Nitrous Oxide, From Discovery to Now - IneedCE.com

More documents

Recommendations

Info

Educational Objectives Upon completion of this course, the clinician will be able to do the following: 1. Be knowledgeable about the history of nitrous oxide 2. Be knowledgeable about the chemistry and action of nitrous oxide 3. Understand the dental treatments where use of nitrous oxide can be useful, as well as know the relative contraindications to nitrous oxide use 4. Be knowledgeable about the considerations and recommendations related to nitrous oxide use and the protocol to be followed on the day of the patient’s appointment Abstract Nitrous oxide was discovered and first prepared in 1793 by an English scientist and clergyman named Joseph Priestley. Since then nitrous oxide has proven to be a safe and popular agent utilized by many dental practices. In this setting, its use is usually as a mild sedative and analgesic. It helps to allay anxiety many patients may have toward dental treatment, and it offers some degree of analgesia. As such, its use for phobic patients has been well established and remains its primary indication. Relative contraindications to the use of nitrous oxide include patients with psychiatric disorders, blocked eustachian tubes, bowel obstruction, colostomy bags, gas in the ocular orbit, and large tubercular lesions in the lung. When administered appropriately, nitrous oxide is a safe and valuable asset in decreasing the pain and anxiety experienced by dental patients. History Nitrous oxide was discovered and first prepared in 1793 by an English scientist and clergyman named Joseph Priestley. He was also the first to prepare and isolate other important gases, such as oxygen, carbon monoxide, carbon dioxide, ammonia, and sulfur dioxide. To prepare nitrous oxide, Priestley heated ammonium nitrate in the presence of iron filings, then passed the gas through water to remove toxic by-products before storing the resultant gas, nitrous oxide. Following Priestley’s work came the study of Humphrey Davy of the Pneumatic Institute in Bristol, England. Davy experimented with the gas’ physiologic properties such as respiration and uptake. Further, he administered the gas to society visitors of the institute and later coined the term “laughing gas” after watching its effects on people who inhaled it. For the next 40 years or so, nitrous oxide’s primary use was for recreational enjoyment (nitrous oxide capers) and public show. Not until the early 1840s did nitrous oxide find its place in clinical dentistry and medicine. A medical school dropout named Gardner Quincy Colton went around the country putting on exhibitions with nitrous oxide. On December 10, 1844, he put on a demonstration in Hartford, Connecticut, and in the audience that day was a local dentist named Horace Wells. Dr. Wells’ interest was sparked when a druggist’s assistant named Samuel Cooley volunteered to inhale the gas. Afterwards, while under the effects of nitrous oxide, he bumped into some nearby benches where he injured his leg. He went back to his seat next to Dr. Wells, but was unaware of the injury until the effects of the gas wore off. This tipped off Dr. Wells to the analgesic qualities nitrous oxide might possess. After the demonstration, Dr. Wells approached Colton and invited him to come to his office the next day. Colton agreed, and the next day he administered nitrous oxide to Dr. Wells while another local dentist, Dr. Riggs, extracted one of Dr. Wells’ molars. This personal demonstration proved successful, since Dr. Wells experienced no pain during the procedure. Subsequently, Colton taught him how to manufacture as well as administer the gas to his patients, which he continued to do throughout the remainder of his private practice. In January 1845, Dr. Wells was given the opportunity to demonstrate the effects of nitrous oxide at the Harvard Medical School in Cambridge, Massachusetts. A patient was anesthetized for a tooth extraction, and during the demonstration the patient responded verbally to the surgical stimulation. The audience booed and hissed their dissatisfaction, and the demonstration was deemed a failure, even though the patient was amnesic during the extraction. This public humiliation eventually led to the downfall of Dr. Wells and his suicide in 1848. Little did he know that 150 years later, he would be recognized as the “Discoverer of Anesthesia.” Nitrous oxide fell out of popularity after Wells’ suicide and would not resurge until about 15 years later, largely due to the efforts and dental practice of Colton in the New England area. Colton later went on to open numerous “dental institutes,” utilizing nitrous oxide throughout the country. During this time, nitrous oxide was administered to patients in a hypoxic mixture (100%), without supplemental oxygen or air, to render the patients unconscious. Fortunately, the procedures were of such a short duration that few suffered significant morbidity. In fact, Colton administered in excess of 120,000 such anesthetics without a fatality. Other contemporary anesthetic agents of the era were ether and chloroform; however, nitrous oxide is the only agent still in clinical use today. Physical Properties Nitrous oxide (N 2 O) is a colorless, almost odorless gas with a molecular weight of 44, a specific gravity of 1.53, and a boiling point of –89°C. It is produced by heating ammonium nitrate crystals to approximately 240°C. The resultant gas is then chemically scrubbed to achieve approximately 99.5% purity, compressed (50 atmospheres) to a liquid form, and dispensed in pressurized tanks. The color of the nitrous oxide tank is blue for universal identification, compared to oxygen tanks, which are 2 www.ineedce.com
