Ventilation

Front 1

Mech of ventilation

Back 1

-to initiate a breath, a drop in pressure in the alveoli must precipitate airflow into the lungs
-according to the laws of physics, the movement of gases is always from an area of higher pressure to lower pressure
-airway pressure: is the pressure in the conducting airways
-intrapleural pressure: is the pressure in the narrow space between the visceral and parietal pleurae
-intra-alveolar pressure: the pressure inside the alveoli
-transpulmonary pressure: the pressure difference between the intra-alveolar pressure and the intrapleural pressure
-intrathoracic pressure: the pressure within the entire thoracic cavity
-pleural pressure, a slightly negative pressure, creates a suction that holds the lungs open to their resting level
-when the glottis is open and no air is flowing, the pressure in the conducting airways and alveoli equals atmospheric pressure

Front 2

Lung compliance

Back 2

-the extent to which the lungs expand is called compliance
-compliance is how easily a tissue is expressed
-if compliance is reduced, it is more difficult to expand the lungs for inspiration
-if compliance is increased, it is easier to expand lung tissue
-collagen fibers resist stretching and make lung inflation difficult, elastin fibers are easily stretched and increase the ease of lung inflation
-when elastin fibers are replaced with scar tissue, such as that which occurs with pulmonary fibrosis or interstitial lung disease, the lungs become stiff and noncompliant
-surfactant lowers the surface tension, increases lung compliance and ease of inflation, provides for stability and more even inflation of the alvioli, and assists in preventing pulmonary edema by keeping the alveoli dry
-surfactant does not mature until the 26th-28th weeks of gestation
-premature infants do not have sufficient amounts of surfactant, which leads to alveolar collapse and severe resp distress
-Compliance refers to the ability of the lung to distend, compared to a balloon
-Initially it is hard to inflate a balloon until it is stretch (noncompliant)
-After repeated inflations, this elastic resistance is lost
-Overly compliant means the balloon (or lungs) becomes very easy to blow up
-As the volume of gas is delivered to a pt on a mechanical ventilator, the ventilators pressure gauge slowly rises from zero to PIP
-The rise in pressure is caused by the airways resistance to flow, as well as by lung and chest wall compliance
-Dynamic pressures and PIP can give an indication of both airway resistance and lung compliance
-When the compliance is low, more pressure will be need to deliver a given volume of gas to a patient. Disease states resulting in low compliance include the Adult Respiratory Distress Syndrome (ARDS), pulmonary edema, pneumonectomy, pleural effusion, pulmonary fibrosis, and pneumonia among others. Emphysema is a typical cause of increased lung compliance.

Front 3

Airway resistance

Back 3

-airflow in the conducting airways is affected by the resistance that air encounters as it moves through the airways
-in the respiratory airways, small changes in airway diameter can have enormous effects on airflow resistance
-conditions that decrease airway diameter, such as those caused by pulmonary secretions or bronchospasm, marked increases in airway resistance occur
-to maintain the same rate of airflow as before the onset of increased airway resistance, people with these conditions must increase the driving pressure, or respiratory rate, to move air

Front 4

Assessment of ventilation

Back 4

-Minute ventilation is the volume of air inhaled and exhaled per minute
-not all the air that enters the airways reaches the alveoli where gas exchange take place
-Dead space ventilation: the part of tidal volume that does not participate in alveolar gas exchange
-dead space ventilation includes anatomical dead space volume and physiological dead space volume
-anatomical dead space: is the amount of air in the conducting airways and is normally about 2ml/kg or about 150ml
-anatomical dead space depends on body posture and disease states
-in certain disease states such as COPD, anatomical dead space is larger than normal
-physiological dead space: occurs when ventilation is normal, but perfusion to the alveoli is reduced or absent
-this can occur with certain disease states, such as reduced CO or PE
-dead space increases the partial pressure of arterial carbon dioxide (PaC02) because blood that is carrying C02 back from the tissue cannot reach the alveoli
-alveolar ventilation: is the volume of fresh gas entering the respiratory zone each minute
-alveolar ventilation is of key importance because it represents the amount of fresh inspired air available for gas exchange
-alveolar ventilation is a product of minute ventilation minus dead space
-it is inversely proportional to PaC02 levels, if one breaths excessively, alveolar ventilation is increased and PaC02 decreases
-if alveolar ventilation is decreased, PaC02 levels will incease
-(Minute ventilation) Ve = Vt X F
-Normal Ve = 4-8 L/min
-Ve should match CO, normal CO is 4-8 L/min

