Physiology of Ventilation and Gas Exchange

Oxygenation & Ventilation

• Oxygenation is the delivery of oxygen to the tissues.
• Ventilation is the movement of air into and out of the lungs.
• Ventilation is primarily driven by carbon dioxide levels.
• Elevated carbon dioxide triggers chemoreceptors, stimulating an increase in ventilation.
• Oxygen delivery is calculated as cardiac output multiplied by oxygen content.
• The oxygen content formula includes components like hemoglobin, saturation, and partial pressure of oxygen (PaO2).

Objectives

• The goal is to understand the difference between ventilation and oxygenation.
• Differentiate factors that stimulate ventilation in healthy and hypoxemic patients.
• Identify and describe the five causes of hypoxemia.

Oxygen Delivery

• Oxygen delivery (DO2) is calculated as cardiac output (CO) multiplied by oxygen content (CaO2).
• The formula for oxygen content is: (Hb)(SpO2)(1.36) + 0.003 x PaO2.
• HR is heart rate, SV is stroke volume, Hb is hemoglobin, SpO2 is oxygen saturation, and PaO2 is partial pressure of oxygen.

Gas Exchange

• Oxygenation involves oxygen transfer across the alveoli.
• This process is dependent on ventilation.
• Ventilation refers to the rate and depth of breathing, facilitating carbon dioxide removal.

Carbon Dioxide (PaCO2)

• Carbon dioxide production drives ventilation.
• A buildup of carbon dioxide in cerebrospinal fluid (CSF) triggers chemoreceptors.
• This, in turn, results in increased ventilation.
• Level of PaCO2 impacts ventilation. The graph shows the relationship.

Effect of PCO2 on Ventilation

• The graph demonstrates the relationship between increasing PaCO2 and minute ventilation. An increase in PaCO2 results in a corresponding increase in minute ventilation up to a point.

Causes of Hyperventilation

• Pain, anxiety, and excitement can all cause hyperventilation.
• Hyperthermia, acidosis, and hypoxemia are also possible causes.

Causes of Hypoventilation

• Central nervous system issues (brain, medulla, pons, cerebrum, cervical spinal disease), like IVDD, Wobblers, AA subluxation and medications (opioids, propofol, anesthesia) can lead to hypoventilation.
• Peripheral neuromuscular diseases (lower motor neuron disease, botulism) and tick paralysis are connected to hypoventilation.

Respiratory System Causes of Hypoventilation

• Respiratory system diseases and airway obstructions can result in hypoventilation.
• Parenchymal disease, pleural space diseases, and thoracic cage/diaphragm disorders are other possibilities.

Oxygen (PaO2) & Hypoxia

• Severe hypoxemia (PaO2 < 60 mmHg) can cause ventilation to be oxygen-driven.
• Aortic and carotid body chemoreceptors respond to low oxygen levels.

Effect of Hypoxemia on Ventilation

• A graph illustrates the impact of decreasing PaO2 on minute ventilation. The relationship demonstrates a direct connection between decreasing PaO2 and increasing minute ventilation initially.

Effect of Minute Ventilation on PO2

• The effect of different inspired oxygen fractions, (FiO2) on the relationship between minute ventilation and partial pressure of oxygen (PaO2) is shown by the graph.

Causes of Hypoxia

• Decreased inspired oxygen (FiO2), alveolar hypoventilation, right-to-left shunts, impaired diffusion, or venous admixture (V/Q mismatch) can cause hypoxemia.

Decreased FiO2

• Causes include anesthesia and high altitudes.
• The calculation for partial pressure of oxygen (PiO2) is FiO2 (Patm - PH2O).

R to L Shunt

• An abnormal circulation leads to deoxygenated blood entering the systemic circulation.
• Conditions like tetralogy of Fallot, transposition of great vessels, persistent truncus arteriosis, and tricuspid atresia can cause R to L shunts.

Venous Admixture (High V/Q)

• An imbalance of blood flow and alveolar ventilation results in an increased V/Q (ventilation-perfusion) ratio.
• Conditions for a high V/Q ratio include pulmonary thromboembolism and pulmonary arterial hypertension.

Venous Admixture (Low V/Q)

• An imbalance between blood flow and alveolar ventilation results in a decreased V/Q (ventilation-perfusion) ratio.
• Conditions for a low V/Q ratio include cardiogenic and non-cardiogenic pulmonary edema, pneumonia, inflammation, atelectasis, and airway obstructions.

Ventilation Compensation

• In cases of high V/Q, local hypocapnia increases pH and triggers local bronchoconstriction.
• Low V/Q conditions cause increased CO2 levels, stimulating higher ventilation and CO2 removal to restore oxygen.
• Hypoxic vasoconstriction redirects blood flow to better-ventilated regions.

Using Respiratory Patterns to Identify Source of Dyspnea

• The presentation emphasizes recognizing abnormal respiratory patterns to determine dyspnea's origin.

Today's Objectives

• Key objectives include recognizing and interpreting abnormal respiratory patterns and signs of respiratory arrest.
• Correlation between breathing patterns and dyspnea's cause and developing a stabilization. plan location-based is important.

