Dyspnea and Exercise

By the end of this lecture, students will be able to:

• Understand the definition of dyspnea.

• List the mechanoreceptors involved in the sensation and relief of dyspnea.

• Highlight the three main pathophysiological factors that impact dyspnea.

• Appreciate common scales used to quantify dyspnea.

• Identify common causes of dyspnea.

• Outline examples of the workup for dyspnea.

• Describe the normal physiological changes that occur during exercise.

Definition of Dyspnea

• American Thoracic Society Definition: Dyspnea is the subjective experience of breathing discomfort.

• Contributing Factors: It relates to the interplay of various physiological, psychological, social, and environmental factors.

• Sensation Description Includes:

  • Air hunger

  • Increased work/effort to breathe

  • Patient descriptions can include:

  • My breathing is shallow.

  • I feel an urge to breathe more.

  • My chest is constricted.

  • My breathing requires effort.

  • I feel a hunger for more air.

  • I feel out of breath.

  • I cannot get enough air.

  • My breath does not go in all the way.

  • My chest feels tight.

  • My breathing requires work.

  • I feel that I am smothering/suffocating.

  • I feel that I cannot get a deep breath.

  • My breath does not go out all the way.

  • My breathing is heavy.

Mechanisms of Perceived Dyspnea

• Dyspnea arises from two main processes:

  • A physiologic or pathophysiologic process that activates sensory afferents.

  • A psychological affective process.

Contributing Factors to Dyspnea

• Air Hunger:

  • Resulting from stimulation of central and peripheral chemoreceptors due to inadequate ventilatory response.

• Increased Work of Breathing:

  • Due to perceived increased airway resistance or decreased lung/chest wall compliance.

  • Respiratory muscle fatigue detected by muscle spindles, joint receptors, tendon receptors, and metaboreceptors.

Hypoxia and Dyspnea

• Chronic Hypoxemia:

  • Alone is a relatively weak stimulus for dyspnea, particularly if the subject can increase ventilation leading to hypocapnia.

• Exercise Impact:

  • If exercising at the same intensity, dyspnea will feel more pronounced under hypoxemia versus normal conditions.

  • Conversely, during hyperoxia, dyspnea will feel less intense under the same exercise conditions.

Mechanoreceptors and Dyspnea

• Key Mechanoreceptors:

  • Slowly adapting stretch receptors.

  • Rapidly adapting stretch receptors.

  • C fibers present in small airways and alveoli.

• Mechanism of Relief:

  • Stimulation of receptors in the face and upper airway (innervated by Cranial Nerve V) may reduce dyspnea.

  • Interventions such as blowing cold air on the face can lead to longer breath-holds, and high-flow oxygen may also reduce dyspnea.

  • Inhaled cold air can alleviate dyspnea, provided it does not provoke bronchospasm.

Pathophysiological Mechanisms of Dyspnea

• Fick's Formula states:

  • VO2 = CO imes (CaO2 - CvO_2)

  • Dyspnea can occur due to mismatches among several factors:

  • Oxygen uptake

  • Oxygenation

  • Ventilation

  • Diffusion

  • Oxygen delivery

  • Perfusion

  • Oxygen carrying capacity

  • Cardiac function

  • Oxygen utilization

• Muscular Involvement:

  • Organs such as muscles and mitochondria play a role in the management of dyspnea.

Assessing Dyspnea

History Taking

• Important aspects to inquire about include:

  • Temporal pattern of dyspnea.

  • Specific triggers for dyspnea.

  • Level of activity associated with dyspnea.

  • Quality of life impacts due to dyspnea.

  • Associated symptoms, if any.

