Comprehensive Notes on Oxygenation and Ventilation Monitoring

Rationale for Non-Invasive Oxygenation and Ventilation Monitoring

• Definitions and Basics:
  • Oxygenation vs. Ventilation: These are two distinct physiological processes. Oxygenation refers to providing $O_2$ for metabolism, while ventilation refers to the removal of $CO_2$ from metabolism.
  • Monitoring Modalities: Pulse oximetry is used for oxygenation monitoring ($SpO_2$), while capnography is used for ventilation monitoring ($EtCO_2$).
• Rationale for Non-Invasive Monitoring:
  • Simplicity: Pulse oximeters are simple bedside devices measuring Hemoglobin ($Hb$) saturation via a non-invasive probe.
  • Patient Comfort: Significantly more comfortable for the patient compared to invasive blood draws.
  • Safety: Lower risk of infection or medical complications.
  • Continuity: Provides a continuous form of monitoring and trending rather than a single point in time.

Principles of Pulse Oximetry

• Spectrophotometry: This is the measurement of the amount of light absorbed as it passes through a substance. Light can be reflected, absorbed, or transmitted.
• Photospectrometry in Pulse Oximetry:
  • Light passes through a capillary bed.
  • Color changes depending on the amount of saturated Hemoglobin ($Hb$).
  • Oxygenated blood is more permeable to red light; the oximeter relates this change to oxygen saturation.
• Light Emitting Diode (LED) Technology: The sensor is composed of two primary parts:
  • A light-emitting diode (LED).
  • A photodetector.
• Measurement vs. Monitoring:
  • Measurement: A one-time "snapshot" (e.g., an Arterial Blood Gas ($ABG$) sample or a "spot-check" $SpO_2$).
  • Monitoring: Ongoing, continuous measurements (e.g., continuous pulse oximetry).

The 40-50-60/70-80-90 Rule

• Relationship between $SpO_2$ and $PaO_2$: To relate Hemoglobin saturation ($Hb$ saturation %) to blood oxygenation ($PaO_2$) without a curve, use this rule (assuming normal $pH$, $PCO_2$, and $Hb$ values):
  • $70\% \text{ saturation} \approx 40\,mm\,Hg \text{ } PaO_2$
  • $80\% \text{ saturation} \approx 50\,mm\,Hg \text{ } PaO_2$
  • $90\% \text{ saturation} \approx 60\,mm\,Hg \text{ } PaO_2$
• Limitations of the Rule:
  • This rule only applies in the middle range of $PO_2$ values where the oxyhemoglobin dissociation curve is most linear.
  • It should not be applied when saturations are greater than $90\%$.
  • A saturation of $100\%$ could represent a $PaO_2$ as high as $200\,mm\,Hg$.

Sensor Application and Accuracy

• The Golden Rule for Safety Monitors: A safety monitor must be SAFE, ACCURATE, RELIABLE, and CONVENIENT. It must follow the principles of "Always detect danger" and "No false goodness."
• Application Sites:
  • Adults & Children: Finger, toe, ear lobe, or forehead.
  • Small Children & Babies: Wrist, upper arm, across the foot, or toe.
• Sensor Misalignment: Inserting a finger too far into the probe can cause errors. A sudden fall in readings without a change in patient condition suggests a probe malfunction or need for reattachment.
• Interference and Artifacts:
  • Absorption Interference: Caused by Anemia, shock/hypotension, skin pigmentation, nail polish (especially blue, beige, purple, and white), carbon monoxide ($CO$), Methemoglobin ($MetHb$), and intravascular dyes.
  • Signal Artifact: Can obscure the signal or cause errors. Causes include low perfusion, hypotension, cold, and motion artifact.
• Diagnostic Considerations:
  • Pulse oximetry reflects changes in oxygenation within $5 \text{ minutes}$.
  • It is not accurate when saturation is below $70\%$.
  • Finger probes placed on the ear or forehead are grossly inaccurate.

