CONTENTS

• General comments on POCIT
  • Preamble
  • Benefits & limitations
  • Normal levels of thermal asymmetry
  • Technical notes
• General
  • Temperature measurement
• Shock & perfusion
  • Skin perfusion in shock
    • Core-to-finger gradient
    • Leg temperature gradients
  • Compartment syndrome
• Infection
  • Cellulitis
  • Subcutaneous abscess
  • Necrotizing soft tissue infection (NSTI)
  • Central line infection
  • Septic arthritis
  • Surgical site infection
• Other
  • Horner's syndrome

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preamble

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Physicians have recognized that skin warmth could indicate disease since antiquity, but they initially lacked any tools to quantify it. In the late 1800s, the thermometer was invented. This was a massive advancement. Dr. Joao Vincent Torres Homem wrote in 1870 that it was no longer “sufficient to ascertain the warmth of the skin with one's hand” since “the physician might easily deceive himself.” (Ganger S)

Nonetheless, to this day, physicians use their hands to assess warmth (e.g., to detect cellulitis or peripheral perfusion). Fortunately, the technology exists to move beyond this practice. Infrared thermography is increasingly inexpensive and accessible. This technology enables quantification of skin temperatures and tracking over time, rather than relying on subjective vibes.

Nonetheless, POCIT (point-of-care infrared thermography) remains at an evolutionary point that may be equivalent to where POCUS was around ~1995-2005. POCIT is not yet a mainstream technique. The technology and evidentiary basis are rapidly improving (e.g., over the past five years, important work has validated normative values in cellulitis and septic shock).

It's reasonable to expect that POCIT will become increasingly valuable and widespread over the coming years. Most people are unlikely to adopt this in the immediate future (nor do they need to). But if you're a gadget nerd who wants to improve your physical examination skills, this might be for you. At the very least, it will force you to spend more time at the bedside and think a little more deeply about the patient.

As with any new technology (e.g., POCUS), there are legitimate concerns about false positives and false negatives. To avoid these pitfalls, it must be remembered that POCIT is merely one additional diagnostic tool in the toolbox, intended to enhance the physical examination. Ultimately, diagnostic and management decisions should always be made based on review of all available data combined with best clinical judgement.

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benefits & limitations

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benefits of POCIT are reminiscent of POCUS

• Noninvasive.
• Fast to perform.
• Inexpensive.
• Portable.
• It's relatively easy to learn how to acquire images.
• Data is immediately available at the point of care.
• It may be performed serially to evaluate changes over time.
• Performing thermography may force clinicians to spend more time at the bedside.

limitations

• The primary limitation is the limited availability of infrared cameras. However, technology is improving and becoming more affordable.
• Limited evidentiary basis regarding normal values and best imaging techniques (but this is rapidly improving).
• Confounding factors:
  • Peripheral vascular disease.
  • Application of ice packs, warm blankets, or other clothing (e.g., leg immobilizer, Rooke protective boot).
  • Unusual ambient temperature conditions (e.g., unusually cold, hot, or humid).
  • Underlying thermal abnormality (e.g., high fever may cause vasodilation, thereby obscuring other pathology). (38983367)

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normal levels of thermal asymmetry

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• Comparing a lesion to the contralateral side of the body is often helpful, allowing the patient to serve as their own “internal control.” This avoids confounding due to shifts in body temperature or ambient temperature.
• Studies on healthy patients indicate that differences between contralateral sides of the body are generally <0.5 ℃. (38138242, 3418388)
• Studies of cellulitis and central line infection have independently established a cutoff of ~0.4-0.5 ℃ as the upper margin of normal variation between contralateral areas of skin. (30699393, 38536160, 30003987) However, as with any cutoff, values near this cutoff may lack significance (discussed further here).

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technical notes

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choosing a device

• (I have no conflicts of interest.)
• The vast majority of published research on POCIT has used a FLIR thermal camera. To follow the best available evidence, it's logical to choose an FLIR camera.
• Various FLIR cameras are available. Some are lightweight and can be attached to a phone. The FLIR Compact series is a stand-alone device that is bulkier but may be more durable (it's drop-tested and less likely to be damaged by falling).
• Devices are improving rapidly. Any FLIR camera you buy today is likely to be superior to the camera used in most of the available literature.

settings and image acquisition

• Set the emissivity value to 0.98 to measure skin temperature. (30699393)
• The angle between the camera and the skin should be ~0-45 degrees. If the skin is viewed at an oblique angle, this may artificially reduce the measured temperature. (35309080)
• Fixed versus dynamic temperature windows:
  • A dynamic temperature window automatically adjusts the maximum and minimum temperatures to achieve optimal contrast within the image.
  • A fixed-temperature window may not be optimal for image contrast, but it allows direct comparisons with other thermal images taken over time.

patient confidentiality

• FLIR cameras have an IR lens and a regular camera lens. They will simultaneously capture both an infrared image and a standard camera image. For our purposes, the standard camera image isn't helpful. I'd recommend covering the regular camera sensor to prevent any patient photographs from being captured. This makes most of your images unidentifiable (they will just be colorized thermal images that could belong to anyone). Identifiable images should be deleted as soon as possible.
• Avoid connecting your camera to the internet or storing images in the cloud.

