Four stages of anesthesia:

  1. Analgesia (conscious       sedation)
  2. Delirium
  3. Surgical anesthesia
  4. Respiratory paralysis

IV sedation and Nitrous oxide results in a patient in stage I.

What are the stages of general anesthesia?
Four stages of general anesthesia is classified by Guedel as:

• Stage of analgesia
• Stage of delirium
• Stage of surgical anesthesia
• Stage of respiratory paralysis

Stage of analgesia, or stage I of anesthesia, is characterized by the patient consciousness and his or her ability to response to command and may or may not become amnesic. It begins with the inital admininistration of CNS depressant drug and continues until the patient loses consciousness. In this stage, pain and pain threshold may be present and unaltered, however, the patient's response is diminished. All the techniques of oral, rectal, intramuscular, submucosal, inhalation, and intravenous sedation, results in a patient in stage I, i.e., stage of analgesia.

Stage II, or stage of delirium or excitement, begins with the patient lose consciousness from stage I. Early in stage II, the breathing patern may become irregular, reflexes may be exaggerated and there is a tendency to overreact to stimulation. In response to stimulation (such as pain), the patient will cry out and attempt to move the extremities. However, he or she suffers from amnesia and will not remember these activities. As stage II deepen, the breathing becomes more regular, with lateral nystagmus (eyeballs oscillate involuntarily), normal pupil reaction to light, muscle tonus first increase at early stage II and more relax in deep stage II, and finally the laryngeal and pharyngeal reflexes become more obtunded as stage II deepens. When performing intravenous sedation, entry into stage II should be avoided as it is the sign of oversedation.

Stage III is the stage of surgical anesthesia. In this stage, the respiration becomes regular, automatic and involuntary. Muscular tonus is lost with patient head and mouth can be manipulated with ease. This stage is furthered divided into four planes by Gillespie in 1943. The following summarizes these planes. Plane 1 is characterized by full, regular, automatic respiration, nystagmus and normal pupil response, swallowing and vomiting reflexes disappear, and finally, tears are secreted throughout plane 1. Plane 2 characterized with cease of nystagmus, thoracic respiration decrease, depth of breathing diminished, and tidal volume decrease. Laryngeal closure disappear, respiratory response to skin incision disappears, and secretion of tears diminishes. Plane 3 is entered when thoracic respiration decreases and abdominal respiration increases. Inspiration now is shorter duration than inhalation. Tidal volume decreases. Plane 4 begins with the paralysis of intercostal muscle until the arrest of respiration. Diaphragm weaken until spontaneous breathing ceases entirely. No muscle tonus and pupils dilate.

Stage IV is the stage of respiratory paralysis (medullary paralysis). This is usually encountered when there is drug overdose. Management of patient in stage IV is basic and advance cardiac life support and oxygen ventilation.

When we perform intravenous sedation in our office, we always stay in stage I. I preferred my patient to be relaxed and stable, with anterograde amnesia (with the use of Midazolam or Versed). He or she should be conscious but coperative with the vital signs barely altered, protective reflexes are intact, and most importantly, the patient is able to maintain a patent airway.

What is nitrous and how does it work?
Nitrous oxide is a nonirritating, nonallergenic, sweet-smelling, colorless gas. It is the least potent of the anesthetic gases. However, it is absorbed rapidly from the lungs into the cardiovascular system. The higher the concentration of gas inhaled, the more rapidly the arterial tension of that agent increase. N2O replaces N2 in the blood and this occurs within 3 to 5 minutes from the onset of nitrous oxide administration. This is why the patient should be remained at a given level of nitrous for 3-5 minutes before increasing the N2O concentration. Also at the completion of nitrous oxide administration, patient should be allowed to breath 100% oxygen for at least 3-5 minutes to avoid the phenomenon of diffusion hypoxia. Diffusion hypoxia refers to rapid diffusion of N2O from blood back into the alveoli (reverse of the beginning) and is responsible for the headache, nausea, and lethargy after nitrous oxide administration. The common term for it is "hang over".

At the therapeutic level, nitrous oxide does not depress the central nervous system. However, it depresses all forms of sensation included sight, hearing, touch, and pain. Memory is affected only to slight degree, as is concentration and thinking. Nausea and vomitting are very uncommon unless diffusion hypoxia occurs. There is no changes in heart rate or cardiac output. Blood pressure should be stable with an insignificant drop due to vasodilation. Nitrous oxide is not irritating to the pulmonary epithelium and therefore it can be used in asthmatic patient. Change in breathing rate or depth is due to the sedation effect. Nitrous oxide has no significant effect on gastrointestinal tract and kidney. Moreover, it can be used on pregnant women. There is no contraindication to the use of nitrous oxide as long as minimum of 25% oxygen is supplied. Finally, to answer the similar question toward oxygen, the inhalation of 100% oxygen has no effect on central nervous system, with a slight fall in heart rate and cardiac output, slight increase in diastolic, no change in systolic and blood pressure. Breathing 100% oxygen for 2 minutes slightly depresses the minute volume.

Our office, Cosmetic Dentist of Katy, is equipped with nitrous oxide and oxygen provided for every operatory. The monitoring equipments includes EKG, pulse oxymeter, and real time blood pressure/temperature monitor. We also have a mobile unit and the oxygen crash cart for our patient.

