Introduction
General anesthesia is a drug-induced, reversible condition, including specific behavioral and physiological components, with concomitant stability of the autonomic, cardiovascular, respiratory, and thermoregulatory system. It can also be perceived as a pharmacological intervention to prevent surgical trauma’s psychological and somatic adverse effects and create favorable conditions for surgery. This chapter will discuss important aspects of general anesthesia under the following headings:
Components of General Anesthesia
Oliver Wendell Holmes coined the term general anesthesia. In 1926, John Lundy described the concept of balanced anesthesia. Balanced anesthesia is achieved by using hypnotic agents, anxiolytics, muscle relaxants, and analgesics. The use of the specific drug helps in achieving the specific target with minimal side effects of each drug. The balanced anesthesia comprises the following:
Concept of Preoxygenation
Under the state of anesthesia, cessation of breathing leads to continuous oxygen uptake with no replenishment of the oxygen store in the lung, which puts the patient at risk of hypoxemia. Preoxygenation is essential to avoid hypoxemia and related end-organ damage. The functional residual capacity (FRC) works as an oxygen buffer, which gets filled during preoxygenation. Preoxygenation can also be considered as denitrogenation, as a high inspired oxygen concentration displaces the nitrogen in the lungs. The aim is to increase the physiological reserve of oxygen to increase the time to desaturation during the apnea time, which can occur with the induction of anesthesia. It is especially desirable if a rapid sequence induction (RSI) is planned, as positive pressure ventilation is avoided in RSI before tracheal intubation. The effectiveness of preoxygenation is assessed by its efficacy and efficiency.
Indices of efficacy include increase in the fraction of alveolar O2 (FaO2), decrease in the fraction of alveolar nitrogen (FaN2), and increase in arterial O2 tension (PaO2).
The efficiency of lung oxygen reserves is a product of the fraction of oxygen within the alveoli (oxygen fraction in expired gas = FeO2) and FRC.
Increased oxygen consumption and a decrease in oxygen reserve can lead to rapid desaturation despite adequate preoxygenation.
Increased oxygen consumption is seen in growing adolescents, fever, sepsis, and pediatric population.
Decreased oxygen reserve is seen in obesity, pregnancy, and patients with
large abdominal mass/ascites.
Preoxygenation Techniques
Before initiating preoxygenation, the following prerequisites must be met:
The inappropriate seal leads to inhalation of ambient air and can cause up to 20 to 40% dilution of the inspired oxygen. The preoxygenation can be done with any of the following techniques:
The traditional tidal volume breathing is an effective technique of preoxygenation. At FiO2 of 1, 3 minutes of TVB ensures maximal preoxygenation in adults with
normal lung function. The various anesthetic circuits (circle system, Mapleson A, and Mapleson D) with a flow rate ranging 5 to 35 L/min have been used successfully, but the circle system with flow rate 5 L/min remains the standard. The classic preoxygenation increases the apnea safety margin by up to 10 minutes.
The deep breathing technique includes the following:
Initial studies found deep breathing as effective as the tidal volume technique in terms of achieving preoxygenation goals. However, later on, it was found to be inferior because of the following reasons:
If the ventilation during the short time (0.5–1.0 min) is higher than the oxygen flow rate, rebreathing is likely to occur and hence lead to a decrease in the inspired concentration of oxygen.
The FRC may get saturated within 0.5 minutes of ventilation, but the tissues and venous compartment need a longer duration to fill.
Since deep breathing techniques provide suboptimal oxygenation, these should be reserved for emergency scenarios where time is lacking, and the patient’s cooperation is inadequate.
The anesthesiologist must target the endpoints of preoxygenation, irrespective of techniques, because the efficiency of preoxygenation gets reflected as the rate of decline in oxyhemoglobin saturation during apnea. The endpoints of preoxygenation are as follows:
The standard technique of preoxygenation is not helpful in patients who require securing airway rapidly and at risk of aspiration. It also does not work in patients who cannot be preoxygenated because of agitation and uncooperativeness. Here, the techniques like RSI and delayed sequence intubation replace the standard technique of preoxygenation.
Rapid Sequence Induction
It is the method of inducing anesthesia in patients who are at risk of aspiration.
After adequate preoxygenation (patient is allowed to breathe spontaneously and no sedatives are given), while cricoid pressure is being applied, a predetermined dose of intravenous anesthetic agent and muscle relaxant is given to intubate the trachea, without attempts at positive pressure ventilation (PPV).
The goal is to intubate the trachea as quickly and as safely as possible.
The application of cricoid pressure requires a force of 10 Newton while the patient is awake and a force of 30 Newton once the patient is induced.
The cricoid pressure occludes the upper esophagus, preventing the regurgitation of gastric content into the pharynx. However, some studies suggest that cricoid pressure lowers the lower esophageal sphincter tone, leading to increased risk of aspiration.
In addition, the application of cricoid pressure may make the visualization of glottis difficult during direct laryngoscopy.
The indications of RSI are as follows:
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