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temperature measurement

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thermography to estimate core temperature

• Core temperature may be estimated by evaluating the maximal thermographic signal along the medial canthus of the eye.
• Accuracy is OK (measurement will probably get you within +/- 1 ℃).

how this is used clinically

• [1] Thermography may be used as a rapid screening tool to evaluate for substantial hypothermia or hyperthermia.
• [2] Core temperature at the medial canthus of the eye may be used to estimate temperature gradients (discussed in the section below). This isn't ideal, but it does allow for the thermal camera to estimate temperature gradients as a stand-alone, immediately actionable bedside technique (without requiring an additional thermometer to measure core temperature).

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skin perfusion

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core-to-finger temperature gradient

• Themography is an advancement compared to the traditional practice of feeling how “warm” or “cool” the extremities subjectively feel:
  • [1] Infrared thermography is a quantified, objective measurement (thereby avoiding errors in relative temperature ascertainment if the examiner's hands are unusually warm or cold). This measurement may be precisely trended out over time.
  • [2] Evaluating the temperature gradient helps avoid erroneous conclusions when the patient's core temperature is abnormal. For example, if the patient's core temperature is 95 ℃ and the fingers are 92 ℃, the patient's fingers may feel cool despite normal perfusion.
• Core-to-periphery temperature gradients have been evaluated using various sites, most commonly the core-to-finger or core-to-toe gradients. The core-to-finger gradient has superior evidentiary support as compared to the core-to-toe gradient. Finger temperature may be less affected by socks and peripheral vascular disease, as compared to the toe temperature.

technical details

• Core temperature may ideally be measured using a bladder or esophageal temperature probe. A dedicated tympanic membrane thermometer may also be used. If these aren't available, infrared thermography may be used to estimate core temperature by measuring the surface temperature at the medial canthus of the eye (as discussed in the section above).
• Finger temperature is measured on the dorsal surface of the index fingertip. To minimize local temperature fluctuations, it might be reasonable to measure this bilaterally and average the two values.

interpretation of core-to-finger temperature gradient

• No prospective studies have validated the use of the core-to-finger temperature gradient as a resuscitative endpoint.
• A gradient of >7 ℃ is associated with increased mortality across studies, so it is reasonable to target a lower value. (32949897, 28002090, 40969181) One study found that a core-to-finger gradient >7 ℃ predicted mortality more accurately than capillary refill time, substantiating this as a valid parameter. (40969181)
• Changes in the core-to-finger temperature gradient appear to track changes in perfusion. (30042287) This could help determine whether a vasopressor, inotrope, norepinephrine, or a fluid challenge has improved perfusion.

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core-to-toe and core-to-knee temperature gradients

The core-to-finger gradient is probably preferred as a global perfusion target (e.g., in shock states). However, lower extremity gradients are included here for completeness. In some situations (e.g., leg compartment syndrome, leg ischemia), it may be useful to know benchmark normative values.

core-to-knee gradient

• In one study of 187 patients in the emergency department with possible sepsis, the core-to-knee gradient was ~5 ℃ with an interquartile range of roughly 4-8 ℃. The core-to-knee gradient tended to be ~2-4 ℃ less than the core-to-finger gradient. (40335928)  

core-to-toe gradient

• In one study of 56 surgical patients with septic shock, the gradient between the medial canthus and the toe was 12.8 +/- 3.8 ℃. In this study, toe temperature was often close to room temperature. (38515021)
• In one study of 187 patients in the emergency department with possible sepsis, the core-to-toe gradient was ~12℃, with an interquartile range of very roughly ~8-16℃. (40335928)  

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compartment syndrome

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• Poikilothermia (a decrease in body temperature to match the room temperature) is a well-known feature of compartment syndrome. However, in practice, this is difficult to measure or track, so it is generally overlooked.
• Unilateral compartment syndrome is easier to diagnose, as there is often a clear difference compared to the contralateral extremity (example above).
• Bilateral compartment syndrome of the legs is more difficult to diagnose because the legs may appear symmetric. This pattern of reduced peripheral temperature could mimic global hypoperfusion. One study of trauma patients with compartment syndrome found that the difference between the proximal and distal temperatures was ~8.8 ± 2 ℃ in legs with compartment syndrome, compared to ~1.2 ± 1 ℃ in unaffected legs (figure below). Notably, this temperature difference was evident during the initial trauma survey in the emergency department (an average of 90 minutes after traumatic injury), thereby enabling immediate diagnosis and triage. (38983367) Unfortunately, these precise skin gradients won't be reproducible with most point-of-care thermal cameras at the bedside (which can't average temperature over customized ellipsoid regions of interest).  

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cellulitis

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scanning technique

• Measure the maximal skin temperature in the erythematous skin as well as the corresponding skin on the contralateral body.  Compare the maximal skin temperature on both sides.