Front 5

Diffusion

Back 5

-after the alveoli are ventilated with fresh air, the next step in the respiratory process is diffusion of oxygen from the alveoli to the pulmonary capillaries and diffusion of C02 from the pulmonary capillaries to the alveoli
-diffusion or movement of molecules occurs from and area of high to low concentration

Front 6

Perfusion

Back 6

-once oxygen has diffused from the alveolus to the pulmonary capillary, it is carried away from the lung by the bloodstream
-the term perfusion is used to describe the flow of blod through the pulmonary capillary bed
-pulmonary capillaries form a dense network around the alveolar wall, making an extremely efficient structure for gas exchange to take place
-when these blood vessels sense a low oxygen content (inc C02) in the alveoli, they vasoconstrict, known as hypoxic vasoconstriction
-hypoxic vasoconstriction has the effect of directing blood flow away from hypoxic areas of the lung
-by diverting blood flow from these areas, harmful effects on gas exchange are reduced
-hypoxic vasoconstriction in the alveoli is opposite to that in the brain
-hypercapnia (inc C02 levels) in the brain is the MOST POTENT VASODILATOR of CEREBRAL VESSELS, causing increased CBF and increased volume, leading to increased ICP

Front 7

Matching of ventilation to perfusion

Back 7

-effective pulmonary gas exchange depends on a balance, or matching of ventilation to perfusion
-2 factors may interfere with the matching of ventilation to perfusion: dead space and shunt
-dead space refers to areas in the resp system that do not participate in gas exchange
-the air in the conducting airways does not participate in gas exchange and is referred to as anatomical dead space
-shunt: refers to blood that bypasses, or shunts by, alveoli without picking up oxygen
-anatomical shunt: blood moves from the right side to the left side of the heart without passing through the lungs
-anatomical shunts occur with congenital heart diseases
-physiological shunt: blood is shunted past alveoli without picking up sufficient amounts of oxgen
-a ventilation-perfusion imbalance occurs when there is inadequate ventilation, inadequate perfusion or both
-three types of ventilation-perfusion imbalances may occur

Front 8

Matching of ventilation to perfusion cont

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Physiological shunt (low ventilation-perfusion ratio):
-when perfusion exceeds ventilation, the ratio is low and a shunt is present
-a shunt means that blood passes by alveoli without gas exchange occurring
-a low ventilation-perfusion ratio is seen with pneumonia, atelectasis, tumor, or a mucous plug
Alveolar dead space (high ventilation-perfusion ratio):
-when ventilation exceeds perfusion
-the alveolus has inadequate perfusion available, and gas exchange cannot occur
-a high ventilation-perfusion ratio is seen with a PE, pulmonary infarction, cardiogenic shock, and mechanical ventilation associated with high Vt
Silent unit
-when both ventilation and perfusion are decreased, a silent unit occurs
-a silent unit is seen with pneumothorax and severe ARDS

Front 9

Oxygen

Back 9

-oxygen is carried in the blood in two forms, dissolved and attached to hemoglobin
-the partial pressure of oxygen in arterial blood (Pa02) represents the level of DISSOLVED oxygen in plasma
-less than %3 of all oxygen is carried in this form
-97% of oxygen is carried in the blood is bound to hemoglobin and is called OXYHEMOGLOBIN
-oxyhemoglobin is transported in arterial blood and made available to the tissues for the use in cell metabolism
-the hemoglobin molecule is said to be fully saturated when oxygen is bound to all four of its oxygen binding sites
-when the affinity (attraction) is high, hemoglobin binds readily with oxygen at the alveolar capillary membrane
-but at the tissue level, hemoglobin does not readily release oxygen
-when the affinity is low, hemoglobin does not bind readily with 02 at the alveolar capillary membrane
-instead when affinity is low, hemoglobin releases oxygen more readily at the tissue level

