Fundamentals

Gas Pressure, Units, and Basic Concepts

• Three main pressure units encountered in respiratory care:

  • millimeters of mercury (mmHg) and torr (torr ∼ mmHg)

  • centimeters of water (cmH₂O) for many respiratory devices

  • pounds per square inch (psi) for cylinders and some equipment

• Other related units you may see (less commonly used in this course): atmospheres (atm), inches of water, inches of mercury

• At sea level, standard atmospheric pressure is: $1~\text{atm} = 760~\text{mmHg}$

• Everyday reference: 1 atmosphere corresponds to about 33 feet of water column; this is a rough physical intuition rather than a practical calculation in therapy screens

• The main pressure concepts you’ll need to select correct values on tests: inspired oxygen partial pressure (PIO₂), alveolar oxygen partial pressure (PAO₂), and total pressures as sums of partial pressures (Dalton’s law)

Oxygen in the Atmosphere and Partial Pressures

• The ambient atmosphere contains 21% oxygen by volume: $\text{FiO₂} = 0.21\,.$

• Partial pressure of inspired O₂ in air depends on barometric pressure (PB) and water vapor pressure (PH₂O):

  • In dry gas at sea level: $P<em>{IO₂} = P</em>B \times FiO₂$

  • When considering humidified air (air entering the lungs after water vapor), use: $P<em>{IO₂} = (P</em>B - P<em>{H</em>2O}) \times FiO₂$

• At sea level, typically: $P<em>B = 760~\text{mmHg}, \quad P</em>{H_2O} \approx 47~\text{mmHg}$

• Example: room air inhalation (FiO₂ = 0.21):

  • Without subtracting water vapor: $P_{IO₂} = 760 \times 0.21 \approx 159~\text{mmHg}$

  • With water vapor subtraction (PB - PH₂O = 713): $P_{IO₂} = 713 \times 0.21 \approx 150~\text{mmHg}$

The Alveolar Gas Equation (PAO₂)

• The alveolar gas equation relates alveolar O₂ to barometric pressure, humidity, FiO₂, and PaCO₂ with a respiratory quotient (R):

  • General form: $P<em>{AO₂} = \big(P</em>B - P<em>{H</em>2O}\big) \cdot FiO₂ - \frac{P_{aCO₂}}{R}$

• Example values and calculation used in lecture:

  • PB = 760 mmHg, PH₂O = 47 mmHg, FiO₂ = 0.55, PaCO₂ = 48 mmHg, R = 0.80

  • Compute: $P_{AO₂} = (760 - 47) \cdot 0.55 - \frac{48}{0.8} = 713 \cdot 0.55 - 60 = 392.15 - 60 \approx 332~\text{mmHg}$

• Why this matters: PAO₂ represents the available oxygen tension at the alveolar site; if measured PaO₂ is lower than expected (e.g., 80–100 mmHg), diffusion, shunt, or ventilation–perfusion mismatch may be implicated

• A related, simpler calculation shown:

  • In atmosphere, with FiO₂ = 0.55, PB − PH₂O = 713, the alveolar equation yields approximately 332 mmHg under the given conditions

The Gas Laws (Foundational Concepts for Respiratory Therapy)

• Avogadro’s Law (brief, less tested): equal volumes of gas at the same T and P contain the same number of molecules. Focus is on molecule count; not a common direct exam question here

• Bernoulli’s Principle (Venturi masks and entrainment)

  • As velocity of gas increases, the surrounding pressure decreases

  • Venturi masks use this to entrain ambient air to achieve a precise FiO₂ by changing the adapter size and the entrained flow

  • Example: a Venturi device may deliver 40% FiO₂ by delivering 12 L/min of 100% O₂ through a narrow jet, entraining room air to reach the target FiO₂

• Boyle’s Law (P↔V, inversely proportional at constant T)

  • In our breathing, diaphragm downward movement expands the lung volume, decreasing intrapulmonary pressure and drawing air in

• Charles’ Law (V ∝ T at constant P)

  • Volume changes with temperature when pressure is held constant

• Gay-Lussac’s Law (P ∝ T at constant V)

  • Pressure changes with temperature when volume is held constant

• Dalton’s Law of Partial Pressures

  • Total pressure is the sum of the partial pressures: P{tot} = P{1} + P{2} + P{3} + \

Gas Pressure, Units, and Basic Concepts

• Three main pressure units encountered in respiratory care:

  • millimeters of mercury (mmHg) and torr (torr

• mmHg). These are commonly used for physiological measurements like blood pressure and arterial gas tensions.

  • centimeters of water (cmH

O) for many respiratory devices. This unit is often seen in ventilator settings, CPAP, and BiPAP devices due to its relevance to airway pressures.

• pounds per square inch (psi) for cylinders and some equipment. This unit indicates the high pressures involved in compressed gas cylinders and related regulators.

  • Other related units you may see (less commonly used in this course): atmospheres (atm), inches of water, inches of mercury. Understanding these units is crucial for accurate equipment operation, monitoring, and patient assessment.