🏆 temperature gradient between the maximal temperature on either side

• This seems to be the best parameter, with a cutoff value of ~0.4-0.5 ℃ (if the erythematous skin is >0.4-0.5 ℃ warmer than the contralateral side, this supports a diagnosis of cellulitis).  
• Data from individual studies:
  • A temperature gradient of ≧+0.4 ℃ had a sensitivity of ~90% and a specificity of ~40% in a study of 204 patients. (38536160)
  • A temperature gradient of ≧+0.47 ℃ had a sensitivity of 87% and specificity of 38% in a study of 67 patients. (30003987)

maximal temperature of the erythematous skin

• Maximal temperature >31.2 ℃ had a sensitivity of ~94% and a specificity of ~40% in a study of 204 patients. (38536160) 
• Absolute temperature should be used cautiously, as it hasn't been as well validated as the temperature differential. Absolute temperature may vary due to ambient temperature and differences in temperature across various parts of the body (e.g., an erythematous area in the upper thigh is more likely to have an absolute temperature >31.2 than an erythematous area in the lower leg, due to normal temperature gradients along the leg). (28951240)

tracking the progression of cellulitis

• Serial thermography may be used to track the progression of cellulitis.
• At baseline, warmth may extend beyond the area of erythema.
• Improvement in warmth may precede improvement in erythema. (37180600)
• For serial imaging, it may be useful to use a fixed temperature window for all images (allowing apples-to-apples comparisons across all images).

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subcutaneous abscess

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• Thermography may help identify the location of a subcutaneous abscess.
• Ultrasonography is preferred for abscess diagnosis, but thermography can help survey an area of erythematous skin to identify epicenters of inflammation for further investigation.

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necrotizing soft tissue infection (NSTI)

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• ⚠️ The use of infrared thermography for the diagnosis of NSTI remains supported by little data. Nonetheless, some findings could be helpful when integrated with the full clinical picture. In general, these findings have low sensitivity, so failure to detect these features should not be used as an argument against the diagnosis of NSTI.
• [1] Skin warmth may support the presence of infection (analogous to the cellulitis data above).
• [2] Rapid advancement of hyperperfused tissue over time supports the presence of necrotizing fasciitis (in contrast, cellulitis shouldn't progress). (30755912) 
• [3] Areas of cool skin may reflect necrosis (non-perfused tissue). (36828267)

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central line infection

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scanning technique & overview

• [1] Qualitative comparison of the central line site with the contralateral side of the body.
  • Is there an increase in temperature around the line?
  • Does the increased temperature seem to center around the line, or is it randomly distributed?
• [2] Quantitative analysis:
  • Measure the maximal skin temperature in regions of skin on both sides of the body, one of which contains the line.
  • Compare the maximal skin temperature on both sides. (30699393)
  • For PICC lines involving the arms, temperature asymmetry >0.5 ℃ may be considered abnormal. (30699393) This study was small, so the precise cutoff value is not well defined. However, a cutoff value of ~0.5 ℃ is consistent across larger datasets involving cellulitis (discussed above).

limitations

• It's conceivable that sterile thrombophlebitis may also cause inflammation and an increased temperature signal. However, thrombophlebitis is often associated with line infection, so in either situation, increased temperature may signal line-associated complications.

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septic arthritis

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septic arthritis involving the knee

• [1] Relative warmth of the knee:
  • Often, the knees are cooler than the surrounding skin.
  • If the skin overlying the knee is warmer than the surrounding skin, this may be abnormal.
• [2] Quantitative comparison to the contralateral knee:
  • One study found that in septic arthritis, the affected knee's peak temperature was 3.2 +/- 1.2 ℃ higher than the contralateral knee, whereas in non-septic arthritis, this temperature differential was 1.3 +/- 1.1 ℃ (expressed as mean +/- sum of squares). (36836106) Further studies are required to define an optimal cutoff point. Note that nonseptic arthritis may elevate skin temperature, but not usually to the same extent as septic arthritis.

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surgical site infection

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prospective surveillance of wounds over time: early areas of hypoperfusion

• 48 hours after cesarean section, the emergence of cold areas along the wound (>2 ℃ cooler than surrounding skin) may predict subsequent wound infection. It's conceivable that these cold areas correspond to small areas of poorly vascularized tissue, which are at risk of infection. (27608511) A study of thoracic surgical wounds yielded similar findings. (36542868)

diagnosis of surgical site infection

• This remains poorly defined.
• It is unclear how much local inflammation is normal versus pathological. This may vary depending on the incision site and the timing after surgery.

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Horner's syndrome

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• Horner's syndrome involves ipsilateral miosis, ptosis, and anhydrosis.
• Anhydrosis may cause the affected side to have a higher skin temperature.
• Thermography has been proposed to help diagnose Wallenberg's syndrome (which causes Horner's syndrome and asymmetric skin temperature, potentially involving the entire body). (29993195)
• (Further discussion of Horner's syndrome is here.)

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questions & discussion

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