Locations of Dyspnea

• The location of dyspnea issues can be extrathoracic airways, intrathoracic airways, pulmonary parenchyma, thoracic cage, pleural space, or vascular regions.

Normal Respiration

• Normal respiration is barely noticeable at rest, with ribs moving cranially and outward, the diaphragm moving caudally, and the abdomen moving passively.
• Chest and abdomen move synchronously.

Recognizing Respiratory Distress

• Increased respiratory rate and effort, open mouth/flared nostrils, tail bobbing (in animals), anxiety/restlessness, orthopnea, cyanosis (a bluish discoloration from low oxygen), noise (e.g., stridor or stertor), and irregular patterns are signs of respiratory distress.

Orthopnea

• Orthopnea is characterized by difficulty breathing when lying down, and easier breathing in an erect position.

Cyanosis

• Cyanosis is an indication of severe oxygen deficiency, and a decrease in PaO2 (to ~40 mmHg) and SpO2 (~75%) is frequently seen.
• Elevated deoxy Hb (~5 g/dL) is a relevant contributor to cyanosis.

Noise

• Types of respiratory noises include stridor (high-pitched, inspiratory sound indicating obstruction in the upper airway), and stertor (snoring, low-pitched sound indicating obstruction in the upper airway or trachea).

Irregular Pattern

• Paradoxical respiration, irregular rhythm, and irregular breathing patterns are abnormal respirations.

Paradoxical Respiration

• Paradoxical respiration leads to chest and abdominal movements in opposite directions.
• This typically results from intercostal muscle weakness (fatigue).
• Ribs/intercostals may move inward during inspiration.

Not in Reptiles

• In normal reptiles, non-ventilatory periods are normal.
• Shortened non-ventilatory periods can be a sign of respiratory disease.

Signs of Impending Respiratory Arrest

• Decreasing mentation, restlessness, changing body posture, vocalization, lateral recumbency(in cats), mydriasis (in cats), puffed tail (in cats), and irregular breathing patterns point to impending respiratory arrest.

In Case of Impending Respiratory Arrest

• Discussion with owners of affected animals is critical.
• Intubation and ventilation are necessary if necessary prior to arrest..
• Use of low doses of drugs during respiratory arrest should be considered.

Birds

• Unidirectional airflow occurs in birds.
• Loss of voice (syrinx) and/or stridor due to upper airway issues.
• History, physical examination, and radiographs are necessary to assess birds.

Reptiles

• Increased respiratory rate and effort (RR/RE), shorter non-ventilatory periods may arise in respiratory disease.
• Visual examination of the larynx and limited auscultation are available.
• Respiratory issues require history, physical examination, and radiographs.

Respiratory Patterns

• Classify respiratory patterns as normal, obstructive, restrictive, or hypoventilation.

Obstructive Pattern

• Obstructive pattern shows airway narrowing, causing slow/deep breaths with good lung expansion and significant chest-wall movement.
• Obstructions can be dynamic or fixed depending on location and timing. Fixed objects will not change during the respiratory cycle.

Extrathoracic Airway

• Extrathoracic airways, including nasal passages, nasopharynx, oropharynx, and extrathoracic trachea, are important locations for potential issues.

Intrathoracic Airway

• Issues in intrathoracic airways, like trachea, and bronchi, must be considered.

Obstruction

• Causes include panic/anxiety, inflammation, or worsening obstructions.

Worsening Obstruction

• Progressive obstruction worsens oxygen and carbon dioxide issues.

Airway: Initial Treatment

• Ensure a patent airway, confirm airflow, and consider intubation, tracheostomy, oxygen use, and anxiolytics as needed for small animals.

Pulmonary Parenchyma: Stabilization

• Provide oxygen and initiate treatment if disease-specific therapy is identified or if empiric therapy is attempted.

Hypoventilation

• Reduced or absent chest-wall movements and shallow abdominal movements are hallmarks of hypoventilation.
• Animals may exhibit few or only mild signs of distress.

Hypoventilation: Differentials

• Potential causes include neurological issues (brain, cervical spine, phrenic nerve), or neuromuscular disorders (e.g., myasthenia gravis, botulism, tick paralysis, hypokalemia, polymyositis).

Hypoventilation Treatment

• Provide oxygen, treat the underlying cause, intubate if necessary, and monitor the animal closely.

Masqueraders

• Conditions masking respiratory problems include hyperthermia, pain, metabolic acidosis, fear/excitement, anemia/hemoglobinopathy, hypoglycemia, purring (cats), and threatening behavior.

Pleural Space: Differentials

• Pleural space issues encompass pneumothorax, pleural effusions (characterized by blood, pus, chyle or transudate), pleuropneumonia in horses and goats, neoplasia, or diaphragmatic hernia.

Pleural Space Disease Stabilization

• Supplementation with oxygen is an initial part of stabilization.

Pulmonary Parenchyma: Differentials

• Pneumonia (viral,bacterial,fungal, parasitic), aspiration pneumonia, heart failure, hemorrhage, contusions, neoplasia, inflammation, and heartworm are all possible issues in the pulmonary parenchyma.

Pulmonary Vascular: Differentials

• Pulmonary thromboembolic disease, pulmonary hypertension, and heartworm disease are vascular-based conditions of the lungs.