  • At sea level, standard atmospheric pressure is: 1~\text{atm} = 760~\text{mmHg}$</p></li><li><p>Everyday reference: 1 atmosphere corresponds to about 33 feet of water column; this is a rough physical intuition rather than a practical calculation in therapy screens. This illustrates the significant force exerted by atmospheric pressure.</p></li><li><p>The main pressure concepts you’ll need to select correct values on tests: inspired oxygen partial pressure (PIO</p></li></ul></li></ul><p>), alveolar oxygen partial pressure (PAO</p><p>), and total pressures as sums of partial pressures (Dalton’s law).</p><h5 collapsed="false" seolevelmigrated="true">Oxygen in the Atmosphere and Partial Pressures</h5><ul><li><p>The ambient atmosphere contains 21$\text{FiO₂} = 0.21\,.$This is a constant value under normal conditions.</p></li><li><p>Partial pressure of inspired O</p><p>in air depends on barometric pressure (PB) and water vapor pressure (PH₂O):</p><ul><li><p>In dry gas at sea level:$\text{P_IO₂} = \text{P_B} \times \text{FiO₂}$</p></li><li><p>When considering humidified air (air entering the lungs after water vapor), use:$\text{P_IO₂} = (\text{P_B} - \text{P_H_2O}) \times \text{FiO₂}$. Water vapor displaces other gases in the air, so its pressure must be subtracted from the total barometric pressure to find the partial pressure of the dry gases.</p></li><li><p>At sea level, typically:$\text{P_B} = 760~\text{mmHg}, \quad \text{P_H_2O} \approx 47~\text{mmHg}$(at body temperature$37^{\circ}\text{C}$).</p></li></ul></li><li><p>Example: room air inhalation (FiO₂ = 0.21):</p><ul><li><p>Without subtracting water vapor:$\text{P_IO₂} = 760 \times 0.21 \approx 159~\text{mmHg}$</p></li><li><p>With water vapor subtraction (PB - PH₂O = 713):$\text{P_IO₂} = 713 \times 0.21 \approx 150~\text{mmHg}$The latter calculation is more physiologically accurate for gases reaching the alveoli.</p></li></ul></li></ul><h5 collapsed="false" seolevelmigrated="true">The Alveolar Gas Equation (PAO₂)</h5><ul><li><p>The alveolar gas equation relates alveolar O₂ to barometric pressure, humidity, FiO₂, and PaCO₂ with a respiratory quotient (R):</p><ul><li><p>General form:$\text{P_AO₂} = \big(\text{P_B} - \text{P_H_2O}\big) \cdot \text{FiO₂} - \frac{\text{P_aCO₂}}{\text{R}}$. The respiratory quotient (R) typically ranges from 0.8 to 1.0 and represents the ratio of carbon dioxide production to oxygen consumption within the body.</p></li><li><p>Example values and calculation used in lecture:</p></li><li><p>PB = 760 mmHg, PH₂O = 47 mmHg, FiO₂ = 0.55, PaCO₂ = 48 mmHg, R = 0.80</p></li><li><p>Compute:$\text{P_AO₂} = (760 - 47) \cdot 0.55 - \frac{48}{0.8} = 713 \cdot 0.55 - 60 = 392.15 - 60 \approx 332~\text{mmHg}$This calculation helps determine the driving pressure for oxygen to diffuse from the alveoli into the blood.</p></li></ul></li><li><p>Why this matters: PAO₂ represents the available oxygen tension at the alveolar site; if measured PaO₂ is lower than expected (e.g., 80–100 mmHg), diffusion, shunt, or ventilation–perfusion mismatch may be implicated. It is a critical value for assessing lung function.</p></li><li><p>A related, simpler calculation shown:</p><ul><li><p>In atmosphere, with FiO₂ = 0.55, PB − PH₂O = 713, the alveolar equation yields approximately 332 mmHg under the given conditions.</p></li></ul></li></ul><h5 collapsed="false" seolevelmigrated="true">The Gas Laws (Foundational Concepts for Respiratory Therapy)</h5><ul><li><p>Avogadro’s Law (brief, less tested): equal volumes of gas at the same T and P contain the same number of molecules. Focus is on molecule count; not a common direct exam question here, but fundamental to understanding gas molar volumes.</p></li><li><p>Bernoulli’s Principle (Venturi masks and entrainment)</p><ul><li><p>As velocity of gas increases, the surrounding pressure decreases. This principle is vital in respiratory devices.</p></li><li><p>Venturi masks use this to entrain ambient air to achieve a precise FiO₂ by changing the adapter size and the entrained flow. This allows for precise oxygen delivery beyond simple flow rates, ensuring accurate FiO₂ for patients.</p></li><li><p>Example: a Venturi device may deliver 40$\text{P_tot} = \text{P_1} + \text{P_2} + \text{P_3} + \text{…}$$ This law is fundamental for understanding gas exchange in the lungs, as