Front 10

Oxyhemoglobin dissociation curve

Back 10

-the oxyhemo curve is a graphic depiction of the relationship between oxyhemoglobin saturation (the percentage of hemoglobin combined with oxygen, or the Sa02) and the arterial oxygen tension (Pa02) to which it is exposed.
-the initial part of the curve is very steep and then flattens at the top
-the flat portion represents the binding of oxygen to hemoglobin in the lungs
-the steep portion of the curve (between 40 and 60 mmHg) represents the release of oxygen from the hemoglobin that occurs in the capillaries
-at an arterial oxygen pressure (Pa02) of 40mmHg, the hemoglobin molecules are still about 70% to 75% saturated with oxygen
-this provides a reserve supply of oxygen that can be given to the tissues in cases of emergency or strenuous exercise
-hemoglobins affinity for oxygen is influenced by pH, C02 concentration, temperature, and 2,3-DPG
-2,3-DPG is a metabolically important phosphate compound found in the blood but in different combination under different metabolic conditions
-hemoglobin binds more readily with oxygen under conditions of inc pH, dec C02, dec body temp and dec 2,3-DPG
-the is represented on the oxyhemo curve as a shift to the left
-with a shift to the left, there is higher oxygen saturation for any given Pa02, inc affinity of hemoglobin for oxygen, and dec release of oxygen to tissues
-hemoglobin more readily releases oxygen under conditions of dec pH, inc C02, inc body temp, and inc 2,3-DPG
-the relationship is represented on the curve by a shift to the right
-with a shift to the right, there is lower oxygen saturation for any given arterial oxygen pressure Pa02, dec affinity of hemoglobin for oxygen, and inc release of oxygen to the tissues

-The oxyhemoglobin dissociation curve is an important tool for understanding how our blood carries and releases oxygen
-Specifically, the oxyhemoglobin dissociation curve relates oxygen saturation (SO2) and partial pressure of oxygen in the blood (PO2), and is determined by what is called "hemoglobin's affinity for oxygen," that is, how readily hemoglobin acquires and releases oxygen molecules from its surrounding tissue.

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Oxyhemoglobin dissociation curve cont

Back 11

-(R)ight shift – (R)aised temp, (R)aised 2-3DPG, (R)aised acid state, (R)educed oxygenation state

-Right shift is indicative of Bohr effect, unloads oxygen at tissue level

-(L)eft shift – (L)ow acid or a(L)kolosis, (L)ow 2-3DPG, (L)ow temp, (L)ots of CO

-Left shift is indicative of reverse Bohr effect, offloading at tissues is impaired

-If Pa02 drops from 100 to 60 mm Hg, the saturation of hemoglobin changes only 7% (from the normal 97% to 90%)
-The hemoglobin remains 90% saturated despite a 40 mm Hg drop in the Pa02
-Pa02 60 mm Hg = 90% saturation
-Patients are adequately oxygenated when the Pa02 is greater than 60 mm Hg

Front 12

Carbon dioxide

Back 12

-is carried in the blood in three forms:
-as dissolved C02 10%, attached to hemoglobin 30%, and as bicarbonate 60%
-C02 is formed as a metabolic byproduct, it diffuses out of the cell and into the capillaries
-most of it diffuses into RBC where it attaches to hemoglobin and most of that is released from the RBC as bicarbonate
-the lung excretes over 10,000 mEq of carbonic acid per day, compared with the kidney which excretes less than 100 mEq of fixed acids per day
-therefore by altering alveolar ventilation and subsequently the elimination of C02, the body is able to exert precise controls over its acid-base balance

Front 13

Respiratory failure

Back 13

-Resp failure-inability to maintain adequate respiration as measured by arterial blood pH, PaC02, and Pa02
-pH less than 7.25
-PaC02 greater than 50mmHg
-Pa02 less than 50mmHg