Physics of the Airway - Clinical Tree

Key Points

An important formula that quantifies the relationship of pressure, flow, and resistance in laminar flow systems is given by the Hagen-Poiseuille equation. This law states that the fluid flow rate through a horizontal straight tube of uniform bore is proportional to the pressure gradient and the fourth power of the radius and is related inversely to the viscosity of the gas and the length of the tube. This law is valid for laminar flow conditions only.
When the flow rate exceeds a critical velocity (the flow velocity below which flow is laminar), the flow loses its laminar parabolic velocity profile, becomes disorderly, and is termed turbulent . If turbulent flow exists, the relationship between pressure drop and flow is no longer governed by the Hagen-Poiseuille equation. Instead, the pressure gradient required (or the resistance encountered) during turbulent flow varies as the square of the gas flow rate. In addition, flow becomes inversely related to gas density rather than viscosity (as occurs with laminar flow).
Clinically, airway-obstructing conditions such as epiglottitis or inhaled foreign bodies are often best modeled as breathing through an orifice. Under such conditions, the approximate flow across the orifice varies inversely with the square root of the gas density, in contrast to laminar flow conditions, in which gas flow varies inversely with gas viscosity. In such conditions, the low density of helium allows it to play a significant clinical role in the management of some forms of airway obstruction.
Laplace’s law predicts that for two alveoli of unequal size but equal surface tension, the smaller alveolus experiences a larger intraalveolar pressure than the larger alveolus, causing the smaller alveolus to collapse. In real life, however, collapse of the smaller alveolus is prevented through the action of pulmonary surfactant, which serves to decrease alveolar surface tension in the smaller alveolus, resulting in equal pressure in both alveoli.
One of the most important gas laws in physiology is the ideal (or perfect) gas law, PV = nRT (where P = pressure of gas, V = volume of gas, n = number of moles of the gas in volume V, R = gas constant, and T = absolute temperature). This is the equation for ideal gases, which experience no forces of interaction. Real gases, however, experience intermolecular attraction (van der Waals forces), which requires that the pressure-volume gas law be written in a more complex form.
Most flowmeters measure the drop in pressure that occurs when a gas passes through a known resistance and correlate this pressure drop to flow. When the resistance is an orifice, resistance depends primarily on gas density. Usually, a given flowmeter is calibrated for a particular gas, such as oxygen or air, with conversion tables available to provide flow data for other gases.
Fick’s law of diffusion, applicable to gas flow across lung and placental membranes, states that the rate of diffusion of a gas across a barrier is proportional to the concentration gradient for the gas and inversely proportional to the diffusion distance over which the gas molecules must travel.
There are five main types of oxygen analyzers: paramagnetic analyzers, fuel cell analyzers, oxygen electrodes, mass spectrometers, and Raman spectrographs. All respond to oxygen partial pressure and not to oxygen concentration.
The concept of minimum alveolar concentration (MAC) does not apply at higher altitudes and should be replaced by the concept of minimal alveolar partial pressure (MAPP).

The Gas Laws

Ideal Gases

Air is a fluid. Understanding the fundamentals of basic fluid mechanics is essential for grasping the concepts of airway flow. Because air is also a gas, it is important to understand the laws that govern its gaseous behavior. Gases are usually described in terms of pressure, volume, and temperature. Pressure is most often quantified clinically in terms of mm Hg (or torr), volume in mL, and temperature in degrees Celsius. However, calculations often require conversion from one set of units to another and therefore can be quite tedious. We have included a small section at the end of this chapter to simplify these conversions.

Perhaps the most important law governing the state of gas is the ideal gas law, which can be written as follows :

PV = nRT

$PV = nRT$

where

P = pressure of gas (pascals or mm Hg)

V = volume of gas (m ³ or cm ³ or mL) n = number of moles of the gas in volume V

R = gas constant (8.3143 J/mol·K, assuming P in pascals, V in m ³ )

T = absolute temperature (in kelvin [K]; 273.16 K = 0°C)

One mole of gas contains 6.023 × 10 ²³ molecules, and this quantity is referred to as Avogadro’s number . One mole of an ideal gas takes up 22.4138 L at standard temperature and pressure (STP); standard temperature is 273.16 K, and standard pressure is 1 atmosphere (760 mm Hg). Avogadro’s law also states that equal volumes of all ideal gases at the same temperature and pressure contain the same number of molecules.