Causes of Dyspnea

• Acute Dyspnea May Be Caused By:

  • Cardiovascular System:

  • Myocardial infarction (MI)

  • Heart failure

  • Cardiac tamponade

  • Respiratory System:

  • Bronchospasm

  • Pulmonary embolus

  • Pneumothorax

  • Pulmonary infection

  • Upper airway obstruction

Quantifying Dyspnea

• Scales Used to Quantify Dyspnea:

  • Borg Scale: Modified Borg scale for assessing intensity of dyspnea or fatigue:

  • 0: Nothing at all

  • 0.5: Very, very slight (just noticeable)

  • 1: Very slight

  • 2: Slight (light)

  • 3: Moderate

  • 4: Somewhat severe

  • 5: Severe (heavy)

  • 6-10: Severity increasing up to maximal intensity

• Modified MRC Scale: Adapted from Fletcher et al. (1959). Description of breathlessness:

  • Grade 0: I only get breathless with strenuous exercise.

  • Grade 1: I get short of breath when hurrying on level ground or walking up a slight hill.

  • Grade 2: On level ground, I walk slower than people of the same age due to breathlessness or need to stop for breath when walking at my pace.

  • Grade 3: I stop for breath after walking about 100 yards (91 meters) or after a few minutes on level ground.

  • Grade 4: I am too breathless to leave the house or am breathless when dressing.

Common Causes of Dyspnea

• Asthma

• Chronic obstructive pulmonary disease (COPD)

• Interstitial lung disease (ILD)

• Cardiac causes

• Anemia

• Deconditioning/obesity

• Neuromuscular diseases

Description of Dyspnea Associated with Disease States

• Qualities of dyspnea are associated with varying disease states:

  • Descriptor: Bronchoconstriction, interstitial edema associated with asthma.

  • Chest Tightness: Myocardial ischemia associated with cardiac conditions.

  • Increased Work: Pulmonary edema associated with heart failure or pulmonary embolism.

  • Air Hunger: Strong urge to breathe, prevalent with interstitial fibrosis.

  • Rapid, Shallow Breathing: Associated with alveolar edema due to pulmonary edema.

  • Heavy Breathing: Indicative of inadequate O2 delivery to the muscles, related to deconditioning.

Differential Diagnoses of Dyspnea

Upper Airway Conditions

• Laryngeal mass

• Vocal fold paralysis

• Inducible laryngeal obstruction (paradoxical vocal fold motion)

• Goiter

• Neck mass compressing airway

Chest/Abdominal Wall Conditions

• Diaphragmatic paralysis

• Kyphoscoliosis

• Late pregnancy

• Massive obesity

• Ventral hernia

• Ascites

• Intra-abdominal processes

Pulmonary Conditions

• Asthma

• Bronchiectasis

• Bronchiolitis

• COPD/emphysema

• Interstitial lung disease

• Mass compressing/occluding airway

• Pleural effusion

• Previous major lung resection (e.g., lobectomy, pneumonectomy)

• Pulmonary right-to-left shunt

• Pulmonary hypertension

• Trapped lung

• Venous thromboembolism (VTE)

Chronic or Recurrent Dyspnea Conditions

• Cardiac Causes:

  • Arrhythmias

  • Constrictive pericarditis, pericardial effusion

  • Coronary heart disease

  • Deconditioning

  • Heart failure (systolic or diastolic dysfunction)

  • Intracardiac shunt

  • Restrictive cardiomyopathy

  • Valvular dysfunction

• Neuromuscular Diseases:

  • Amyotrophic lateral sclerosis

  • Phrenic nerve disease/dysfunction

  • Glycolytic enzyme defects (e.g., McArdle syndrome)

  • Mitochondrial diseases

  • Polymyositis/dermatomyositis

• Toxic/Metabolic/Systemic Causes:

  • Anemia

  • Metabolic acidosis

  • Renal failure

  • Thyroid disease

• Miscellaneous Causes:

  • Anxiety

  • Early pregnancy (effects of progesterone)

Workup Examples for Dyspnea

Basic Workup

• Complete blood count: Hemoglobin levels

• Thyroid Stimulating Hormone (TSH) test

• Complete Metabolic Panel (CMP)

• Pulmonary function tests

• Six-minute walk test with pulse oximetry

• Chest X-ray (CXR)

• Electrocardiogram (ECG)/Echocardiogram

• Plasma B-type natriuretic peptide (BNP) levels

Advanced Workup

• Chest CT to assess for ILD

• Diffusing capacity of the lungs for carbon monoxide (DLCO) test

• Methacholine challenge test to rule out asthma or trial of bronchodilators (e.g., symbicort)

• Myocardial stress test to assess cardiac disease

• Cardiopulmonary exercise testing

Cardiopulmonary Exercise Testing

• Overview: Incremental exercise test usually performed on a stationary bike.

