Neuro

NEUROLOGIC CONTROL OF VENTILATION

INTRODUCTION

  • Ventilation is both a voluntary and involuntary process.

    • Most of the time, ventilation occurs without conscious awareness.

    • The body alters the ventilatory pattern in response to physiological changes without the individual's awareness.

    • Individuals can consciously control their breathing (e.g., taking a deep breath or holding their breath).

    • However, this control is not perfect; prolonged breath-holding leads to involuntary neural control that resumes ventilation.

PRIMARY COMPONENTS OF NEUROLOGIC CONTROL

  1. Receptors

    • Sensors that collect information and transmit it to the brain.

  2. Control Centers of the Brain

    • Interpret information from receptors and direct instructions to effectors.

  3. Effectors

    • Pulmonary muscles that influence breathing patterns.

LOCATION OF VENTILATORY RHYTHM ORIGIN

  • Two key parts of the brainstem involved:

    • Medulla Oblongata

    • Pons

COMPONENTS WITHIN THE MEDULLA OBLONGATA

  • Medulla Oblongata is the lower part of the brainstem.

    • Contains three groups of neurons regulating ventilation:

    1. Pre-Bötzinger Complex

      • Responsible for the rhythmic nature of ventilation.

      • Located in the ventrolateral portion of the medulla oblongata.

    2. Dorsal Respiratory Group (DRG)

      • Activates inspiratory muscles.

      • Sends impulses to the phrenic nerve (innervating the diaphragm) and external intercostal motor nerves.

      • Stimulates diaphragm contraction, increasing thoracic cavity volume.

      • Based on Boyle’s Law, increased volume results in decreased pressure in the lungs, creating a pressure gradient for ventilation.

    3. Ventral Respiratory Group (VRG)

      • Primarily activates exhalation.

      • Located bilaterally in the medulla, anteriorly and laterally to the DRG.

NORMAL VENTILATORY PATTERN

  • Depth:

    • Tidal Volume (VT): Normal rate is 5-7 mL/kg Ideal Body Weight (IBW), average adult male VT ~ 500 mL.

    • IBW = Ideal Body Weight (also known as PBW or Predicted Body Weight).

  • Rate:

    • Frequency: Normal range is 12-18 breaths/min for adults.

  • Timing:

    • Inhalation to Exhalation ratio (I:E): Normally 1:2.

PONS CONTROL CENTERS

  • Pons is located in the upper brainstem with two ventilation control centers:

    1. Apneustic Center

    • Located in the lower pons, just above the medulla oblongata.

    • Sends signals to the VRG and DRG.

    1. Pneumotaxic Center

    • Located in the upper pons.

    • Regulates the inspiratory phase and fine-tunes the ventilatory rhythm by shortening the inspiratory cycle.

APNEUSTIC BREATHING

  • Occurs if there is injury to the pneumotaxic center, allowing the apneustic center to take over.

  • Characterized by prolonged inspiration with pauses at full inspiration.

  • Clinical associations include:

    • Stroke

    • Cerebral edema

    • Medications leading to drug-induced respiratory depression.

TYPICAL BREATHING PATTERNS

  • Normal Breathing (Eupnea):

    • Tidal volume of 5 - 7 mL/kg IBW

    • Respiratory rate of 12 - 18 bpm

    • I:E ratio of 1:2.

  • Apneustic Breathing:

    • Deep, gasping inspirations indicating possible brainstem damage.

  • Biot's Breathing (Ataxic Breathing):

    • Irregular patterns of rate and depth along with periods of apnea, highlighting neurological issues.

  • Cheyne-Stokes Breathing:

    • Pattern alternates between deeper and faster breathing, followed by diminished depth and rate, then apnea.

    • Potentially represents neurological problems or congestive heart failure (CHF).

  • Kussmaul's Breathing:

    • Rapid, deep breathing commonly associated with diabetic ketoacidosis (DKA).

  • Not Breathing (Apnea):

    • Complete or temporary cessation of breathing.

    • A true apnea occurs when breathing stops for 20 seconds or more.

  • Agonal Respirations:

    • A brainstem reflex caused by severe hypoxia, where gasping occurs in an attempt to restore oxygen.

    • Not effective breaths and require manual ventilation.

BRAINSTEM STRUCTURE AND PATHOPHYSIOLOGY

  • Components of the brain include:

    • Brain Parenchyma (80%)

    • Cerebral Spinal Fluid (CSF) (10%)

    • Blood (10%)

  • Elevated intracranial pressure (ICP) may lead to brainstem herniation through the foramen magnum due to:

    • Significant swelling or edema

    • Bleeding or hemorrhage

    • Increased CSF.

