The first study called, A randomized, double-blind,
placebo-controlled trial comparing pethidine to metamizol for treatment of
post-anaesthetic shivering, was designed to evaluate and compare the effectiveness
of pethidine (meperidine) and metamizol in the treatment of post-operative
shivering1. The study included 104 patients with complications of
post-anesthetic shivering1. All patients were required to undergo pre-operative
evaluation and were only included if they provided written consent. Patients
were excluded if they presented with comorbidities and if they were treated
with certain medications that would interfere with the results of the study. The
patients were randomized to receive one treatment of placebo (no drug), meperidine,
or metamizol1.

The patients’ responses to treatment were
measured in terms of severity at intervals of 5, 15, 30 and 45 minutes after
administration1. Severity of shivering was measured on a scale of
0-2, 0 as absent, 1 as shivering present in action, and 2 as shivering present
in action and rest1. Shivering response to treatment was assessed as
null, shivering not changed; improvement, shivering decreased; and disappearance,
absence of shivering1. Subjective information was also recorded, such
as the presence of muscle contractions and/or the sensation of feeling cold1.

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The continuous variables were analyzed with a chi square test and ANOVA, as
well as the Kruskall-Wallis test1. The results concluded that drug
treatment was useful in comparison to placebo. Meperidine appeared to have the
highest efficacy and quickest response to shivering than placebo or metamizol1.

The study did not exhibit selection bias when
choosing patients to receive treatment. Inclusion criteria allowed patients to
participate that presented with post-operative shivering for at least 5 minutes
or longer during the first hour of recovery from anesthesia1. Internal
validity was threatened because the study did not specify when the treatment began
after surgery. The results could have varied if treatment was initiated 1 hour
or 2 hours after surgery. The study was completed on ethical measures with each
patient informed of the study and having written consent. The sample size
required for the study (n=90) was appropriately calculated with an alpha of 0.05
and a power of 0.80; n=104 was used in the study1. According to the
results, the differences between the three groups proved to be statistically significant.

When compared to placebo, metamizol showed a better response (P= 0.03), but meperidine showed to be
the most effective (P

There was no statistical significance between both drugs (P=0.16)1. The results of this study show to exhibit
external validity and can be extrapolated into real life situations or other studies
involving patients undergoing general anesthesia.

The second study called, A Comparison of Tramadol, Amitriptyline, and
Meperidine for Postepidural Anesthetic Shivering in Parturients, is a
randomized, double- blinded, study which was designed to evaluate the
effectiveness of meperidine, tramadol, and amitriptyline in the treatment of
post-anesthetic (epidural) shivering2. The study involved 115 obstetric
female patients ranging from ages 18 to 40 years old that were scheduled for a cesarean
delivery2. Patients were excluded if they had comorbidities, a
history of alcohol or substance abuse, or were taking other medications that
would interfere with the results of the study2. Shivering was
assessed with a scale of 0-4, 0, no shivering; 1, peripheral vasoconstriction;
2, muscular activity in one group of muscles; 3, muscular activity in many
muscle groups; 4, shivering across the whole body2. Patients with a
shivering score of 3 or 4 for at least 3 minutes were included in the study;
only 45 patients requested treatment for shivering 2. Each patient
received one treatment of tramadol, amitriptyline, or meperidine.