Local anesthetic
Without epinephrine
With epinephrine (1:200,000)
Lidocaine
3–4 mg/kg (300 mg)
7 mg/kg (500 mg)
Mepivacaine
4 mg/kg (300 mg)
7 mg/kg (500 mg)
Prilocaine
5–6 mg/kg (400 mg)
8–9 mg/kg (600 mg)
Articaine
5–6 mg/kg (400 mg)
Ropivacaine
3–4 mg/kg (250 mg)/to 37.5 mg/h continually
Bupivacaine
2 mg/kg (150 mg)/up to 0.4 mg/kg/h continually
2–3 mg/kg (150–225 mg)
By using ultrasound guidance during the placement of the block, a smaller volume of local anesthetic can be used to obtain the same effect, which results in improved patient safety [5]. Prilocaine is an aminoamide and is metabolized hepatically and extrahepatically.
>> Just as the last milliliter of local anesthetic was injected, Mr. Graham was pushed into the OR. Dr. Blue quickly injected 2 mg midazolam. Whether or not the block had fully taken effect could not be checked, but Dr. Blue was quite confident in her success. The inflatable cuff for the tourniquet was wrapped in cotton, because no one was sure whether or not it contained latex. Mr. Graham slept, but as the surgeons made their first incision, he jolted. “Look, at least make sure the patient stays still and doesn’t feel any pain, OK?” called the surgeon.
20.1.6 What Options Do You Have Now?
Mr. Graham could possibly have moved by chance, just at the same moment of the incision, but a check of the block needs to be performed immediately. If it is insufficient, then the surgeons can try to supplement it with more local anesthesia, being careful not to exceed the maximum dose. Apart from the additional local, it is also possible to administer systemic pain relief, for example:
Bolus administration of fentanyl
Continuous remifentanil infusion
Bolus administration of ketamine (in combination with a benzodiazepine)
One should not attempt to rescue an inadequate nerve block at any cost. Such action is unethical and traumatic for the patient. General anesthesia should be immediately induced. If the patient moved only by chance, then the sedation can be increased, such as by:
Bolus of midazolam
Continuous low-dose propofol infusion
>> Dr. Blue gave 3 mg of midazolam IV, and anesthesia tech Maria brought knee rolls, but neither helped for long. Dr. Blue was annoyed that she had actually believed the procedure would only take 1.5 hr. She was now informed that the surgery would take at least another hour. A continuous infusion of propofol 50 μg/kg/min was started. Mr. Graham slept so deeply that Dr. Blue had to hold up his jaw with one hand, while she filled out the anesthetic record with the other. Anesthesia tech Maria placed an oxygen face mask on Mr. Graham, with 4 l O 2 /min. With these measures, the pulse oximetry showed (S P O 2 ) 93 %.
20.1.7 Which Inspiratory Oxygen Concentration (FiO2) Can Be Reached with a Face Mask?
Through the basic nasal cannula system, a maximum FiO2 of 30–40 % can be reached (see Table 15.1). With a simple oxygen mask without a reservoir, the maximal FiO2 value can be 50 %. In order to prevent inhalation of exhaled air, a fresh gas flow of >6 l/min needs to be set. With an oxygen mask with a reservoir bag and unidirectional valve, FiO2 values of 85 % can be obtained, if the fresh oxygen flow is at 10–15 l/min.
>> With the propofol infusion, Mr. Graham was now very still and tolerated the surgery well. Every now and then, he took an extra deep breath. The hemodynamic parameters were stable, but the S P O 2 value dropped to 88 % despite the increasing O 2 flow – now at 10 l/min.
20.1.8 What Noninvasive Techniques/Physical Checks Should You Try Now?
The decrease in saturation could have a number of causes. The following points must be checked:
Fresh gas flow to the mask and correct connections.
Technical defects of the pulse oximeter should be ruled out.
An arterial perfusion disorder in the location of SpO2 measurement must be ruled out.
Hypothermia should be ruled out.
Movement artifacts should be ruled out.
Sleep apnea with undulating levels of saturation due to partial obstruction of the upper airways, aggravated by propofol administration.
Auscultation and inspection of the lungs to rule out:
Pneumothorax, status post-accident
Aspiration of gastric contents
Exacerbation of chronic bronchitis
Bronchospasm
If at hand, reevaluate chest X-ray. If not at hand, consider ordering one
Check hemodynamics.
>> After checking possible causes of the decrease in S P O 2 , Dr. Blue was no smarter than when she began. Everything seemed normal. She reduced the propofol in the meantime to 25 μg/kg/min, and she woke the patient many times to ask him to breathe deeply and cough. Dr. Blue had hoped that there would be an error on the pulse oximeter reading itself, but it remained unchanged after checking everything. The saturation decreased slowly to 85 %, and she became very nervous.
20.1.9 What Invasive Actions Could You Now Take?
The pathophysiological causes of arterial hypoxia have been discussed in Case 1 (see Sect. 1.1.8). If time permits, further diagnostics should be done, such as an arterial blood gas and a chest X-ray. The airway and ventilation – if the patient is in danger of hypoxemia – must be secured by intubation and/or PEEP ventilation. Then, bronchoscopy can be done to rule out aspiration of gastric contents.
>> Dr. Blue decided to intubate and ventilate Mr. Graham. Anesthesia tech Maria had already prepared everything for a rapid sequence induction. “Hopefully he won’t aspirate,” thought Dr. Blue, as she began the RSI. The endotracheal tube was just placed as her attending anesthesiologist, Dr. Eldridge, walked through the door. “I thought the times of intubation and plexus anesthesia were over, thanks to ultrasound” was his sinister comment. Silently, Dr. Blue was annoyed by her attending, because she felt that the plexus anesthesia was working very well.
Despite intubation and ventilation with PEEP and a F i O 2 of 1.0, the SpO 2 decreased to 84 %. Dr. Blue recruited her attending for assistance and advice. He glanced at the monitor, then on the anesthesia record, and then rolled his eyes. “Draw an arterial blood gas with hemoglobin co–oximetry,” he said to Dr. Blue. After a few minutes, the blood gas came back with the following values:
P a CO 2 : 34 mmHg (reference 35–46 mmHg)Related posts:
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