BRAINSTEM HERNIATION (CUSHING’S TRIAD)

  1. Systemic Hypertension with wide pulse pressure

  2. Bradycardia

  3. Very abnormal ventilatory pattern (neuro breathing)

BLOOD-BRAIN BARRIER (BBB) & CEREBRAL SPINAL FLUID (CSF)

  • The Blood-Brain Barrier (BBB) is a semipermeable barrier protecting the CNS from contaminants in the blood.

  • Functions include:

    • Blocking harmful particles from entering the CNS.

    • Allowing necessary molecules for metabolism into the CNS.

    • Secreting substances regulating information exchange between the CNS and the body.

  • Chemoreceptors monitor and respond to changes in the chemistry of surrounding fluids (blood, CSF, ECF).

    • Two types control ventilation:

    1. Central Chemoreceptors

    2. Peripheral Chemoreceptors

CENTRAL CHEMORECEPTORS

  • Located bilaterally along the ventrolateral medulla oblongata.

  • Monitor changes in H+ ion concentration within the CSF:

    • Increased PaCO2 causes increased carbon dioxide in the CSF, which leads to more H+ ions and decreased pH.

    • When pH falls, central chemoreceptors stimulate increased ventilation.

  • Reaction equation:
    CO2 + H2O \rightleftharpoons H2CO3 \rightleftharpoons HCO_3^- + H^+

PROLONGED HYPOVENTILATION

  • Chronic hypoventilation leads to diminishing stimulatory effects of CO2 on central chemoreceptors after 1-2 days due to renal compensation:

    • Kidneys increase HCO3- in response to respiratory acidosis, which raises CSF pH back to normal, reducing chemoreceptor stimulus and decreasing ventilation.

  • Additionally:

    • When PaCO2 falls, carbon dioxide influx into CSF decreases, H+ decreases, and pH increases, leading to decreased ventilation.

PERIPHERAL CHEMORECEPTORS

  • Located in:

    • Carotid Bodies (at common carotid artery bifurcation)

    • Aortic Bodies (above and below the aortic arch):

    • Respond primarily to changes in PaO2 and, to a lesser extent, PaCO2.

    • Carotid bodies also respond to decreases in arterial blood pH; aortic bodies do not.

  • When PaO2 drops below 60 mm Hg, impulses are sent to the medulla to increase ventilation.

LIMITATIONS OF PERIPHERAL CHEMORECEPTORS

  • Peripheral chemoreceptors do not respond effectively when:

    • Hemoglobin is displaced (carbon monoxide poisoning).

    • Hypoxia occurs due to chronic anemia.

CLINICAL IMPLICATIONS

  • Patients with advanced lung disease (CO2 retainers) have a high PaCO2 and low PaO2, leading to a failure of central chemoreceptors to respond. However, peripheral chemoreceptors may still control breathing in these patients.

  • NBRC Tip for Hypoxic Drive Theory (Oxygen-Induced Hypoventilation):

    • If PaO2 rises above 70 mm Hg and PaCO2 begins to rise in a patient with severe COPD during O2 therapy, ventilatory drive is suppressed; thus, O2 percentage should be decreased.

IRRITANT RECEPTORS

  • Irritant Receptors:

    • Vagal sensory fibers located in the airway epithelium (nose, trachea, pharynx, bronchi).

    • Stimulated by inhaled irritants (e.g., gases, dust, smoke), cold air, or mechanical stimulation during medical procedures.

    • Response includes bronchoconstriction, coughing, laryngospasm, tachypnea, and bradycardia.

J RECEPTORS (JUxtacapillary Receptors)

  • Juxtacapillary Receptors (J receptors):

    • Located adjacent to pulmonary capillaries, supplied by pulmonary blood and innervated by vagus nerve.

    • Stimulated by hypoxemia-related conditions (e.g., pulmonary edema, pneumonia).

    • Result in rapid shallow breathing and bradycardia.

OTHER RECEPTORS

  • Peripheral Proprioceptors:

    • Located in muscles, joints, and tendons; activated by movement/pain or sudden changes (e.g., cold water).

    • Increase ventilation response during exercise.

  • Muscle Spindle Fibers:

    • Present in the intercostal muscles and diaphragm; monitor muscle elongation.

    • Send proportional impulses to muscle contraction strength based on demand.