4.4 Oxygen Delivery Modalities, Capnography & Mechanical Ventilation Basics

Oxygen Delivery Modalities, Capnography & Mechanical Ventilation Basics

Core Principle: Oxygenation and ventilation are distinct physiological processes. Oxygenation is the transfer of oxygen from inspired gas into arterial blood ($PaO_2, SpO_2$), whereas ventilation is the mechanical elimination of carbon dioxide ($PaCO_2, ETCO_2$). The critical care specialist must master both modalities to optimize cellular respiration while preventing iatrogenic ventilator-induced lung injury (VILI) and oxygen toxicity.

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1. Oxygen Delivery Systems & Fraction of Inspired Oxygen (FiO2)

Selecting the appropriate oxygen delivery interface depends on patient tolerance, required $FiO_2$, underlying pathophysiology, and invasiveness.

Oxygen Toxicity & Absorption Atelectasis

• Mechanism: Prolonged high concentrations of inspired oxygen generate excessive reactive oxygen species (ROS) (superoxide anions, hydroxyl radicals, hydrogen peroxide) that overwhelm endogenous antioxidant enzymes (superoxide dismutase, catalase). ROS induce alveolar endothelial lipid peroxidation, surfactant inactivation, and pulmonary fibrosis.
• Absorption Atelectasis: Breathing 100% O2 washes out inert alveolar nitrogen ($N_2$). When all oxygen is rapidly absorbed into pulmonary capillaries, the alveolus loses its structural gas scaffold and collapses.
• Safe Threshold Rule: Avoid delivering $FiO_2 > 60%$ ($>0.60$) for longer than 24 to 48 hours (and $100% \text{ for }>12\text{--}24\text{ hours}$). Titrate $FiO_2$ down to the lowest level that maintains $SpO_2 \ge 94\text{--}98%$ ($PaO_2 \ge 80\text{ mmHg}$).

2. Pulse Oximetry Principles & Diagnostic Pitfalls

Pulse oximetry provides continuous, non-invasive estimation of arterial hemoglobin oxygen saturation ($SpO_2$) via spectrophotometry.

Physical Principles

• Uses a sensor with two light-emitting diodes: Red Light (660 nm) and Infrared Light (940 nm).
• Oxygenated hemoglobin ($HbO_2$) absorbs more infrared light (940 nm) and allows more red light to pass through.
• Deoxygenated hemoglobin ($Hb$) absorbs more red light (660 nm) and allows more infrared light to pass through.
• The microprocessor measures the ratio of pulsatile (arterial) light absorption to calculate $SpO_2$.

The Sigmoidal Oxyhemoglobin Dissociation Curve

• The Critical Cliff ($90% / 60\text{ mmHg}$ Rule): Because of the sigmoidal shape of the curve, as long as $PaO_2 > 80\text{ mmHg}$, $SpO_2$ remains stable at $95\text{--}100%$. However, below $SpO_2$ of $90%$, the curve plunges precipitously. An $SpO_2$ of $90%$ represents a $PaO_2$ of $60\text{ mmHg}$. Any further minor drop in $PaO_2$ causes a catastrophic plunge in arterial oxygen saturation and systemic tissue oxygen delivery ($DO_2$).

Clinical Pitfalls & Artifacts

• Hypoperfusion & Vasoconstriction: Severe hypovolemic shock, hypothermia, or administration of alpha-2 agonists (dexmedetomidine) causes peripheral vasoconstriction, leading to loss of pulsatile signal and erroneous readouts.
• Carbon Monoxide (CO) Poisoning: Carboxyhemoglobin ($COHb$) has an absorption spectrum at 660 nm virtually identical to $HbO_2$. The pulse oximeter reads falsely normal ($99\text{--}100%$) despite fatal tissue hypoxia.
• Methemoglobinemia: Methemoglobin absorbs equally at both 660 nm and 940 nm, locking the $SpO_2$ reading at approximately $85%$ regardless of the true arterial $PaO_2$.
• Tissue Pigmentation & Motion: Dark melanin pigment in the oral mucosa or shivering/motion artifact impairs optical transmission.