The ideal gas law incorporates the laws of Boyle and Charles. Boyle’s law states that, at a constant temperature, the product of pressure and volume (P × V) is equal to a constant. Consequently, P is proportional to 1/V (P ∝ 1/V) at constant T. However, gases do not obey Boyle’s law at temperatures approaching their point of liquefaction (i.e., the point at which the gas becomes a liquid).

Boyle’s law concerns perfect gases and is not obeyed by real gases over a wide range of pressures (see the following section for a discussion of nonideal gases). However, at infinitely low pressures, all gases obey Boyle’s law. Boyle’s law does not apply to anesthetic gases and many other gases because of the van der Waals attraction between molecules (i.e., they are nonideal gases).

Charles’ law states that, at a constant pressure, volume is proportional to temperature (i.e., V ∝ T at constant P). Gay-Lussac’s law states that, at a constant volume, pressure is proportional to temperature (i.e., P ∝ T at constant V). Often, these two laws are shortened for convenience to Charles’ law. When a gas obeys both Charles’ law and Boyle’s law, it is said to be an ideal gas and obeys the ideal gas law.

In clinical situations, gases are typically mixtures of several “pure” gases. Quantifiable properties of mixtures may be determined using Dalton’s law of partial pressures. Dalton’s law states that the pressure exerted by a mixture of gases is the sum of the pressures exerted by the individual pure gases ^, :

P_{total} = P_{A} + P_{B} + P_{C} + \dots + P_{N}

$P_{total} = P_{A} + P_{B} + P_{C} + \dots + P_{N}$

where P _A , P _B , and P _C are the partial pressures of pure ideal gases.

Nonideal Gases: The van der Waals Effect

Ideal gases have no forces of interaction, but real gases have intermolecular attraction, which requires that the pressure-volume gas law be rewritten as follows ^, :

(P + \frac{a}{V^{2}}) \times (V - b) - nRT

$(P + \frac{a}{V^{2}}) \times (V - b) - nRT$

where

P = pressure of gas (pascals or mm Hg)

V = volume of gas (m ³ or cm ³ or mL) n = number of moles of the gas in volume V

R = gas constant (8.3143 J/mol⋅K, assuming P in pascals, V in m ³ )

T = absolute temperature (K) a and b = physical constants for a given gas

The values of a and b for a given gas may be found in physical chemistry textbooks and other sources. This equation, provided by van der Waals, accounts for intramolecular forces fairly well.

Diffusion of Gases

Clinically, diffusion of gases through a membrane is most applicable to gas flow across lung and placental membranes. The most commonly used relation to govern diffusion is Fick’s first law of diffusion, which states that the rate of diffusion of a gas across a barrier is proportional to the concentration gradient for the gas. Fick’s law may be expressed mathematically as follows :

Flux = - D \frac{Δ C}{Δ X}

$Flux = - D \frac{Δ C}{Δ X}$

where

Flux = the number of molecules crossing the membrane each second (molecules/cm ² ⋅s)

ΔC = the concentration gradient (molecules/cm ³ )

ΔX = the diffusion distance (cm)

D = the diffusion coefficient (cm ² /s)

In general, the value of D is inversely proportional to the gas’ molecular weight, as well as intrinsic properties of the membrane.

Because gases partially dissolve when they come into contact with a liquid, Henry’s law becomes important in some instances. It states that the mass of a gas dissolved in a given amount of liquid is proportional to the pressure of the gas at constant temperature. As a result, the gas concentration (in solvent) is equal to a constant × P (at constant T).

Pressure, Flow, and Resistance

The laws of fluid mechanics dictate an intricate relationship between pressure, flow, and resistance. Pressure is defined as force per unit area. It is usually measured clinically in mm Hg or cm H ₂ O, but it is most commonly measured scientifically in pascals (Pa), or newtons of force per square meter (1 Pa = 1 N/m ² ).

