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21 Cards in this Set
- Front
- Back
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What is respiratory failure
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A syndrome, not a disease
-Signifies severity of respiratory ailment A condition in which the respiratory system fails in either: -Oxygenation of venous blood -Carbon dioxide elimination from venous blood Onset is either: -Quickly is ACUTE respiratory failure -Slowly is CHRONIC respiratory failure |
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Hypercapnic Respiratory Failure
Hypoxic Respiratory Failure |
Hypercapnic:
PaCO2>45 mmHg Acute Develops in min to hrs Chronic Develops over several days or longer Hypoxic: PaO2 < 55 mmHg Acute Develops in min to hrs Chronic Develops over several days or longer Categories are not mutually exclusive! |
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Explanations for Hypoxemia
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Hypoventilation
Ventilation-Perfusion/Shunt -gross mismatch of capillary blood-capillary air -low fraction of inspired oxygen (altitude-FIO2) -heart failure (cardiac pump failure) Impaired Diffusion -thickened lung parenchyma – alveolar lumenal to capillary lumenal thicknesses Impaired tissue utilization of O2 (eg cyanide) |
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PaCO2 and ventilation
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Removal of CO2 is proportional to VA (alveolar ventilation)
Relationship is a straight line Changes in VA , often a response to CO2 production |
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Increases in PaCO2
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When CO2 production increases more than VA can eliminate
When dead space increases(pts with ventilation-perfusion/shunt problems) When there is a fall in respiratory rate (f) = hypoventilation |
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p(A-a)O2 gradient: calculation
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Differences between Alveolar and arterial pO2 values
1st determine the PAO2 (Alveolar partial O2 tension) The PaO2 and PaCO2 values are taken from the arterial blood gas (ABG) sample. PAO2 = PIO2 - PaCO2/R PIO2 is partial pressure of inhaled oxygen = atm pressure x % of air that is oxygen (21%) = 760 x 0.21 = approx 150 mm Hg R = respiratory exchange ratio = 0.8 pAO2 = 150 - (40/0.8) = 100 For a young healthy person: paO2 is 90-95 mmHg and the normal A-a gradient will be 5-10 mmHg The P(A-a)O2 gradient is a sensitive indicator of respiratory disease that interferes with gas exchange It can help distinguish extrapulmonary from intrapulmonary causes of hypercapnia & hypoxemia |
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Hypoxic respiratory failure: physiological processes
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Alveolar hypoventilation
-Normal A-a gradient -Eg: neuromuscular diseases Ventilation-perfusion mismatch -Widening of A-a gradient Shunt -Widening of A-a gradient Diffusion limited -Widening of A-a gradient |
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Acute respiratory failure: clinical, and treatment
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Clinically, it implies that respiratory muscles are no longer capable of moving necessary Ve required to keep pO2/pCO2 nl
Mechanical ventilation -By mask -By endotracheal tube |
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Mechanical ventilation limitations
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They only provide bulk flow
Convection or bulk flow in larger airways Diffusion starts - about level of alveolar ducts |
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Ventilation perfusion mismatch
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understanding how the lack of “re”-oxygenating
some of the blood as a % of cardiac output, Some alveolar capillary blood never sees fresh O2 |
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Oxygen content
Oxygen delivery |
Oxygen content =
% of hemoglobin saturated with oxygen (i.e., oxygen saturation) & actual hemoglobin concentration [grams/dL] + dissolved oxygen -But: O2 content of blood has little to do with dissolved oxygen. Its all about hemoglobin O2 delivery = O2 saturation of hgb x hgb conc x cardiac output O2 delivery = CO x [(Hgb x SaO2 x 1.36) + (PaO2 x 0.0031)] -AKA oxygen content*cardiac output |
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Shape of oxyhemoglobin dissociation curve
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The shape of the oxyhemoglobin dissociation curve provides protection against tissue hypoxia
But only to a degree Over the range of PaO2 =60 mmHg to 100 mmHg, Oxygen Saturation of Hemoglobin remains > 90% |
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Hemoglobin-O2 saturation
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Arterial
-Normal=approximately 100% --O2 content high Mixed venous -Normal=approximately 75% -O2 content low If demand for O2 increases -Arterial remains 95-100% -Venous can go very low ~50% --Oxygen content is very low |
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Right to left shunting
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When mixed venous blood goes directly into the arterial circulation without having first been exposed to alveolar gas.
When the shunted blood mixes with the rest of the arterial blood it lowers the average oxygen content and therefore the average PaO2 is lowered The P(A-a)O2 is increased in these patients Three Types of Shunt: -cardiac or great vessel -pulmonary vascular -pulmonary parenchymal |
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Respiratory failure and effector components
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Respiratory Failure can arise from an abnormality in any of the “effector” components of the respiratory system:
- CNS - peripheral nervous system - respiratory muscles and chest wall - airways - alveoli |
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Oxygenation failure
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NOT ON TEST
a) ventilation -movement of gases between the environment and lungs b) intrapulmonary gas exchange -mixed venous blood releases CO2 & becomes oxygenated c) gas transport -adequate delivery of oxygenated blood to metabolizing tissue d) tissue gas exchange -extraction or use of O2 and release of CO2 by peripheral tissues [c and d may fail independently of the performance of the lung or ventilatory pump] |
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Hypercapnic respiratory failure
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NOT ON TEST
Hypercapnic Respiratory Failure = failure of factors that reduce ventilatory supply or increase ventilatory demand: Brain Injury Narcotics Fever Exercise Pain |
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CNS abnormalities and respiratory failure
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NOT ON TEST
Suppression of the central neural drive to breathe Pharmacologic: overdose Structural: encephalitis, tumors, vascular abnormalities of medulla Metabolic: derangements producing hypercapnia through depression of respiratory control centers - severe myxedema, hepatic failure, advanced uremia |
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PNS abnormalities and respiratory failure
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NOT ON TEST
Abnormalities of the peripheral nervous system & chest wall Neurologic: Guillain - Barre, myasthenia gravis, polymyositis Chest wall: Severe Kyphoscoliosis, morbid obesity |
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Airway abnormalities and respiratory failure
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NOT ON TEST
Large airways obstructions: epiglottitis, aspirated foreign body, tracheal tumor, narrowing of trachea with scar tissue “Microscopic” (small) airways obstruction: asthma, COPD, cystic fibrosis Both produce a greater transthoracic pressure gradient requirement for inspiratory airflow. The resistive component of the work of breathing is increased, the respiratory muscles fatigue, a shallow breathing pattern ensues. |
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Alveoli abnormalities and respiratory failure
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NOT ON TEST
Although diseases characterized by diffuse alveolar filling frequently result in hypoxemic respiratory failure, hypercapnia may complicate the picture. Diffuse alveolar filling creates a large right-to-left shunt as pulmonary blood flows through poorly ventilated regions of the lung, such as pneumonia or congestive heart failure |