Hypoxemia and Hypoxia

• Definitions:
  • Hypoxemia: Abnormally low arterial oxygen tension ($PaO_2$). It may suggest hypoxia but does not always mean absolute tissue hypoxia (e.g., low oxygen may be offset by high cardiac output).
  • Hypoxia: Inadequate oxygen for aerobic cellular metabolism, leading to anaerobic metabolism, lactic acid production, and acidosis. Clinical signs include tachycardia, hypertension, dizziness, and mental confusion.
• Hypoxemia Classifications (Rule of Thumb):
  • Normal: $80-100\,mm\,Hg$
  • Mild Hypoxemia: $60-80\,mm\,Hg$
  • Moderate Hypoxemia: $40-60\,mm\,Hg$
  • Severe Hypoxemia: $< 40\,mm\,Hg$
• Types of Hypoxia:
  • Hypoxic (Hypoxemic) Hypoxia: Low arterial oxygen tension ($PaO_2$). Causes: hypoventilation, high altitude, diffusion impairment (fibrosis, edema), V/Q mismatch, shunting.
  • Anemic Hypoxia: Normal $PaO_2$, but low oxygen-carrying capacity. Causes: anemia, hemorrhage, carboxyhemoglobin, methemoglobin.
  • Circulatory (Stagnant/Hypoperfusion) Hypoxia: Inadequate blood flow to tissues. Causes: slow peripheral blood flow, arterial-venous shunts.
  • Histotoxic Hypoxia: Impaired cellular ability to metabolize oxygen. Cause: Cyanide poisoning.

Consequences of Chronic Hypoxia

• Polycythemia: Chronic hypoxia stimulates the bone marrow via hormones to increase Red Blood Cell ($RBC$) production (erythropoiesis). While this increases oxygen-carrying capacity, it increases blood viscosity.
• Pulmonary Effects:
  • Hypoxic Vasoconstriction: The lungs direct blood away from hypoxic regions. Large-scale vasoconstriction in end-stage disease increases resistance and leads to pulmonary hypertension.
• Cor Pulmonale: Right heart failure resulting from chronic pulmonary disease. Mechanics include:
  • Increased blood viscosity from polycythemia.
  • Increased pulmonary resistance from vasoconstriction.
  • Destruction of the pulmonary vascular bed.
  • Physical Signs: Jugular vein distension ($JVD$), peripheral/pitting edema in extremities, and liver enlargement.

Carbon Monoxide (CO) Poisoning

• Properties of CO: Colorless, odorless, tasteless, and toxic gas resulting from incomplete combustion of carbon-based materials.
• Physiology:
  • $CO$ affinity for hemoglobin is greater than $200 \times$ that of oxygen.
  • Forms Carboxyhemoglobin ($SpCO$ or $COHb$), reducing oxygen-carrying capacity.
  • Intracellular toxin: binds with myoglobin in muscle, interferes with heart and skeletal muscle, and alters oxygen release to cells.
• Clinical Manifestations of $SpCO$ Levels:
  • $0-4\%$: None - Normal
  • $5-9\%$: Minor Headache
  • $10-19\%$: Headache, Shortness of Breath
  • $20-29\%$: Headache, Nausea, Dizziness, Fatigue
  • $30-39\%$: Severe Headache, Vomiting, Vertigo, Altered Level of Consciousness ($ALOC$)
  • $40-49\%$: Confusion, Syncope, Tachycardia
  • $50-59\%$: Seizures, Shock, Apnea, Coma
  • $60\% \text{ and up}$: Coma, Death
• Normal vs. Poisoning Levels:
  • Normal: $1-3\%$
  • Smokers: $5-10\%$
  • Heavy Smokers: $10-15\%$
  • $CO$ Poisoning: $> 20\%$
• Treatment and Half-Life of CO:
  • Breathing Room Air: Half-life is approx. $240\text{ minutes}$.
  • High-flow $100\% O_2$ (NRM): Half-life decreases to $40-60\text{ minutes}$.
  • Hyperbaric Oxygen Treatment (HBOT): at $2.5\text{ to } 3 \text{ Atmospheres}$, half-life decreases to $20-24\text{ minutes}$.
  • Thresholds for HBOT: Adults $> 25\%$, Pediatrics and Pregnant females $> 15\%$, or presence of neurologic compromise.
• Pulse CO-Oximetry: The Masimo RAD-57 uses Signal Extraction Technology ($SET$) and Rainbow SET Technology (analyzing $7+$ wavelengths) to measure carboxyhemoglobin non-invasively.