Flow (i.e., the rate of flow) is equal to the change in pressure (pressure drop or pressure difference) divided by the resistance experienced by the fluid. For example, if the flow is 100 mL/s at a pressure difference of 100 mm Hg, the resistance is 100 mm Hg/(100 mL/s), or 1 mm Hg⋅s/mL. In laminar flow systems only, the resistance is constant, independent of the flow rate. ^,

An important formula that quantifies the relationship of pressure, flow, and resistance in laminar flow systems is given by the Hagen-Poiseuille equation. Poiseuille’s law states that the fluid flow rate through a horizontal straight tube of uniform bore is proportional to the pressure gradient (ΔP) and the fourth power of the radius (π) and is inversely proportional to the viscosity of the gas (µ, in g/cm⋅s) and the length of the tube (L, in cm). This law, which is valid for laminar flow only, may be stated as follows ^, :

Δ P = \frac{8 μ L}{π^{4}} \times Flow

$Δ P = \frac{8 μ L}{π^{4}} \times Flow$

See the discussion in the section Laminar Flow for further details.

When the flow rate exceeds a critical velocity (the flow velocity below which flow is laminar), the flow loses its laminar parabolic velocity profile, becomes disorderly, and is termed turbulent ( Fig. 4.1 ). If turbulent flow exists, the relationship between pressure drop and flow is no longer governed by the Hagen-Poiseuille equation. Instead, the pressure gradient required (or the resistance encountered) during turbulent flow varies as the square of the flow rate. See the discussion in the section Turbulent Flow for further details.

Viscosity, µ, characterizes the resistance within a fluid to the flow of one layer of molecules over another (shear characteristics). Blood viscosity is influenced primarily by hematocrit, so that at low hematocrit blood flow is easier—that is, blood is more dilute. The critical velocity at which turbulent flow begins depends on the ratio of viscosity (µ) to density (ρ), which is defined as the kinematic viscosity (υ)—that is, υ = µ/ρ. (This is illustrated with an example in the section entitled Turbulent Flow.) The unit for viscosity is g/cm⋅s (poise). The typical unit for kinematic viscosity is cm ² /s.

The viscosity of water is 0.01 poise at 25°C and 0.007 poise at 37°C. The viscosity of air is 183 micropoise at 18°C.

Density is defined as mass per unit volume (g/cm ³ or g/mL). The density of water is 1 g/mL. The general relation for the density of a gas is given by the following equation:

D = D_{0} (\frac{T_{0} P}{{TP}_{0}})

$D = D_{0} (\frac{T_{0} P}{{TP}_{0}})$

where D ₀ is a known density of the gas at temperature T ₀ and pressure P ₀ , and D is the density of the gas at temperature T and pressure P. For dry air at 18°C and 760 mm Hg (atmospheric pressure), D = 1.213 g/L.

The fall in pressure at points of flow constriction (where the flow velocity is higher) is known as the Bernoulli effect ( Fig. 4.2 ). ^, This phenomenon is used in devices employing the Venturi principle, such as gas nebulizers, Venturi flowmeters, and some oxygen face masks. The lower pressure related to the Bernoulli effect sucks in (entrains) air to mix with oxygen.

Fig. 4.2, Bernoulli effect. (A) Diagram shows fluid flow through a tube with varying diameters. At the point of flow constriction, fluid pressure is less than at the distal end of the tube, as indicated by the height of the manometer fluid column. This effect is described by the Bernoulli equation. In the case of a horizontal pipe, the distance between the centerline of the pipe and an arbitrary datum at two different points will be the same (z). (B) Venturi tube. The lower pressure caused by the Bernoulli effect entrains air to mix with oxygen. P, Pressure; V, velocity.

One final consideration that is important in the study of the airway is Laplace’s law for a sphere ( Fig. 4.3 ). It states that, for a sphere with one air-liquid interface (e.g., an alveolus), the relation between the transmural pressure difference, surface tension, and sphere radius is described by the following equation :

P = \frac{2 T}{r}

$P = \frac{2 T}{r}$

Fig. 4.3, Laplace’s law for a sphere. (A) Laplace’s law dictates that for two alveoli of unequal size but equal surface tension, the smaller alveolus experiences a larger intraalveolar pressure than the larger alveolus. This causes air to pass into the larger alveolus and causes the smaller alveolus to collapse. (B) Collapse of the smaller alveolus is prevented through the action of pulmonary surfactant. Surfactant serves to decrease alveolar surface tension in the smaller alveolus, which results in equal pressure in both alveoli. P, Transmural pressure difference; r, sphere radius; T , surface tension.

