Comprehensive Notes – Spirometry, Oxygen & CO₂ Monitoring, and Indirect Calorimetry

Application Scenarios – Spirometry & Lung-Volume Measurements

• Bellows spirometer reads lower exhaled volumes than hospital lab
  • No leaks & patient unchanged → most likely error: failure to correct to BTPS (Body Temperature, Pressure, Saturated) conditions.
  • Exhaled gas cools on the way to bellows ⇒ volume shrinks.
  • Correcting to $37\,^{\circ}\text C$ restores true lung volume.
• Hand-held turbine (respirometer) used for FVC
  • Patient blows an apparent $5\,\text L$.
  • Device accurate only when peak flow ≤ $300\,\text{L·min}^{-1}$.
  • Higher flows add inertia → vanes overspin → falsely high volume.
• Post-op abdominal patient, shallow breaths on bed rest
  • Start incentive spirometry to give visual feedback & encourage deep breaths.
  • Adequate analgesia remains essential to prevent splinting.

Objective 1 – Oxygen Analysis

• Clinical need: know exact $FiO_2$ delivered.
• General accuracy: ±$2\%$ of reading (e.g., 30 % ⇒ actual 28–32 %).

Polarographic Analyzer (Clark electrode)

• Electrochemical cell; measures partial pressure, converts to %O₂.
• O₂ diffuses across Teflon membrane → current proportional to $P<em>{O</em>2}$ → display %.
• Pressure-dependent: higher ambient/barometric or circuit pressure exaggerates reading.
• Calibrate at both $21\%$ & $100\%$ O₂; failure ⇒ replace electrode.
• Battery-powered; continuous or spot use.

Galvanic Cell Analyzer

• Also electrochemical & pressure-dependent.
• Generates its own current; no external power.
• Slower response but often longer cell life.

Electrical (paramagnetic/thermal) O₂ analyzers

• Common in PFT labs (e.g., helium dilution), less in bedside care.

Objective 2 – Pulse Oximetry

• Provides continuous or intermittent SpO₂; detects only functional hemoglobin (HbO₂ + Hb).

Physics

• Spectrophotometry: measures differential absorption at two λ (usually 660 nm red, 940 nm IR) by HbO₂ vs. Hb.
• Photoplethysmography: isolates pulsatile change in light absorption to distinguish arterial blood from static tissues.
• Accuracy ±$2–4\%$ for SpO₂ 70–100 % (good perfusion, no interference).

Probe Considerations

• Secure, limit ambient light; choose site (finger, earlobe, forehead) with adequate perfusion.

Interferences

• Intravascular dyes: methylene blue, indigo carmine ↓ SpO₂; indocyanine green minimal effect.
• Motion, low perfusion, vasoconstriction, dark skin, nail polish, ambient light all degrade accuracy.
• Dysfunctional Hb (COHb, MetHb) not differentiated → CO poisoning falsely high SpO₂.

SpO₂ vs. PaO₂ Relationship

• Follows O₂-Hb dissociation curve.
  • $PaO<em>2 = 60\,\text{mmHg} \Rightarrow SpO</em>2 \approx 90\%$.
  • Curve plateau: changes in PaO₂ >100 mmHg barely alter SpO₂.

Pulse-Ox vs. Co-oximetry (lab gold standard)

• Co-oximetry uses ≥4 wavelengths, measures SaO₂ + COHb + MetHb; accuracy ±1–2 %.
• Pulse-ox cheap, continuous, but blind to dyshemoglobins.

CPG Highlights

• Indications: monitor O₂Hb saturation; evaluate response to therapy; guide during bronchoscopy.
• Contra-indications: need for pH/PaCO₂/total Hb; abnormal Hb.
• Record: probe type/site, FiO₂, patient appearance, HR agreement, simultaneous ABGs.

Objective 3 – Capnometry & Capnography

• Measures $P<em>{CO</em>2}$ in exhaled gas; waveform = capnogram.

Technologies

1. Colorimetric (disposable): pH-indicator paper turns yellow with >$0.5\%$ CO₂ → confirms tracheal intubation.
2. Infrared Spectroscopy (most common)
   • CO₂ absorbs IR at $4.26\,\mu m$.
   • Types: single-beam with rotating chopper cells; double-beam with reference gas.
   • Interference: $H<em>2O$, $N</em>2O$ absorb nearby wavelengths → drying agents & filters used.
   • Configurations:
     • Sidestream—small sample suctioned (≈0.5 L·min⁻¹); delay, clog risk.
     • Mainstream—sensor inline at airway; zero delay but adds dead space & weight.
3. Mass Spectrometry: ions separated by mass/charge; expensive; multi-patient ORs.
4. Raman scattering (rare).