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3. Capnography Waveforms & Clinical Interpretation

Capnography is the continuous graphic recording of the carbon dioxide concentration in respiratory gases over time. It is the gold standard for monitoring ventilation, pulmonary perfusion, and equipment integrity.

                  NORMAL TIME-BASED CAPNOGRAM WAVEFORM

        CO2 (mmHg)
          50 ┤                       (ETCO2 Point)
             │                         ▼
          40 ┤             ┌───────────┐ (Phase III - Alveolar Plateau)
             │            / (α angle)   │ (β angle)
          20 ┤           / (Phase II)   │ (Phase 0 / IV - Inspiration)
             │          /               │
           0 └─────────┴────────────────┴─────────── Time
             (Phase I - Baseline)

The 4 Phases & Angles of the Normal Capnogram

• Phase I (Inspiratory Baseline): Represents exhalation of $CO_2$-free anatomical dead space gas (trachea, pharynx). Normal value is $0\text{ mmHg}$.
• Phase II (Expiratory Upstroke): Rapid ascent representing the mixing of dead space gas with carbon dioxide-rich alveolar gas.
• Alpha ($\alpha$) Angle: The angle between Phase II and Phase III (normally $100^{\circ}\text{ to }110^{\circ}$). Increases with bronchospasm or dynamic airway obstruction.
• Phase III (Alveolar Plateau): Exhalation of pure alveolar gas. The highest point at the very end of Phase III is the End-Tidal $CO_2$ ($ETCO_2$) (normal $35\text{ to }45\text{ mmHg}$).
• Beta ($\beta$) Angle: The angle between Phase III and the start of inspiration (Phase 0/IV), normally $\approx 90^{\circ}$. Increases with rebreathing.
• Phase 0 / IV (Inspiratory Downstroke): Rapid descent back to $0\text{ mmHg}$ as fresh, $CO_2$-free gas is inhaled.

Pathological Waveform Analysis

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4. Indications & Initial Settings for Positive Pressure Mechanical Ventilation (PPMV)

Mechanical ventilation is life-support therapy instituted when conservative medical and oxygen therapies fail to maintain adequate gas exchange or when the mechanical work of breathing becomes unsustainable.

                 INDICATIONS FOR MECHANICAL VENTILATION

  ┌─────────────────────────────────────────────────────────────┐
  │ 1. VENTILATORY (HYPERCAPNIC) FAILURE                        │
  │    • PaCO2 > 60 mmHg with progressive acidemia (pH < 7.20)  │
  │    • Unresponsive to medical therapy                        │
  └─────────────────────────────────────────────────────────────┘
  ┌─────────────────────────────────────────────────────────────┐
  │ 2. OXYGENATION (HYPOXEMIC) FAILURE                          │
  │    • PaO2 < 60 mmHg on FiO2 > 50% (P/F ratio < 200-300)     │
  │    • SpO2 < 90% despite aggressive oxygen supplementation   │
  └─────────────────────────────────────────────────────────────┘
  ┌─────────────────────────────────────────────────────────────┐
  │ 3. INTRACTABLE WORK OF BREATHING & FATIGUE                  │
  │    • Impending respiratory arrest / Diaphragmatic fatigue   │
  └─────────────────────────────────────────────────────────────┘

Initial Lung-Protective Ventilator Settings

To prevent Ventilator-Induced Lung Injury (VILI)—encompassing barotrauma (excessive pressure), volutrauma (alveolar overdistension), atelectotrauma (repeated alveolar collapse and reopening), and biotrauma (inflammatory cytokine release):