where

P = transmural pressure difference (dynes/cm ² ; 1 dyne/cm ² = 0.1 Pa = 0.000751 torr)

T = surface tension (dynes/cm) r = sphere radius (cm)

The key point in Laplace’s law is that the smaller the sphere radius, the higher the transmural pressure. However, real (in vivo) alveoli do not obey Laplace’s law because of the action of pulmonary surfactant, which decreases the surface tension disproportionately compared with what is predicted on the basis of physical principles. When pulmonary surfactant is missing from the lungs, the lungs take on the behavior described by Laplace’s law.

Example: Transtracheal Jet Ventilation

Transtracheal jet ventilation (TTJV) can be used to oxygenate and ventilate patients in an emergent “cannot intubate, cannot oxygenate” (CICO) scenario. It is a temporizing measure that is used only until an airway can be secured. It is usually employed using equipment commonly available in the operating or emergency room and often using the 50-psi wall oxygen source. ^,

Analysis

The gas flow through a catheter depends on both the resistance of the catheter–connection hose assembly and the driving pressure applied to it. If the resistance of the assembly is R, the flow (F) from the catheter is F = P _d /R, where P _d is the pressure difference between the ends of the catheter-connection assembly. R itself certainly depends on F when the flow becomes turbulent, but the flow relationship still holds. However, P _d is very close to the driving pressure (P) applied to the ventilation catheter, because the lung offers little relative back pressure. (At back pressures greater than 100 cm H ₂ O, the lung is likely to burst, and P is often chosen to be 50 psi, or about 3500 cm H ₂ O.) Therefore, the flow relationship may be simplified to F = P/R.

Next, TTJV is applied through a sequence of “jet pulses,” each resulting in a given tidal volume (e.g., 500 mL). Ignoring entrained air effects, the delivered tidal volume is equal to catheter flow × pulse duration. For a catheter flow of 30 L/min, a jet pulse lasting 1 second results in a tidal volume of 30 L/min × $\frac{1}{16}$ $\frac{1}{16}$ min = 0.5 L.

In a TTJV setup consisting of a 14-G angiocatheter connected to a regulated oxygen source by a 4.5-foot polyvinyl chloride (PVC) tube of $\frac{7}{32}$ $\frac{7}{32}$ -inch inner diameter (ID), for oxygen flows between 10 and 60 L/min, the resistance was found to be relatively constant between 0.6 and 0.8 psi/L/min.

Many systems for TTJV choose 50 psi for convenience (50 psi being the oxygen wall outlet pressure), although a regulator is very often used to permit lower pressures. However, 50 psi may not be an optimal pressure choice for TTJV. Using the preceding data, the pressure required for TTJV for a tidal volume of 500 mL can be calculated. Assuming that the setup resistance is 0.7 psi/L/min and the desired flow rate is 30 L/min, the driving pressure should be 0.7 × 30 = 21 psi.

Similar analyses can be carried out for other arrangements derived from experiments to obtain resistance data.

Gas Flow

Laminar Flow

In laminar flow, fluid particles flow along smooth paths in layers, or laminas, with one layer gliding smoothly over an adjacent layer. Any tendencies toward instability and turbulence are damped out by viscous shear forces that resist the relative motion of adjacent fluid layers. Under laminar flow conditions through a tube, the flow velocity is greatest at the center of the tube flow and zero at the inner edge of the tube ( Fig. 4.4 ; see also Fig. 4.1 ). The flow profile has a parabolic shape. Under these conditions in a horizontal tube, the relation between flow, tube, and gas characteristics is given by the Hagen-Poiseuille equation ( Eq. 5 ), restated as follows :

\overset{\cdot}{V} = \frac{π Δ \overset{}{\Pr^{4}}}{8 μ L}

$\overset{\cdot}{V} = \frac{π Δ \overset{}{\Pr^{4}}}{8 μ L}$

where

V ˙ = flow rate (cm ³ /s)

π = 3.1416

ΔP = pressure gradient (Pa) r = tube radius (cm)

L = tube length (cm)

µ = gas viscosity (g/cm⋅s)

Typical units are shown in parentheses. The dot indicates rate of change: V represents volume, and V ˙ represents the rate of change of volume , or flow rate . Another way of looking at this concept is that, under conditions of laminar flow through a tube of known radius, the pressure difference across the tube is given by the following proportionality (which is also essentially the same as Eq. 5 ):

Δ Pressure \propto \frac{Flow \times Viscosity \times Length}{{Radius}^{4}}

$Δ Pressure \propto \frac{Flow \times Viscosity \times Length}{{Radius}^{4}}$

The pressure gradient through the airway increases proportionately with flow, viscosity, and tube length but increases exponentially as the tube radius decreases.