Capnography Fundamentals

• Definitions:
  • Capnometry: Measurement of $CO_2$ at the airway during ventilation.
  • Capnometer: The machine that displays the numeric values.
  • Capnography: The graphical display of the $CO_2$ waveform against time using infrared absorption.
  • $PetCO_2$: Partial pressure of $CO_2$ at the end of an exhaled tidal volume ($Vt$). It is usually $5-10\,mm\,Hg$ lower than arterial $PaCO_2$.
• Sensors: Mainstream vs. Sidestream:
  • Mainstream:
    • Advantages: Fast response, no sample flow required (no tidal volume reduction), real-time readings.
    • Disadvantages: Secretions/humidity block sensor, bulky at the airway, difficult for non-intubated patients, weight/deadspace issues (especially in neonates).
  • Sidestream:
    • Advantages: No bulky heater/sensor at airway, disposable sample line, can be used for non-intubated patients.
    • Disadvantages: Secretions block tubing, requires water trap, slow response time, sample flow may decrease delivered tidal volume in intubated neonates.

The Capnogram Waveform

• Four Phases of the Normal Capnogram:
  • Phase I: Start of exhalation via anatomic dead-space emptying.
  • Phase II (Transitional): Beginning of $CO_2$/alveolar plateau; progressive emptying from proximal alveoli and central airways.
  • Phase III (Alveolar Plateau): Exhalation of alveolar gas. The end-point is the $PetCO_2$ reading.
  • Phase IV (Inspiratory Phase): End of expiratory phase and start of inhalation; $CO_2$ drops back to zero.
• Bronchospasm ("Shark Fin"): Loss of the sharp upslope in Phase II and alteration of Phase III. Suggests asthma, COPD, or airway obstruction.

Clinical Interpretations of EtCO2 Changes

• Sudden/Exponential Decrease in $EtCO_2$: Sudden hyperventilation, sudden decrease in cardiac output (hypovolemic shock), cardiac arrest, massive pulmonary embolism, ventilator disconnection, or tube obstruction.
• Sudden Increase in $PetCO_2$: Sudden increase in cardiac output, sudden release of a tourniquet, or injection of Sodium Bicarbonate.
• Gradual Decline: Hyperventilation, decrease in oxygen consumption, or decreased pulmonary perfusion.
• Gradual Increase/Persistently High: Hypoventilation/insufficient ventilation, metabolic alkalosis, increased metabolism, or shivering (increased muscular activity).
• CPR Applications:
  • $PetCO_2 < 10\,mm\,Hg$ indicates a need to improve compression quality (rate/depth).
  • Abrupt and sustained increase in $PetCO_2$ is an indicator of Return of Spontaneous Circulation ($ROSC$).
• Intubation Verification:
  • $PetCO_2$ should be used to confirm Endotracheal Tube ($ETT$) placement.
  • A reading of $0\%$ immediately after intubation indicates esophageal placement (with the exception of prolonged cardiac arrest without compressions).

Questions & Discussion

• Scenario 1: A disoriented post-op male exhibit tachypnea and cyanosis with an $SpO_2$ of $93\%$.
  • Recommendation: Oxygen therapy or assessment for ventilation issues is required as physical symptoms (cyanosis/tachypnea) contradict the seemingly borderline $SpO_2$.
• Scenario 2: In conditions of low blood flow, which site is more accurate than a finger?
  • Answer: The forehead (or ear).
• Scenario 3: What is a problem with mainstream sensors?
  • Answer: Accumulation of moisture, secretions, and debris.
• Scenario 4: During a Code Blue with no cardiac compressions, the $CO_2$ detector reads $0\%$. Is the tube definitely in the esophagus?
  • Discussion: Not necessarily. Without cardiac compressions, there is no circulation to bring $CO_2$ to the lungs for exhalation, which can result in a false zero reading even if the tube is in the trachea.
• Scenario 5: A mechanically ventilated patient's expired $CO_2$ drops to near zero, but bilateral breath sounds are good and connections are tight. What is the problem?
  • Answer: Likely a sudden sensor failure or total obstruction of the sampling line/chamber.