Medical Temperature Measurement Oesophagus esophagus esophageal Rectal Oral Tympanic membrane Axilla Skin medical notes Copyright 2005 All rights reserved   EXACON SCIENTIFIC

Medical Temperature Measurement Oesophagus esophagus esophageal Rectal Oral Tympanic membrane Axilla Skin medical notes Copyright 2005 All rights reserved   EXACON SCIENTIFIC
 
 
 
Notes regarding Medical Temperature Measurement
 
 
ESOPHAGUS MEASUREMENTS
This highly accurate part of the body is considered invasive and is commonly used during surgery or in critical care areas.
Due to the length of the esophagus, the placement of the sensor is critical. If it is too high in the esophagus the reading will be affected by tracheal air.
Proper placement is in the lower third of the esophagus which will allow the sensor to be closer to the heart and aorta, and that it will accurately reflect the core temperature. It also indicates changes in core temperature significantly faster than peripheral sites.
 
 
 
RECTAL MEASUREMENTS
For many years rectal temperature measurement was considered the "Standard" especially in pediatric patients. Rectal temperature is a good approximation of body core temperature only if the patient is in thermal balance. Many studies have however shown that rectal temperatures fail to track rapid changes in body core temperature because the rectum has no thermoreceptors. In fact, because of the delayed response, core temperature may be changing in the opposite direction, and the lag time may be up to one hour. It is documented that heat passes from the rectum into the blood, not vise versa. Dwell time is significant here also. The temperature probe require a three to five minutes dwell time. It is discussed otr possible causes of inaccurate rectal readings, which include:
 
Heavy exercise of the large muscles in the buttocks and thighs
The insulating effect of fecal material in rectum
Coliform bacterial action
Improper depth of probe -
 
 
AXILLA & GROIN
These two sites are popular with the lay public due to their non-invasiveness and accessibility, but the clinical accuracy at these sites is suspect. The temperature probe must remain in position between 8 and 10 minutes. These sites are not located near a major artery or thermoreceptor and may not reflect temperature fluctuations. These factors may alter readings as much as 1,2° C ~ 2,2° F lower than actual core temperature. Lastly, if the patient is in shock the peripheral vaso-constriction will adversely affect the reading.
 
 
 
ORAL CAVITY
The most commonly used site is the sublingual area. It is considered a fairly accurate site due to its close proximity to the lingual and external carotid arteries, however, on average it runs lower than core temperature by approximately 0.5C° ~ 0.8° F. Correct placement of the oral probe is important for accuracy.
Differences in readings can vary by as much as 0.95° C ~ 1.7° F from the rear sublingual pocket to beneath the frenum in front of the floor of the mouth. Dwell time is also important, a probe require a three to five minute dwell time. Readings can be affected if:
 
The patient ate, drank, chewed gum, or smoked within 15 minutes of the reading.
If the probe is not kept the properly placed under the tongue.
The patient is an oral breather.
The patient talks during the reading.
 
It is documented that changes in oral temperature reflect changes in blood flow not necessarily changes in core temperature.It is furthermore stated that because of variable conditions oral temperature should not be considered equivalent to core temperature, unless studies are performed under strict controls.
 
 
 
URINARY BLADDER
Studies have shown a strong correlation between bladder and other core temperatures, because the urine is a filtrate of the blood and the kidney’s receive 20% of cardiac output. This method is considered minimally invasive, as it requires a urinary catheter with a thermistor tip to be inserted into the bladder. It is explained that bladder temperatures track core temperature changes better than rectal, but he readings may be altered due to urinary volume or if the patient is receiving bladder irrigations.
 
 
 
 
SKIN & SURFACE
The difference between core (Tc) and skin temperature (Tsk), interacts physically with total skin blood flow. The greater the (Tc - Tsk) difference, the greater the amount of heat transferred to the body surface. To a first approximation, the heat transfer is proportional to the product of the temperature difference and the blood flow.
 
A change in either Tc or Tsk affects heat transfer through the effect on the (Tc - Tsk) difference and also skin blood flow, through the thermo regulatory reflex influence of both Tc and Tsk. A Tsk change that reduced the temperature difference (a change in the direction of reducing heat transfer) would simultaneously induce an increase in skin blood flow (a change in the direction of increasing heat transfer. Arrows in the diagram show effects of an increase in skin temperature.
The first line is the physical effect. Increased skin temperature decreases the difference between core and skin temperatures. Consequently, less heat is delivered to the skin surface by a given amount of skin blood flow.
 
The bottom line is the reflex effect. Increased skin temperature increases skin blood flow with the consequence of increased heat transfer. The two effects could balance. If the gradient was reduced by, say, 30% percent and the increase in blood flow was also 30%, the net effect of the skin temperature change on heat transfer could be zero. Core temperature would remain steady despite the changes in skin temperature and skin blood flow. If the physical and reflex effects did balance one another, skin temperature could be driven from one extreme of the neutral zone to the other with no effect on core temperature. The whole job of regulation of thermal balance would be accomplished through this reflex control from skin temperature that perfectly balanced the physical effect of the altered gradient.
If the effects do not balance, then core temperature will seek a new equilibrium after a skin temperature change. What you expect intuitively is that core temperature should follow skin temperature. Don't you expect that, if skin temperature fell from the high side of the neutral range to the low extreme, core temperature would fall at least a little? It would not if the reflex reduction of skin blood flow were proportionately greater than the increase in core:skin gradient. Core temperature would change the wrong direction -- it would increase in response to a decrease in skin temperature.
 