The conditions under which flow through a tube is predominantly laminar can be estimated from critical flow rates. The critical flow is the flow rate below which flow is predominantly laminar in a given airflow situation.

Laminar Flow Example

Assume a tube of uniform bore that is 1 cm in diameter and 3 m in length. A pressure difference of 5 cm H ₂ O exists between the ends of the tube, and air is the fluid flowing through the tube. Assuming laminar flow, what flow rate should be expected?

Answer:

The relevant variables are expressed in the centimeter-gram-second (CGS) system of units: r = 0.5 cm

L = 3000 cm

µ = 183 micropoise = 183 × 10 ⁻⁶ poise = 183 × 10 ⁻⁶ g/cm⋅s

ΔP = 0.5 cm H ₂ O = 490 dynes/cm ²

Using the Hagen-Poiseuille equation, the laminar flow is determined as follows:

Flow = \frac{π \times 490 \times {(0.5)}^{4}}{8 \times 183 \times 10^{- 6} \times 3000} = 219.06 {cm}^{3} /s

$Flow = \frac{π \times 490 \times {(0.5)}^{4}}{8 \times 183 \times 10^{- 6} \times 3000} = 219.06 {cm}^{3} /s$

Turbulent Flow

Flow in a tube below the critical flow rate remains mostly laminar. However, at flows greater than the critical flow rate, the flow becomes increasingly turbulent. Under turbulent flow conditions, the parabolic flow pattern is lost, and the resistance to flow increases with flow itself. Turbulence may also be created where sharp angles, changes in diameter, and branches are encountered ( Fig. 4.5 ). The flow-pressure drop relationship is given approximately by the following equation ^, :

V \propto \sqrt{Δ P}

$V \propto \sqrt{Δ P}$

Fig. 4.5, Turbulent flow. Four circumstances likely to produce turbulent flow.

where

V = mean fluid velocity (cm/s)

ΔP = pressure (Pa)

Reynolds Number Calculation Example

The Reynolds number (Re) represents the ratio of inertial forces to viscous forces. ^, ^, It is useful because it characterizes the flow through a long, straight tube of uniform bore. It is a dimensionless number having the following form:

Re = \frac{V \times D \times ρ}{μ} = \frac{V \times D}{v} = \frac{2 \times \overset{\cdot}{V} \times ρ}{π \times r \times μ}

$Re = \frac{V \times D \times ρ}{μ} = \frac{V \times D}{v} = \frac{2 \times \overset{\cdot}{V} \times ρ}{π \times r \times μ}$

where

Re = Reynolds number

V ˙ = flow rate (mL/s)

ρ = density (g/mL)

µ = viscosity (poise or g/cm⋅s) r = radius (cm)

υ = kinematic viscosity (cm ² /s) = µ/ρ

D = diameter (cm)

V = mean fluid velocity (cm/s)

Typical units are shown in brackets. For tubes that are long compared with their diameter (i.e., length ÷ diameter >0.06 × Re), the flow is laminar when Re is less than 2000. For shorter tubes, flow is turbulent at Re values as low as 280.

When a tube’s radius exceeds its length, it is an orifice; flow through an orifice is always turbulent. Under these conditions, the flow is influenced by the density rather than the viscosity of the fluid. This characteristic explains why a helium-oxygen mixture (heliox) flows better in a narrow edematous glottis: as the following data suggest, helium has a very low density and thus presents less resistance to flow through an orifice.