What is intravenous sedation and how does it work?
Intravenous sedation is a procedure of which sedation is achieved with the drug administered via parenteral. It is beneficial because the rapid onset, ability for drug titration for the desired clinical effect, easily application of emergency medicine as we have the IV line already established. The dangerous effect is due to the rapid onset, the action of the drugs are more pronounced. As mentioned above, the rapid onset of the drugs allows the dosage to be tailored to fit the need of the patient. The level of sedation can be fine tuned, and also the recovery of the intravenously administered drugs will be significantly shorter than any other route except for nitrous oxide administration. The patient vein is maintained during the entire procedure to facilitate injection of any drugs and also for emergency drugs should the complication arises. The side effect of nausea and vomiting are very uncommon with IV sedation, as is the gag reflex, motor disturbance (seizure). The salivary secretion can be controlled easily by addition of other medication through IV. Disadvantages are the procedure of venipuncture, thrombophlebitis may occur at the venipuncture site, and more intensive monitoring process required. Recovery from IV drugs is not complete at the end of dental treatment and patient must be escorted to and from the dental office by a responsible companion. Keep in mind that it is not easily reversed the intravenous drug. The rule of thumb is try NOT to over sedate the patient if possible.

Contraindication for IV sedation are:

• Dentist must be trained and certified in performing IV sedation
• Pregnancy is relative contraindication because most CNS depressant will cross the placenta into the fetus and may produce birth defect
• History of significant hepatic disease is a contraindication due to the fact that liver is the place where the drugs undergo biotransformation thus the effect of medication might be prolonged or profounded.
• Thyroid dysfunction is a relative contraindication for the use of IV sedation. Hypothyroid patient are particularly sensitive to CNS depressants such as sedative hypnotic, antianxiety agents, and narcotic analgesic. Clinically hypothyroid patient should not be IV. Hyperthyroid patient, on the other hand, is very extremely difficult to sedated due to the high metabolism and heart rate. Atropine and scopolamine therefore should be avoided in these patients.
• Adrenal insufficiency, as in patients who received chronic corticosteroid therapy or those of Addison's disease, is a relative contraindication for the use of IV sedation. They will not be able to handle the stress thus light to moderate sedation only is recommended.
• Patients with MAOIs or tricyclic antidepressants should be carefully evaluated for potential synergistic action with narcotic agonist and barbiturates.
• IV sedation is not contraindicated in patient with psychiatric disorders
• Obsese patient may pose the venipunture problem and also the decrease in cardiovascular and pulmonary reserves. Other forms of sedation (such as nitrous) should be considered first.
• IV sedation is contraindicated if the patient is allergic to the drugs used in sedation. Patient may experience asthma with the narcotics and barbiturates and pophyria with the barbiturates. Other contraindication is the use of anticholinergics in glaucoma and prostatic hypertrophy patient.

In summary, IV sedation provides amnesia (use of midazolam, diazepam) and anxiety management which are desired in a complicated dental procedure. However, even though IV sedation is a very effective form of sedation, I only employed the sedation when there is a specific indication for it. After all, the best patient is the patient that can be managed without the use of any drugs. I have removed countless number of complete bony third molar extraction under just local anesthesia, and the patient behaves very well (Over 500 cases, perhaps only two or three cases with IV sedation). I performed IV sedation on numerous medical compromises patients of which the stress reduction protocol is mandatory. I choose a narcotic agonist and a sedative hypnotics, in conjunction of oxygen administration. My favorite narcotic agonist is meperidine (Demerol) and that of the sedative hypnotics is midazolam (Versed).

What do we expect from the drug interaction and the intravenous procedure using these drugs?

Midazolam (Versed) is a benzodiazepine. Its average sedative dose is 2.5-7.5mg. Its maximal dosage is 10mg. It posesses several favorable characteristic such as anterograde amnesia (patient will not remember from the point of administration on), short duration (1 hour typical), and has minimal effect on cardiovascular and respiratory system. Its most usual side effect is dizziness. The medical history of the patients receiving diazepam should be checked for benzodiazepine allergy, acute pulmonary insufficiency, and respiratory depression.

Meperidine (Demerol) is a narcotic agonist which is the most frequently used IV narcotic in dentistry. Its onset is approximately 2-4 minutes, and its duration of action is 30-45 minutes. The recommended dose is 50mg (maximum), with the average sedative dose is 37.5mg-50mg. Meperidine produce atropine properties such as reduction of salivary secretion and increase heart rate (minimal with 50mg maximal dose). The notable side effect of meperidine is the phenomenon of "tracking", i.e., the skin overlying the vein into which meperidine is injected will appear red and iching is present. The reddening may continue following the path of the vein. Management of meperidine-induced histamine release is simply to allow it to dissipate spontanneously, which occurs over the next 10-15 minutes. The medical history of patiets receiving meperidine should be checked for narcotic anagesics allergy, MAO inhibitors taken within 14 days (synergistic effect), Asthma, and COPD and decreased respiratory reserve.

I always preferred to use EKG, pulse oximeter, and real time blood pressure on the intravenous sedation of patient. Oxygen administration is mandatory (in my opinion) as to maintain oxygen saturaion pressure at 96-99%. Most importantly, the medical history should be screen and checked with the patient carefully prior to any sedation procedure.

 These points of discussion reflect Dr. Bui's view point on the topic of sedation in dentistry. He has been performing intravenous sedation since 1998. He is certified in oral, inhalation, IM, and IV sedation. His office equipped with EKG, pulse oxymeter, real time blood pressure and temperature monitor, and finally, oxygen and nitrous are plumbed to every operatory.