As a matter of fact, that happens all the time. Afterdrop is a familiar phenomenon in which core temperature falls when skin temperature increases. You hear less often of afterrise, but many have observed that a sudden fall in skin temperature causes a rise in core temperature. These observations are usually at skin temperatures far outside the neutral zone, but the same phenomenon can be seen within that narrow range.
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As discussed earlier there are several sites besides the inaccessible hypothalamus for measuring temperature. How do you choose which site to use? The issues of accessibility and accuracy are the critical thinking points of contention. You would not put in a central monitoring urinary bladder catheter probe just to measure temperature. Other sites are affected by treatments and environmental factors, for example the mouth and forehead are affected by ambient air and oral medications or topical lotions. The rectum is affected by stool content, and disease states such as hemorrhoids or colitis.
 
 
 
TYMPANIC MEMBRANE
Obtaining a patient’s temperature by tympanic temperature probes became available in the 1960’s. Originally, this was done by anesthesiologists during surgery with a probe placed directly against the membrane.
 
The tympanic site was chosen because it is located in close proximity to the internal carotid artery, which supplies the hypothalamus. Since the tympanic membrane and auditory canal are relatively devoid of metabolic activity, the primary determinant of the temperature is that of the carotid artery. Tympanic temperatures are more effective in tracking changes in temperatures than rectal which can lag up to 60 minutes or more behind, which in particular is important for discovering an initial MH. The temperature of the tympanic membrane is relatively protected from the influence of ambient temperatures and is unaffected by smoking, respirations, eating or drinking.
 
Malignant Hyperthermia - MH
MH is a life threatening, acute pharmacogenetic disorder, developping during or after a general anaesthesia. Both a genetic predisposition, and one or more triggering agents are necessary to evoke MH. Triggering agents include all volatile anaesthetics (Chloroform, Ether, Halothane, Enflurane, Isoflurane, Sevoflurane, Deflurane) and depolarizing muscle relaxants (Suxamethonium). The classical MH crisis shows a hypermetabolic state, caused primarly by the muscles of the sceletal system. Besides this classical form of MH exist abortive forms with unspecific signs like tachycardia, arrhythmia and a raise in temperature. Modern monitoring, better knowledge of MH by the anaesthetists and the therapy using dantrolene reduces the incidence of the classic MH crisis. Nevertheless, MH is a dangerous disease, and anyone who is involved with anaesthesia and anaesthetics should have up to date knowledge about diagnosis and therapy of MH.
 
Survey Over Body Temperature Positions 

Site

Variation from Core Temp

Pro

Contra

Reliability

Complications

Uses

Oesophagus

Core reference

Reflects temp of body core

Tempe varies according to depth of probe placement

Placement is key

Needs to be in lower third of oesophagus

Used during anesthesia

Rectal

<0,3ºC

Preferred by MDs

Lags behind other core sites when temp is changing rapidly

Reading may be delayed from core temperature changes

Fecal material
Care with Neonates

Often requested by MDs as the "Most accurate" site for core temp

Nasopharynx

 

Reflective of brain temp

Affected by breathing. Invasive and uncomfortable

Variable

Risk of nose bleeding

Used under anesthesia

Oral

<0,4ºC

Easy access - Familiar. Minimally invasive

Affected by eating, drinking etc. Temp varies with oral cavity.

Affected by placement

Continuous sublingual placement

Most common for instant clinical use in adult and children over 5.

Bladder

<0,2ºC

Reflects temp of body core

Affected by the amount of throughput. Lags behind other core temp sites

Placement is key

Affected by urine volume or bladder irrigations

Used in surgery and emergency or critical care

Tympanic

 

Reflective of brain temp

Invasive and uncomfortable

Placement is key

Care for penetration

Used during anesthesia Emergency team at accidental hypothermia

Axilla

<1,2ºC

Easy access - Familiar.

Reflects skin temp. Not always a good indicator     of core temp.

Variable

Dwell time important for accurate reading

Most common site in children under 5.

Groin

<1,2ºC

Easy access in infants and   small children

Skin temp requires leg to be drawn up against domen

Variable

Dwell time important for accurate reading

Used in infants and neonates

Pulmonary Artery

 

Reflects temp of body core

Affected by temperature of infused fluids

Variable

Introduction

Used in surgery and critical care

Skin

<0,5ºC

Easy placement. Non-invasive

Variable with placement

Affected by placement and subcutaneous blood flow

Blood flow - Ambient temp. radiation

OP - ICU . Incubators

Great Toe

Depending

Easy access. Non-invasive can be informative when used with core temp

Peripheral skin temp very remote from body core

Affected by placement and subcutaneous blood flow

Blood flow - Ambient temp. radiation

OP - ICU . Incubators

 
Medical Temperature Measurement Oesophagus esophagus esophageal Rectal Oral Tympanic membrane Axilla Skin medical notes Copyright 2005 All rights reserved   EXACON SCIENTIFIC