	Viscosity at 20°C	Density at 20°C
Helium	194.1 micropoise	0.179 g/L
Oxygen	210.8 micropoise	1.429 g/L

How can one predict whether a given gas flow through an endotracheal tube (ETT) is laminar or turbulent? One approach is first to identify the physical conditions. For example, consider the case of an ETT with a 6-mm ID and a length of 27 cm through which 60 L/min of air is passing. In this setting,

L = 27 cm r = 0.3 cm (size 6.0 ETT) flow ( V ˙ ) = 60 L/min = 1000 mL/s viscosity (µ) = 183 micropoise = 183 × 10 ⁻⁶ g/cm⋅s (air at 18°C) density (ρ) = 1.21 g/L = 0.001213 g/mL (dry air at 18°C)

With this information, one can calculate the Reynolds number:

Re = \frac{2 \times 1000 \times 0.001213}{π \times 0.3 \times 183 \times 10^{- 6}} = 1.41 \times 10^{4}

$Re = \frac{2 \times 1000 \times 0.001213}{π \times 0.3 \times 183 \times 10^{- 6}} = 1.41 \times 10^{4}$

Because this number greatly exceeds 2000, flow is probably quite turbulent.

Critical Velocity

The critical velocity is the point at which the transition from laminar to turbulent flow begins. This point is reached when Re becomes the critical Reynolds number, Re _crit . Critical velocity, the flow velocity below which flow is laminar, is calculated by the following equation :

V_{crit} = V_{c} = \frac{{Re}_{crit} \times Viscosity}{Density \times Diameter}

$V_{crit} = V_{c} = \frac{{Re}_{crit} \times Viscosity}{Density \times Diameter}$

where, Re _crit = 2000 for circular tubes.

As can be seen from this equation, the critical velocity is proportional to the viscosity of the gas and is related inversely to the density of the gas and the radius of the tube. Viscosity has the dimensions of pascal-second (Pa⋅s) (equivalent to N⋅s/m ² , or kg/m⋅s).

The critical velocity at which turbulent flow begins depends on the ratio of viscosity to density—that is, µ/ρ. This ratio is known as the kinematic viscosity , υ, and has typical units of centimeters squared per second (cm ² /s). The actual measurement of viscosity of a fluid is carried out with the use of a viscometer, which consists of two rotary cylinders with the test fluid flowing between.

Critical Velocity Calculation Example

Using the same data as in the previous Reynolds number calculation, one can calculate the critical velocity at which laminar flow starts to become turbulent:

\begin{matrix} V_{c} = \frac{2000 \times (183 \times 10^{- 6} poise)}{(0.001213 g / {cm}^{3}) \times (2 \times 0.3 cm)} \\ V_{c} = 502.8 \frac{poise}{({g/cm}^{3}) \times cm} = 502.8 \frac{cm}{s} \end{matrix}

$\begin{matrix} V_{c} = \frac{2000 \times (183 \times 10^{- 6} poise)}{(0.001213 g / {cm}^{3}) \times (2 \times 0.3 cm)} \\ V_{c} = 502.8 \frac{poise}{({g/cm}^{3}) \times cm} = 502.8 \frac{cm}{s} \end{matrix}$

Flow Through an Orifice

Flow through an orifice (defined as flow through a tube whose length is smaller than its radius) is always somewhat turbulent. Clinically, airway-obstructing conditions such as epiglottitis or swallowed obstructions are often best viewed as breathing through an orifice. Under such conditions, the approximate flow across the orifice varies inversely with the square root of the gas density:

\overset{\cdot}{V} \propto \frac{1}{\sqrt{Gas density}}

$\overset{\cdot}{V} \propto \frac{1}{\sqrt{Gas density}}$

This is in contrast to laminar flow conditions, in which gas flow varies inversely with gas viscosity. The viscosity values for helium and oxygen are similar, but their densities are very different ( Table 4.1 ). Table 4.2 provides useful data to allow comparison of gas flow rates through an orifice.

TABLE 4.1

Viscosity and Density Differences of Anesthetic Gases

	Viscosity at 300 K (µPa × s)	Density at 20°C (g/L)
Air	18.6	1.293
Nitrogen	17.9	1.250
Nitrous oxide	15.0	1.965
Helium	20.0	0.178
Oxygen	20.8	1.429

Data from Haynes WM. CRC Handbook of Chemistry and Physics . 91st ed. CRC Press; 2010, and Streeter VL, Wylie EB, Bedford KW. Fluid Mechanics . 9th ed. McGraw-Hill; 1998.