• Ventilation Mode: Volume-Controlled Ventilation (VCV) or Pressure-Controlled Ventilation (PCV).
• Tidal Volume ($V_T$): $8\text{ to }10\text{ mL/kg}$ (delivers physiological volume without overdistending alveoli; reduce to $6\text{--}8\text{ mL/kg}$ in ARDS/contusions).
• Respiratory Rate ($RR$): $12\text{ to }20\text{ breaths/min}$, titrated to maintain $PaCO_2$ at $35\text{--}45\text{ mmHg}$.
• Positive End-Expiratory Pressure (PEEP): $4\text{ to }8\text{ cmH}_2\text{O}$. Prevents end-expiratory alveolar collapse, recruits atelectatic lung units, and increases functional residual capacity (FRC).
• Peak Inspiratory Pressure ($PIP$): Limit to $<20\text{ cmH}_2\text{O}$ in dogs and $<15\text{ cmH}_2\text{O}$ in cats (maximum limit $25\text{ cmH}_2\text{O}$).
• Inspiratory-to-Expiratory ($I:E$) Ratio: $1:2\text{ to }1:3$ (Inspiratory time $T_I \approx 0.8\text{ to }1.2\text{ seconds}$) to ensure complete alveolar exhalation and prevent intrinsic "auto-PEEP".
• Inspired Oxygen ($FiO_2$): Start at $1.0$ ($100%$) during induction and stabilization, then rapidly wean to $<0.60$ ($<60%$) within $2\text{ to }4\text{ hours}$ as soon as $SpO_2 \ge 94%$ is achieved.

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5. Comprehensive ICU Nursing Care for the Ventilated Critical Patient

Mechanically ventilated patients require continuous 1-on-1 dedicated critical care nursing. The patient is entirely dependent on nursing interventions for survival.

                 ICU VENTILATOR NURSING PROTOCOLS

     ┌────────────────────────────────────────────────────────┐
     │ • Eye Lubrication: Sterile artificial tears q2h        │
     │ • Oral Swabs: 0.05% Chlorhexidine q4-6h (VAP defense)  │
     │ • Recumbency Changes: Rotate q2-4h (Sternal / Lateral) │
     │ • Endotracheal Suction: Closed sterile technique <10s  │
     │ • Heat-Moisture Exchanger (HME): Check & change q24h   │
     │ • Urinary Catheter: Closed Foley with UOP q1-2h        │
     │ • Passive ROM: Limb flexion/extension q4-6h            │
     └────────────────────────────────────────────────────────┘

1. Ocular Protection: Mechanically ventilated, heavily sedated patients lose the corneal blink reflex and tear production. Apply sterile ophthalmic lubricating ointment (artificial tears) to both eyes every 2 hours to prevent exposure keratitis and corneal ulceration.
2. Oral Hygiene & Subglottic Suctioning: Clean the oral cavity with 0.05% chlorhexidine gluconate solution swabs every 4 to 6 hours and suction the oropharynx above the cuff. This eliminates colonization of oral pathogens, significantly reducing the incidence of life-threatening Ventilator-Associated Pneumonia (VAP).
3. Closed Endotracheal Suctioning:
   • Pre-oxygenate with $100% \text{ O}_2$ for 2 minutes.
   • Advance a sterile suction catheter into the endotracheal tube.
   • Apply continuous vacuum for $<10\text{ seconds}$ while rotating and withdrawing the catheter. Prolonged suctioning causes catastrophic hypoxemia, atelectasis, and vagal bradycardia.
4. Patient Repositioning & Pulmonary Physical Therapy:
   • Rotate the patient between sternal, right lateral, and left lateral recumbency every 2 to 4 hours to prevent dependent hypostatic atelectasis and decubital pressure sores.
   • Perform passive range-of-motion (PROM) exercises and gentle massage on all four limbs every 4 to 6 hours to promote lymphatic and venous return.
5. Humidification & Circuit Management: Ensure in-line Heat-Moisture Exchangers (HME) or active heated circuit humidifiers are operational. Inspect breathing circuits for condensate accumulation and change circuits every 24 to 48 hours under sterile technique.
6. Indwelling Urinary Catheter Care: Maintain a sterile closed Foley catheter drainage system; measure urine output (UOP) hourly (target $>1\text{ to }2\text{ mL/kg/hr}$); clean the prepuce/vulva with dilute antiseptic every 8 hours.