Comprehensive Notes on Atmospheric Pressure and Pulmonary Ventilation and Respiratory Mechanics

Definitions of Ventilation and Respiration

Ventilation Definition: Ventilation is defined as the bulk movement of air or gas in and out of the lungs and the airway. This process involves inhaling oxygen to replenish the body’s supply and exhaling carbon dioxide ( $CO_2$ ) to remove waste gas.
Patterns of Ventilation: The effectiveness of gas exchange is influenced by the pattern, depth, and rate of ventilation.
Respiration Definition: Respiration refers specifically to the process of gas exchange, which is distinct from the mechanical movement of air (ventilation).
Internal vs. External Respiration: * External Respiration: This is the gas exchange that occurs between the alveoli in the lungs and the surrounding pulmonary capillaries. * Internal Respiration: This is the gas exchange that occurs between the systemic capillaries and the body tissues.
Professional Requirements for Respiratory Therapists (RTs): Future Registered Respiratory Therapists (RRTs) must have a comprehensive understanding of pulmonary ventilation mechanisms, atmospheric gases, atmospheric pressures, the elastic properties of the lung and chest wall, and the dynamic characteristics of moving gas. They must also recognize normal and abnormal ventilatory patterns.

Composition of Atmospheric Gases

Room Air Oxygen Levels: The air in the current atmosphere consists of approximately $21\%$ oxygen. More specifically, the concentration is $20.95\%$ .
Supplemental Oxygen: In cases of lung trauma or blood disorders that impede oxygen transport, supplemental oxygen is provided via devices such as nasal cannulas or tracheal tubes. These provide oxygen at levels exceeding the atmospheric $21\%$ .
Fraction of Inspired Oxygen ( $FiO_2$ ): This term describes the fractional concentration of oxygen in the inspired gas.
Main Constituents of Atmosphere: * Nitrogen ( $N_2$ ): Makes up the vast majority of the atmosphere at $78\%$ . * Oxygen ( $O_2$ ): Makes up approximately $21\%$ . * Argon: Makes up $0.9\%$ .
Trace Gases: These are gases present in concentrations less than $1\%$ , including: * Carbon Dioxide ( $CO_2$ ): While textbooks often reference $0.03\%$ , the speaker notes a value of $0.4\%$ . * Neon, Helium, Methane, Krypton, and Nitric Oxide.

Atmospheric Pressure and Altitude

Atmospheric Layers: Gas layers in the atmosphere are divided based on temperature and are held in place by the Earth’s gravitational pull.
Definition of Atmospheric Pressure: The weight of the gas molecules in the column of air above us exerts a force known as atmospheric pressure.
Baseline at Sea Level: Sea level (flat regions like Shreveport, Louisiana, or New Orleans) is characterized by a standard barometric pressure of $760\,mmHg$ (millimeters of mercury).
Dynamics of High Altitude: * Cities at high altitudes include Denver (Colorado), Montana, and Lima (Peru). * Air at high altitudes is described as "thin" and "dry." * The percentage of oxygen remains constant at $21\%$ regardless of altitude. However, the total atmospheric pressure (the weight of the gas column above) is significantly lower, which decreases the total pressure of oxygen available to be pushed into the lungs. * Mount Kilimanjaro Example: At the tip of the mountain, the barometric pressure ( $P_B$ ) is approximately $345\,torr$ . This results in a much lower partial pressure of oxygen than at sea level ( $345 \times 0.21 = 72.45\,mmHg$ ). * Clinical Significance: Lower oxygen pressure at high altitudes makes it difficult for oxygen to cross the alveolar-capillary membrane into the blood, leading to altitude sickness or requiring supplemental oxygen, especially for those with pulmonary disease.

Dalton’s Law of Partial Pressures

Dalton’s Law: This law states that each gas in a mixture exerts a partial pressure that is proportional to its fractional concentration in the total volume.
Total Pressure at Sea Level: Total pressure is $760\,mmHg$ , which is considered $100\%$ of the atmospheric gas pressure.
Calculating Partial Pressure: Partial pressure is calculated as $P = \text{Fraction of Gas} \times P_B$ . * Nitrogen ( $P_{N_2}$ ): $0.78 \times 760\,mmHg = 592.8\,mmHg$ (often rounded to $593\,mmHg$ ). * Oxygen ( $P_{O_2}$ ): $0.21 \times 760\,mmHg = 159.6\,mmHg$ (often rounded to $160\,mmHg$ ).
Oxygen in the Lungs: Once oxygen enters the lungs, its partial pressure changes because the body warms the air to $37^{\circ}\text{C}$ and adds $100\%$ relative humidity (water vapor), which takes up space and pressure (referred to as the Alveolar Air Equation).