green in the United States. When dispensed in a pressurized tank, the nitrous oxide exists in two forms—gas and liquid. The pressure of a full tank, regardless of size, will read approximately 760 psi. Unlike other compressed gases which exist in only a gas form, the pressure inside the tank does not decrease with the amount of gas used. For example, oxygen, which is only in a gas form, will read approximately 2000 psi for a full tank. When the tank is 50% full, the pressure will read 1000 psi, 25% full will read 500 psi, and so on. Nitrous oxide tanks will register 760 psi for as long as a liquid phase exists. When the tank has been used to the point where a liquid phase no longer exists (after about 75–80% consumption), then the pressure will start to drop. When this occurs, the provider will know that approximately 20–25% of the tank still remains. If a more accurate account of the tank’s volume is necessary, then one can weigh the tank to make the determination. Other important physical properties of the gas include its partition (solubility) coefficients, some of which are blood/gas = 0.47, brain/blood = 1.1, fat/blood = 2.3, and oil/gas = 1.4. The minimum alveolar concentration (MAC) of nitrous oxide is 104%, which is defined as the minimum alveolar concentration of the gas necessary to render 50% of patients motionless following a painful surgical stimulus, such as skin incision. For inhalational agents, both MAC and the oil/gas coefficient help to quantify potency. The blood/gas coefficient best quantifies uptake, speed of onset and recovery characteristics. Physiologic Effects Nitrous oxide is a very weak anesthetic agent (as its MAC of 104% would indicate) when used alone, and it is unable to produce general anesthesia unless used in hypoxic mixtures (of 100% or above) under hyperbaric conditions. Most current administration devices do not allow the delivered concentrations of nitrous oxide to exceed 75–80%. Its principal use in dentistry is as a mild sedative and analgesic agent to help allay fear and anxiety for phobic patients, and many times it is used in combination with local anesthesia as well as other sedative, hypnotic agents. Its analgesic qualities are quite potent, with a 30–40% delivered concentration equivalent to 10–15 mg. of morphine. In fact, some EMS services take advantage of these qualities when transporting acute myocardial infarction patients to the emergency room by administering mixtures of 30% nitrous oxide with 70% oxygen. It has minimal effects on either the respiratory or the cardiovascular system in normal healthy patients, which adds to its safety. As any sedative medication will have some degree of respiratory depressive properties, nitrous oxide’s degree is only minimally so, mostly due to its lack of potency. Further, all anesthetic medications have some degree of myocardial depressant activity, but again, nitrous oxide’s degree is only minimally so. This is due not only to its lack of potency, but also to its mild central sympathetic stimulatory activity, which helps to offset its direct myocardial depressant effects. These effects may be exaggerated, however, in patients with severe cardiac disease and function. While nitrous oxide also possesses some mild peripheral vasodilatory properties, this effect may not be consistent in all patients. Its principal use in general anesthesia is as an adjunct or supplement to other, more potent inhalational anesthetics such as Fluothane®, isoflurane, and enflurane. In this setting, nitrous oxide’s additive effect can decrease the necessary concentrations of the other agents, thereby decreasing some of their negative physiologic effects. In this setting, nitrous oxide concentrations of 50–60% are routinely used and can decrease the MAC of the other potent inhalational agents by as much as half. Also, for these cases, some physical properties of nitrous oxide can be advantageous. One such property is termed the concentration effect. As mentioned above, the blood/gas partition coefficient is very low (0.47), which means not much dissolves in blood. However, nitrous oxide is very diffusible and will saturate the blood and reach an equilibrium very quickly. This helps to explain its very fast onset and recovery characteristics. When administered in high concentrations (50–75%), the nitrous oxide is quickly taken from the alveoli into the pulmonary circulation. Because this occurs in a very rapid fashion, a relative vacuum of gas occurs in the alveoli, which acts to pull more fresh gas into the lungs, thereby increasing alveolar ventilation and increasing the concentration of the administered gas faster than would otherwise occur. Another phenomenon called the second gas effect piggybacks upon this property. This occurs when a more potent gas anesthetic agent is co-administered with the nitrous oxide. It, too, is virtually sucked into the lungs at a higher rate and will be taken up in the pulmonary circulation to exert its effect faster than if it were not co-administered with nitrous oxide. Finally, just as nitrous oxide is quickly taken up by the bloodstream during administration, it is also given up quickly by the blood-stream upon cessation. When the administration of nitrous oxide is terminated, it quickly diffuses back into the lungs across the concentration gradient. This happens at quite a fast pace, which can lead to a relative decrease in the alveolar concentration of other gases such as oxygen. This can cause diffusion hypoxia. Although this rarely produces clinical hypoxia, it is prudent to administer 100% oxygen to patients for about 3–5 minutes following the termination of nitrous oxide. As discussed above, nitrous oxide diffuses very rapidly. In fact, it diffuses more rapidly than many other gases such as nitrogen. As a result, nitrous oxide tends to diffuse across concentration gradients into air-filled cavities faster than the nitrogen can diffuse out, which causes the cavity to expand. This can be a problem under certain circumstances. www.ineedce.com 3
Page 1: Earn 4 CE credits This course was w
Page 5 and 6: Past reports of possible increased
Page 7 and 8: 1. Nitrous oxide was first isolated

Nitrous Oxide, From Discovery to Now - IneedCE.com

Create successful ePaper yourself

Delete template?

Save as template?