an early short-term exposure study by Pepke-Zaba et al. (1991), NO to a final concentration of 40 ppm was added to a Douglas bag about 15 min before the patients inhaled the gas for 5 min. At present, such a strategy should be considered outdated, due to significant levels of NO2 formation before inhalation. In Japan long-term continuous exposure of mice for over 6 months was accomplished by controlling the atmosphere of live-in boxes (Oda et al., 1976 Nakajima et al., 1980). Inspired gas was passed over soda lime in order to minimize the NO2 content. 

Monitoring of gaseous NO and NO2 levels is an increasingly complex issue that is only briefly described here. At the outset we must state that we believe any acceptable monitoring system must reliably measure NO and NO2 in a gas mixture with 50-90% oxygen characterized by cyclic pressure variations and at both 100% (expired gas) and 0% humidity (inspired gas immediately following the ventilator). 

Figure 23-19 a shows the fundamental structure of the light barrier with a pulsed light source to be used in the case of simple interface electronics such as a lock-in-amplifier. The filter for the spectral dispersion is placed in a parallel light beam. The band pass filter for the 4.26 pm region can consist of two components. The one-channel method either works without periodic automatic calibration or calibrates on C02-free inspiration gas as, eg, during normal ventilation in intensive care. Zero-point calibration uses gas blends without CO2. Full-scale or span calibration is carried out by use of specific test gas blends. 

Inspired gas partial pressure A high partial pressure of the gas in the lungs results in more rapid achievement of anesthetic levels in the blood. Advantage is taken of this effect by the initial administration of gas concentrations higher than those required for maintenance of anesthesia. 

Minimum alveolar concentration (MAC) is the anesthetic concentration that eliminates the response in 50% of patients exposed to a standardized painful stimulus. In this table, MAC is expressed as a percentage of the inspired gas mixture. 

During inspiration, gas enters the lung by the process of bulk flow, in... 

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