Measurement Units and Conversions

Standard Pressure Units: * Millimeters of Mercury ( $mmHg$ ). * Torr: A unit equivalent to $mmHg$ ( $1\,torr = 1\,mmHg$ ). * Kilopascals ( $kPa$ ): Used in the International System of Units (SI), commonly found in Europe. * Centimeters of Water Pressure ( $cmH_2O$ ): The primary unit used for respiratory and ventilating pressures.
Conversion Factors: * $1\,mmHg = 1.36\,cmH_2O$ * $1\,cmH_2O = 0.74\,mmHg$
Mathematical Examples: * Converting $5\,mmHg$ to $cmH_2O$ : $5 \times 1.36 = 6.8\,cmH_2O$ . * Converting $5\,cmH_2O$ to $mmHg$ : $5 \times 0.74 = 3.7\,mmHg$ .

Pulmonary Pressure Baselines

Atmospheric Baseline: In pulmonary medicine, atmospheric pressure ( $760\,mmHg$ ) is treated as a baseline of zero. This is one Atmosphere ( $1\,ATM$ ).
Positive Pressure Example: If a patient is placed on a CPAP (Continuous Positive Airway Pressure) of $10\,torr$ , the actual pressure in the lung is $760 + 10 = 770\,torr$ .
Negative Pressure (Suction) Example: If a negative pressure of $-8\,mmHg$ is generated in the trachea (e.g., during suctioning), the actual pressure is $760 - 8 = 752\,mmHg$ .

Mechanism of Breathing: Gas Movement and Boyle’s Law

Pressure Gradients: Gases (and liquids) move from areas of high pressure to areas of low pressure. This movement is called moving "down the pressure gradient."
Primary Principle of Respiration: For air to move, a pressure gradient must be established between the atmosphere and the alveoli. * Inspiration Requirement: Alveolar pressure must be lower than barometric pressure ( $P_{alv} < P_B$ ). * Expiration Requirement: Alveolar pressure must be higher than barometric pressure ( $P_{alv} > P_B$ ).
Boyle’s Law: States that the volume ( $V$ ) of a gas varies inversely with its pressure ( $P$ ) at a constant temperature. The formula is $P_1 \times V_1 = P_2 \times V_2$ . * If volume decreases, pressure increases. * If volume increases, pressure decreases.
Mechanics of Inspiration: * Diaphragm: The primary muscle of inspiration. At rest, it is dome-shaped. * Phrenic Nerve: Innervates the diaphragm; originates at cervical spine levels $C3$ , $C4$ , and $C5$ ("3, 4, 5 keep the diaphragm alive"). * Process: The phrenic nerve triggers the diaphragm to contract and descend, which increases the volume of the thoracic cavity (closed system). Increasing the volume causes the pressure in the pleural space to become more negative. This negative pressure is transmitted to the thin-walled alveoli, making the alveolar pressure lower than atmospheric pressure, causing air to rush into the lungs.
Mechanics of Expiration: * Passive Process: There is no primary muscle of expiration. It relies on the elastic recoil of the lungs. * Process: The diaphragm stops contracting and returns to its dome shape, and the elastic lungs recoil. This decreases the volume of the thoracic cavity, which increases the pressure. The pleural pressure becomes less negative, making alveolar pressure greater than barometric pressure, which forces air out of the lungs. * Accessory Muscles: While not used in normal expiration, accessory muscles can be utilized in forced expiration.
COPD Variations: Patients with COPD often have flattened diaphragms and must use accessory muscles to pull the chest upward and outward to increase thoracic volume.

The Respiratory Cycle

One Respiratory Cycle: Defined as the sequence of Inspiration -> End-Inspiration -> Expiration -> End-Expiration.
Points of Equilibrium: At "End-Inspiration" and "End-Expiration," there is zero gas flow because the barometric pressure and the alveolar pressure have equalized.
Physiological Significance of Air Retention: The lungs never lose all their air (volume) because it would take too much energy to re-inflate them from a collapsed state. This is related to concepts of Positive End-Expiratory Pressure (PEEP). When someone has "the breath knocked out of them," they have lost a significant portion of this residual volume and must work hard to reinflate their lungs.

Questions & Discussion

Question: Does humidity affect breathing?
Response: While high humidity makes the air feel "thicker" or heavier to breathe, the oxygen percentage remains the same. The upper airway is designed to heat and humidify air to optimal levels for the lower airway. Barometric pressure (altitude) has a greater impact on the actual pressure of oxygen in the lungs than humidity does.
Discussion on Mountain Altitudes: The class discussed that more people can climb Mount Kilimanjaro than Mount Everest. A barometric pressure check for Mount Kilimanjaro confirmed it is approximately $345\,torr$ .
Question: Is there gas flow at the end of a breath?
Response: No, there is no gas flow at the very end of inspiration or expiration because the pressures are equal. Continuous movement of gas would require too much energy.