Capnogram Phases

• I : anatomic dead space (no CO₂).
• II : rapid up-stroke (mix dead + alveolar).
• III : alveolar plateau; slope indicates V/Q inhomogeneity.
• IV : sharp drop on inspiration.

Abnormal Waveforms & Causes

• Prolonged Phase III, no plateau → obstructive diseases (e.g., COPD).
• Rebreathing, oscillations, curare cleft, hyper/hypoventilation, Cheyne-Stokes all have distinctive shapes.

Clinical Uses

1. Confirm airway placement & monitor supraglottic devices.
2. Evaluate CPR quality (EtCO₂ ∝ pulmonary perfusion).
3. Optimize ventilation, detect circuit disconnections.
4. Trend pulmonary disease severity & treatment response.
5. Estimate metabolic rate (volumetric CO₂ elimination).

Objective 4 – Transcutaneous $P<em>{O</em>2}$ & $P<em>{CO</em>2}$

• Heated skin electrodes (42–$45^{\circ}\text C$) increase perfusion & melt lipids → better diffusion.

Electrode Types

• PtcO₂: Clark polarographic with Teflon membrane.
• PtcCO₂: Stowe-Severinghaus pH glass with bicarbonate buffer.

Accuracy & Influences

• Best in neonates (thin skin, good perfusion).
• Hypotension, vasopressors, edema, thick skin ↓ accuracy.
• PtcCO₂ slightly overestimates PaCO₂ (local metabolism); PtcO₂ often underestimates PaO₂ (skin consumption).

Technical Points

• Calibrate before each placement; change site q 4–6 h to prevent burns.
• Heater power ↑ implies ↑ perfusion → usually good; ↓ may indicate poor blood flow or sensor fault.
• Validate against ABG at initiation & periodically.
• Hazards: skin injury, false results leading to wrong therapy.

Indications

1. Trend adequacy of oxygenation/ventilation.
2. Evaluate therapy; early hypoperfusion index (PtcO₂/FiO₂).
3. Wound care, peripheral revascularization assessment.

Objective 5 – Calorimetry (Nutrition–Respiration Interface)

Importance

• Malnutrition ⇒ diaphragm weakness, ↓ immune defense.
• Overfeeding ⇒ ↑ $VCO_2$, harder to ventilate; excessive carbohydrate especially raises CO₂ burden.

Indirect Calorimetry

• Measures $VO<em>2$ & $VCO</em>2$ to compute energy expenditure.
• Respiratory Quotient (RQ)$RQ = \frac{VCO<em>2}{VO</em>2}$
  • Carbohydrate ≈ 1.0, Fat ≈ 0.7, Protein ≈ 0.8.
  • RQ > 1 ⇒ lipogenesis/overfeeding; RQ < 0.7 ⇒ underfeeding/ketosis.

Open-Circuit Method

• Separate inspiratory/expiratory flows; FiO₂ stable (<0.60 ideal); leaks invalidate data.

Closed-Circuit Method

• Patient rebreathes O₂-filled bag; CO₂ absorbed; O₂ use inferred from volume drop; bulky but no FiO₂ limit.

Advantages over Predictive Equations

• Real-time, patient-specific; prevents over/underfeeding; works on ventilated pts.

Supplementary Clinical Scenarios

• Blender set 80 %, analyzer reads 70 % → analyzer not calibrated at 21 % & 100 %.
• Smoke-inhalation pt. with SpO₂ 100 % on 5 L NC
  • Do not wean O₂.
  • Pulse-ox misreads COHb as HbO₂ → false high.
  • Treat with 100 % high-flow O₂ to displace CO.
• COPD pt. on ventilator shows slow-rising Phase III → high airway resistance causing slow alveolar emptying.
• Transcutaneous monitor power ↑ 5 % → heater compensating for ↑ local perfusion; beneficial, expect improved accuracy.

Key Numerical & Formula Summary

• Polarographic/galvanic O₂ analyzer accuracy: ±$2\%$.
• Turbine respirometer accurate flows ≤$300\,\text{L·min}^{-1}$.
• Pulse-ox accuracy: ±$2–4\%$ for SpO₂ 70–100 %.
• Capnograph sidestream sample flow ≈$500\,\text{mL·min}^{-1}$.
• Heater temp for transcutaneous electrodes: $42–45^{\circ}\text C$.
• Cardiac index effects: >$2.2\,\text{L·min}^{-1}\text m^{-2}$ ⇒ PtcO₂/PaO₂ ≈0.5; <$1.5$ ⇒ ≈0.1.

Practical, Ethical & Safety Notes

• Always correlate non-invasive monitors with ABGs when clinical picture disagrees.
• Misinterpretation (e.g., CO poisoning, poor calibration) may cause grievous harm—ensure calibration, understand device limitations.
• Regularly document settings, sites, patient status to maintain continuity of care & meet infection-control standards (clean probes per manufacturer).