• Purposes: Useful for diagnosing issues related to lung, heart, circulation, and leg muscles.

• Parameters Measured:

  • Cardiovascular and gas exchange parameters including:

  • Flow, O2, CO2 (e.g., respiratory rate (RR), tidal volume (Vt), minute ventilation (VE), oxygen uptake (VO2), peripheral oxygen saturation (SpO2), respiratory quotient (RQ))

  • ECG and blood pressure responses

  • Work output

Normal Physiological Changes During Exercise

• Resting Oxygen Consumption (VO2): Approximately 250 ml O2/min.

• During Exercise:

  • Increased muscular activity and cellular respiration lead to increased oxygen demand and oxygen consumption (VO2).

  • Steady state of VO2 corresponds with a steady state of CO2 elimination from metabolism.

  • Decreased mixed venous oxygen pressure (PvO2).

  • Anaerobic Threshold: Indicates the point where anaerobic metabolism becomes significant. Lactate is produced, exacerbating CO2 production contributing to increased partial pressure of carbon dioxide (PvCO2).

  • Respiratory compensation point is triggered, which stimulates further respiration due to the development of acidemia.

VE to VO2 Plot Interpretation

• First Vertical Line: Marks the anaerobic threshold.

• Second Vertical Line: Marks the respiratory compensation point.

Ventilatory Threshold

• Defined as the point at which minute ventilation (Ve) increases disproportionately compared to oxygen consumption (VO2).

• This usually occurs at exercise intensities above 40% of VO2max, which is higher in fit individuals and lower in those with cardiovascular or peripheral vascular diseases.

Cardiac Response During Exercise

• Cardiac Output: Increases 4-5 times from baseline (from approximately 5-6 L/min to about 25 L/min in young healthy individuals). This increase limits maximal performance capabilities in healthy individuals.

• Mechanism of Increase:

  • Increase in stroke volume and heart rate.

  • Stroke volume increases hyperbolically, with heart rate primarily driving cardiac output at peak exercise levels.

• V/Q Ratio Adjustments: Base of the lung typically shows low ventilation-to-perfusion (V/Q) ratio, which increases during exercise.

Heart Rate Changes During Exercise

• Increase in Heart Rate:

  • Increases linearly and can reach 2 to 3 times maximum heart rate, calculated using the formula 220 - age.

Blood Pressure Dynamics During Exercise

• Systolic Blood Pressure: Increases during exercise.

• Diastolic Blood Pressure: Remains unchanged.

• Pulmonary Vascular Resistance: Rises modestly during exercise due to recruitment and distention of the pulmonary circulatory system, but subsequently decreases.

Respiratory System Responses During Exercise

• Minute Ventilation Increase: From a resting volume of 6 liters to over 100 liters in healthy young adults.

• Components of Ventilation Increase:

  • Both tidal volume and respiratory rate increase.

  • Tidal volume usually increases hyperbolically, plateauing at 50-60% of the resting vital capacity.

  • The respiratory rate increases linearly, at least doubling, potentially reaching a maximum of 50 breaths/min.

  • The dead space fraction (Vd/Vt) decreases along with a reduction in physiologic dead space.

• Increase in Tidal Volume: Reflects improved breathing efficiency during exercise.

Conclusion

These notes encapsulate the vast information presented regarding the physiology of dyspnea and exercise, providing clarity on definitions, mechanisms, assessment scales, and physiological dynamics during various exercise intensities.