Abstract
Physical factors such as temperature, pressure, altitude, and humidity affect gases in particular and, thus, should be well understood by the critical care practitioner. A number of useful tables, formulas, and figures follow. Thermal injuries are commonly considered environmental events, and, thus, these formulas and figures are included in this chapter as well.
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Physical factors such as temperature, pressure, altitude, and humidity affect gases in particular and, thus, should be well understood by the critical care practitioner. A number of useful tables, formulas, and figures follow. Thermal injuries are commonly considered environmental events, and, thus, these formulas and figures are included in this chapter as well.
1 Temperature
Temperature conversion calculations are often done in the management of critically ill patients. Degrees Celsius (°C) and Fahrenheit (°F) are most commonly utilized:
°C to °F
°F to °C
Occasionally, the Kelvin (K) temperature scale is used, primarily in gas law calculations:
K to °C
°C to K
2 Humidity
Relative Humidity
Relative humidity (RH) is usually measured by hygrometers, thus eliminating the need of extracting and measuring the humidity content of the air samples:
Humidity Deficit
The humidity deficit (HD) represents the maximum humidity capacity at body temperature:
where
-
capacity = amount of water the alveolar air can hold at body temperature (also known as absolute humidity)
-
content = humidity content of inspired air (see Table 3.1):
3 Pressure
Pressure is defined as force per unit area, and there are various ways of measuring this force. One way is that force can be recorded in a form of the height of a column as in the mercury barometer; therefore, it can be recorded in milliliters of mercury (mmHg) pressure or centimeters of water pressure.
To Convert cmH2O to mmHg
To Convert mmHg to cmH2O
Pressure Per Square Inch
A less commonly used conversion in clinical medicine includes converting Psi (pressure per square inch) to mmHg:
Pressure-Related Formulas
Other useful pressure-related formulas/facts include:
Pressure/Volume Relationships
Useful pressure/volume relationships that can be used in the management of critically ill patients include:
where
-
VolumeBTPS = gas volume saturated with water at body temperature (37 °C) and ambient pressure [BTPS = barometric temperature pressure saturation]
-
VolumeATPS = gas volume saturated with water at ambient (room) temperature and pressure [ATPS = ambient temperature pressure saturation]
-
Factor represents the factors for converting gas volumes from ATPS to BTPS:
See also Table 3.2:
4 Altitude
As altitude varies, changes in atmospheric pressure produce alterations in gas density (see Table 3.3):
5 Burns
To estimate the extent of burn, the rule of nines for body surface area (BSA) is commonly used:
-
Adults: Arms 9 % each; legs 18 % each; head 9 %; trunk 18 % anterior, 18 % posterior; genitalia 1 %.
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Children: Arms 9 % each; legs 16 % each; head 13 %; trunk 18 % anterior, 18 % posterior; genitalia 1 %.
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Infants: Arms 9 % each; legs 14 % each; head 18 %; trunk 18 % anterior, 18 % posterior; genitalia 1 %.
-
In addition, the Lund and Browder chart (Fig. 3.1) can be used (more accurate in children).
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© 2014 Springer Science+Business Media New York
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Varon, J., Fromm, R.E. (2014). Environmental Facts and Formulas. In: Acute and Critical Care Formulas and Laboratory Values. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-7510-1_3
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DOI: https://doi.org/10.1007/978-1-4614-7510-1_3
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