TABLE 4.2

Gas Flow Rates Through an Orifice

	%	Density (g/L)	(Density) ^−1/2	Relative Flow
Air	100	1.293	0.881	1.0
Oxygen	100	1.429	0.846	0.96
Helium (He)	100	0.179	2.364	2.68
He-oxygen	20/80	1.178	0.922	1.048
He-oxygen	60/40	0.678	1.215	1.381
He-oxygen	80/20	0.429	1.527	1.73

From Rudow M, Hill AB, Thompson NW, et al. Helium-oxygen mixtures in airway obstruction due to thyroid carcinoma. Can Anaesth Soc J. 1986;33:498.

Helium-Oxygen Mixtures

The low density of helium allows it to play a significant clinical role in the management of some forms of airway obstruction. For instance, Rudow and colleagues described the use of heliox mixtures in a patient with severe airway obstruction related to a large thyroid mass (see the following section for clinical examples).

The available percentage mixtures of helium and oxygen are typically 80:20 and 70:30. These mixtures are usually administered by a rebreathing face mask to patients who have an increased work of breathing due to airway pathology (e.g., edema) but for whom it is preferable to withhold endotracheal intubation at the time.

Although the use of heliox mixtures in patients with upper airway obstruction has had considerable success, the hope that this approach would also work well for patients with severe asthma has not been borne out. In a systematic review of seven clinical trials involving 392 patients with acute asthma, the authors cautioned, “existing evidence does not provide support for the administration of helium-oxygen mixtures to emergency department patients with moderate-to-severe acute asthma.” A similar study noted that “heliox may offer mild-to-moderate benefits in patients with acute asthma within the first hour of use, but its advantages become less apparent beyond 1 hour, as most conventionally treated patients improve to similar levels, with or without it”; however, the authors suggested that its effect “may be more pronounced in more severe cases.” They concluded, “There are insufficient data on whether heliox can avert endotracheal intubation, or change intensive care and hospital admission rates and duration, or mortality.”

Clinical Vignettes

Rudow and colleagues reported the following clinical illustration of heliox therapy. A 78-year-old woman with both breast cancer and ophthalmic melanoma developed airway obstruction from a thyroid carcinoma that extended into her mediastinum and compressed her trachea. She had a 2-month history of worsening dyspnea, especially when positioned supine. On examination, inspiratory stridor and expiratory stridor were present. The chest radiograph showed a large superior mediastinal mass and pulmonary metastases. A solid mass was identified on a thyroid ultrasound scan. Computed tomography revealed a large mass at the thoracic inlet and extending caudally. Clinically, the patient was exhausted and in respiratory distress.

A 78:22 heliox mixture was administered and provided almost instant relief, with improvements in measured tidal volume and oxygenation. Later, a thyroidectomy was carried out to alleviate the obstruction. For this procedure, topical anesthesia was applied to the airway and awake laryngoscopy and intubation were performed with the patient in the sitting position. After the airway was secured with the use of an armored tube, the patient was given a general anesthetic by intravenous induction. Extubation after the surgery was performed without complication.

Another interesting clinical scenario was published by Khanlou and Eiger. They presented the case of a 69-year-old woman in whom bilateral vocal cord paralysis developed after radiation therapy. Heliox was successfully used for temporary management of the resultant upper airway obstruction until the patient was able to receive a tracheostomy.

A final clinical vignette was reported by Polaner, who used a laryngeal mask airway (LMA) and an 80:20 heliox mixture to administer anesthesia to a 3-year-old boy with asthma and a large anterior mediastinal mass. Clinical management involved an unusual combination of management strategies: The child was kept in the sitting position, spontaneous ventilation with a halothane-in-heliox inhalation induction was used, and airway stimulation was minimized by use of an LMA. However, the author cautioned that cases such as these can readily take a deadly turn, noting that “one must, of course, always be prepared to intervene with either manipulations of patient position in the event of airway compromise (including upright, lateral, and prone) or more aggressive strategies, such as rigid bronchoscopy and even median sternotomy (in the case of intractable cardiovascular collapse), or to allow the patient to awaken if critical airway or cardiovascular compromise becomes evident at any time during the course of the anesthesia.”

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