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Shortness of breath causes and reasons codified

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Shortness of breath causes are too numerous to commit to rote.  The best way to recall and understand many shortness of breath causesof them is to group the main reasons for shortness of breath by the involved body systems or organs.  Then link the mechanisms to the reasons in each group.  Finally, learn the more common disorders in each group.

The information that follows is not all-inclusive. But it does include the more common causes of shortness of breath and the reasons. In addition, the approach lends itself to seamless insight on this and related topics.                                                         

RESPIRATORY SYSTEM REASON  
MECHANISM
Disease
Category
Disease
or
Condition
Increased Work of Breathing Low Oxygen High Carbon Dioxide  
Reduced Oxygen
Carrying Capacity
 
Pathophysiology
Obstructive lung disease COPD/asthma/ bronchiectasis/tumor obstructing bronchus
 
  V/Q mismatch
±
Diffusion
±
Hypoventilation
Intrinsic restrictive lung disease
 
Interstitial lung disease,
sarcoidosis, etc.
    V/Q mismatch
±
↓ Diffusion
Extrinsic restrictive lung disease Obesity/chest wall deformity   Hypoventilation
±
V/Q mismatch from atelectasis
Pulmonary vascular disease Pulmonary embolism/pulmonary hypertension       V/Q mismatch
±
↓ Diffusion
±
Intrapulmonary Shunt
±
Cardiac Output
Alveolar lung disease Pneumonia/alveolar cell lung cancer       V/Q mismatch
±
↓ Diffusion
±
Intrapulmonary Shunt
 
Alveolar + interstitial lung disease
 
Pulmonary edema/ARDS
      V/Q mismatch
±
↓ diffusion
±
Intrapulmonary shunt

Alveolar hypoventilation in the above table refers to the decreased expulsion of CO2 from the body by means of exhalation.  Raised CO2 in the blood is the best clue.  CO2 does not increase from V/Q mismatch or decreased diffusion in the absence of hypoventilation though.  The reasons are: 1) low O2 stimulates chemoreceptors to cause an increase in the breathing rate; 2) CO2 is more diffusible than O2.

It is a given that low O2 or high CO2 in many instances increase the work of breathing because of a compensatory increase in the rate and/or depth of breathing; but work of breathing is not listed as a cofactor in this setting since it most likely is not the main reason.  

CARDIOVASCULAR SYSTEM REASON       
MECHANISM
Disease
Category
Disease
or
Condition
Increased Work
of Breathing
Low Oxygen High Carbon Dioxide Reduced Oxygen
Carrying Capacity
Left-sided heart failure Pulmonary edema  (due to systolic dysfunction or diastolic dysfunction of the left ventricle)     V/Q mismatch
±
↓ Diffusion
±
Intrapulmonary Shunt
Acute coronary syndrome MI, unstable angina or gray area without know ventricular dysfunction         Not well-defined
Congenital heart disease Tetrology of Fallot
Pulmonary valve atresia
Tricuspid atresia
Hypoplastic left heart syndrome
Eisenmenger syndrome
      Right-to-left Shunt

The low O2 in blood that causes shortness of breath with the congenital heart diseases listed in the above table is due to blood flow from the right side of the heart to the left side of the heart or from the right side of the heart into the systemic circulation. In either case O2 depleted blood in veins bypasses the lungs before returning to the systemic circulation.  The hypoxemia that results causes shortness of breath.  In the case of infants who cannot express shortness of breath it causes cyanosis.

NERVOUS SYSTEM REASON       
Mechanism
Disease
Category
Disease
or
Condition
Increased Work
of Breathing
Low Oxygen High Carbon Dioxide Reduced Oxygen
Carrying Capacity
Extrinsic restrictive lung disease Neuromuscular   disease   Hypoventilation

The reason for shortness of breath above can be either or a combination of the ones checked.  It depends on the cause and severity of the disorder.

HEMATOLOGIC SYSTEM REASON       
MECHANISM
Disease
Category
Disease
or
Condition
Increased Work
of Breathing
Low Oxygen High Carbon Dioxide Reduced Oxygen
Carrying Capacity
Anemia Various types of anemia       ↓ O2 to tissues
Hemoglobin abnormalities Hemoglobinopathy/
thalassemia
      ↓ O2 to tissues

In the above group the O2 partial pressure is normal.  In other words the amount of oxygen dissolved in blood is normal.  But the amount of O2 that the blood delivers to tissues is less than normal.

KIDNEYS, LIVER & OTHER REASON       
MECHANISM
Disease
Category
Disease
or
Condition
Increased Work
of Breathing
Low Oxygen High Carbon Dioxide Reduced Oxygen
Carrying Capacity
Kidney disease Acute kidney failure
Chronic kidney disease
      pH due to metabolic acidosis
or
Pulmonary edema from fluid retention
Liver disease Acute or chronic liver failure/Hepatopulmonary syndrome       ↓ pH due to lactic acidosis

V/Q mismatch

Deconditioning Sedentariness/
Obesity w/o RLD
        Not well-defined
Psychological Anxiety/panic disorder         Not well-defined


In some cases the reason for shortness of breath is less well-understood.   Hopefully, future research will shed light in these areas. 

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