Florida Hospital Resident Presentation Oxygenation Florida Hospital Resident Presentation

Outline Structure and Function as Pertains to Oxygenation. Oxygenation Oxygenation Failure Scenarios Remedies

Structure and Function of the Respiratory System 2 components Conduction System and Gas Exchange System.

Gas Conduction System All airways utilized to move gas into and out of the lungs. Begins at the trachea and branches out progressively from there until the gas exchange system is reached. After about 16 divisions these airways become the respiratory zone airways with special properties. Massive increase in cross sectional area - 3m2 compared to 2.5cm2 at the trachea Loss of support

Massive cross sectional area At the level of the respiratory bronchioles and acini there is no longer tidal movement of gas - diffusion becomes the primary means of gas transport. You can image that air is at a standstill at these levels. The majority of air in the chest is at this level.

Gas Exchange System Formed of the structures that allow exchange of gases - the respiratory bronchioles, Acini, and Alveoli. Gas brought in by the conduction system is brought into very close proximity with blood to allow exchange.

The Setup Deoxygenated blood enters the lungs through the pulmonary artery and percolates through the alveolar capillaries. Breathing constantly refreshes the gas in the larger airways; Oxygen diffuses from the larger airways into the acini. Oxygen is carried away by the blood for utilization.

Ventilation Perfusion Matching The blood leaving the alveolus is oxygenated to the PaO2 of the gas in the acinus. The PaO2 of the gas in the acinus is determined by a balance of 2 things. How much oxygen is diffusing into the acinus (ventilation) How much oxygen is being sucked out of the acinus by blood (perfusion)

Ventilation Perfusion Matching This is a dynamic process. While oxygen is diffusing in; it is being sucked out. Imagine it as a bathtub being constantly filled by a faucet (oxygen diffusing in) while at the same time being drained (perfusion)

Alveolar Gas Equation

Diffusion Diffusion is an efficient process. The blood reaches equilibrium with the alveolar gas within ⅓ of the distance along the alveolar capillary. The rest of the distance is just a safety zone.

Oxygenation Failure Can occur due to failures of VQ matching Shunting Failures of diffusion

Oxygenation Failure VQ mismatches and shunts are the most common causes of hypoxemia. Diffusion as a cause of hypoxemia is rare.

VQ mismatch When ventilation is poor RELATIVE to perfusion the PaO2 in the alveolus decreases. The bathtub is draining faster than it can fill. The level of water is low. Blood reaches equilibrium with the alveolar gas and is hypoxic leaving the alveolus.

Shunt A shunt is a special case of VQ mismatch were the ventilation is essentially nil. Blood passing through a shunt is not exposed to oxygen and so cannot be oxygenated at all. This becomes important when we discuss remedies to hypoxemia.

Diffusion failures In disorders affecting the alveolar membrane it takes longer for the blood to reach equilibrium with the alveolar gas. The safety margin for diffusion can be eaten away when the blood moves faster such as with exertion. Diffusion failure is rare as a cause of hypoxemia in the ICU as compared to VQ mismatching/shunts.

Scenarios Blood enters the lung at the pulmonary artery saturating around 70%. Blood percolates through the lung picking up oxygen. Drains from the lung into the pulmonary vein. All blood coming out of lung mixes and gets pumped systemically.

Key point Blood cannot be saturated beyond 100%. Once blood is fully saturated it cannot carry any more oxygen. Segments that are hyperoxic (normal or high VQ) CANNOT correct for hypoxic segments (low VQ).

Pulmonary Embolism

Pneumonia

ARDS Shunt

Hypoxic Vasoconstriction Constitutive NOS generates a continuous infusion of NO into the pulmonary vasculature - as long as there is Oxygen available. In an area of lung that is poorly ventilated, NO is withdrawn and the vessels constrict.

Remedies to Hypoxemia Supplemental Oxygen Recruitment of lung Positioning Pulmonary vasodilators Paralytics ECMO

Supplemental Oxygen Adding water to the tub. Corrects the hypoxemia caused by poor ventilation in low VQ segments.

Recruitment of Lung Improving ventilation in low VQ segments with positive pressure.

Position to improve VQ matching Low VQ Segments Blood is taken from the low VQ segments to high VQ segments

Inhaled vasodilators Low VQ Segments Blood is taken from the low VQ segments to high VQ segments

Paralytics/Sedation

ECMO

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