Environmental Facts and Formulas

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.

These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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

$$ ^\circ {\bf F}=(^\circ{\rm C}\times 9/5)+32$$

°F to °C

$$ ^\circ {\bf C}=(^\circ{\rm F}-32)\times 5/9$$

Occasionally, the Kelvin (K) temperature scale is used, primarily in gas law calculations:

K to °C

$$ {\bf K}=^\circ{\rm C}+273$$

°C to K

$$ ^\circ {\bf C}={\rm K}-273$$

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:

$$ \mathbf{RH}=\frac{{\mathrm{Content}\;[\mathrm{mg}/\mathrm{L}\ \mathrm{or}\ \mathrm{mm}\ \mathrm{Hg}]}}{{\mathrm{Capacity}\;[\mathrm{mg}/\mathrm{L}\ \mathrm{or}\ \mathrm{mm}\ \mathrm{Hg}]}}=\% $$

Humidity Deficit

The humidity deficit (HD) represents the maximum humidity capacity at body temperature:

$$ {\bf HD}=\text{Capacity}-\text{content}=\text{mg}/\text{L}$$

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):

Table 3.1 Humidity capacity of saturated gases from 0 to 43 °C

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

$$ {\bf mmHg}={\text{cmH}}_{2}\text{O}\times 0.735$$

To Convert mmHg to cmH₂O

$$ {\bf cm{H}_{2}O}=\text{mmHg}\times 1.36$$

Pressure Per Square Inch

A less commonly used conversion in clinical medicine includes converting Psi (pressure per square inch) to mmHg:

$$ {\bf mmHg}=\text{Psi}\times 51.7$$

Pressure-Related Formulas

Other useful pressure-related formulas/facts include:

$$ \mathbf{Total}\;\mathbf{pressure}={P_1} + {P_2} + {P_3}+\cdots\,(\mathrm{Dalton}\hbox{'}\mathrm{s}\ \mathrm{Law}) $$

$$ \begin{array}{ll} {\mathbf{1}\;\mathbf{atmosphere} = 760\ \mathrm{mmHg}} {= 29.921\ \mathrm{in}\ \mathrm{Hg}}{ = 33.93\ \mathrm{ft}\ {{\mathrm{H}}_2}\mathrm{O}} { = 1,034\ \mathrm{cm}\ {{\mathrm{H}}_2}\mathrm{O}} \\ \qquad\qquad\qquad\quad{ = 1,034\ \mathrm{g}/\mathrm{c}{{\mathrm{m}}^2}} { = 14.7\ \mathrm{lb}/\mathrm{in}{.^2}} \end{array} $$

Pressure/Volume Relationships

Useful pressure/volume relationships that can be used in the management of critically ill patients include:

$$ {\bf {Volume_{BTPS}}}={\text{Volume}}_{\text{ATPS}}\times \text{Factor}$$

where

• Volume_BTPS = gas volume saturated with water at body temperature (37 °C) and ambient pressure [BTPS = barometric temperature pressure saturation]

• Volume_ATPS = 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:

$$ \mathbf{Conversion}\;\mathbf{factor}=\frac{{{P_{\mathrm{B}}}-P{{\mathrm{H}}_2}\mathrm{O}}}{{{P_{\mathrm{B}}}-47}}\times \frac{310 }{{\left( {273+^{\circ}\mathrm{C}} \right)}} $$

See also Table 3.2:

Table 3.2 Factors for converting gas volumes from ATPS to BTPS

4 Altitude

As altitude varies, changes in atmospheric pressure produce alterations in gas density (see Table 3.3):

Table 3.3 Changes in density with altitude assuming a constant temperature

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 %.

• Children: Arms 9 % each; legs 16 % each; head 13 %; trunk 18 % anterior, 18 % posterior; genitalia 1 %.

• 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).

  Fig. 3.1

  

  Lund and Browder chart for estimation of burn extent