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Respiratory Monitoring

Respiratory Monitoring. Dr Arthur Chun-Wing LAU Associate Consultant Department of Intensive Care PYNEH 28 Sep 2007. Gas exchange. Pulse oximetry Capnometry Continuous blood gas analysis Transcutaneous monitoring (PTCO2 and PTCCO2). Question. Which patient is more hypoxemic, and why?.

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Respiratory Monitoring

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  1. Respiratory Monitoring Dr Arthur Chun-Wing LAU Associate Consultant Department of Intensive Care PYNEH 28 Sep 2007

  2. Gas exchange Pulse oximetry Capnometry Continuous blood gas analysis Transcutaneous monitoring (PTCO2 and PTCCO2)

  3. Question • Which patient is more hypoxemic, and why? • Answer: A (Patient A, with the higher PaO2 but the lower hemoglobin content, is more hypoxemic) • Patient A: Arterial oxygen content = 7 x 1.36 x .95 + 0.0031 x 85 = 9.3 ml O2/dl • Patient B: Arterial oxygen content = 15 x 1.36 x .85 + 0.0031 x 55 = 17.5 ml O2/dl

  4. Arterial oxygen content • CaO2 = • (Hgb x 1.36 x SaO2) • + (0.0031 x PaO2) • Total oxygen content in the arterial blood in ml O2/dL (N: 18 – 21) • Oxygen bound to Hb • SpO2: measured by pulse oximeter • SaO2: calculated from PaO2 • Oxygen dissolved in plasma • negligible amount • Measured by electrode

  5. Question • Which of the following situation(s) would be expected to lower PaO2? • anemia • CO poisoning • Abnormal hemoglobin that holds oxygen with half the affinity of normal hemoglobin • Abnormal hemoglobin that holds oxygen with twice the affinity of normal hemoglobin • lung disease with intra-pulmonary shunting. • Answer: only 5 • 1 affects only content, not oxygen saturation or PO2. • 2 through 4 affect only oxygen saturation and content, not PO2

  6. Important • Neither the quantity nor quality of hemoglobin should affect the amount of dissolved oxygen, and hence should not affect the PaO2. • PaO2 is not a function of hemoglobin content or of its characteristics, but only of the alveolar PO2 and the lung architecture (alveolar-capillary interface).

  7. How to measure oxygen saturation • Calculated from pO2: as in most automated blood gas analyzers (Clark electrodes) • calculated from the measured parameters pO2 and pH, on the basis of standard oxygen-dissociation curves • Assumption: there is an otherwise normal hemoglobin dissociation curve, also provided that nothing else is binding to the Hb except O2 • Differential spectrophotometry: as in oximeter • Tonometry • process of exposing a liquid to an ambient gas phase in such a way that each gas in the gaseous phase partitions to an equilibrium between the liquid and gas phases. • For QC samples, checking linearity of electrodes, or for various hemoglobin studies. • Transcutaneous monitors • rely on the oxygen content of capillary blood • measured by heating skin locally to dilate capillaries • agrees well with arterial blood pO2 when tissue perfusion is adequate, but not in states of hypoperfusion

  8. Pulse Oximetry

  9. What is oxygen saturation % • Numerator • oxygenated Hemoglobin • carboxyhemoglobin (usu not present) • methemoglobin (usu not present) • Denominator • total oxygenated and deoxygenated hemoglobin

  10. Based on two physical principles • Photoplethysmography: “pulse” • to distinguish pulsatile arterial blood from venous blood • Spectrophotometry: “oximeter” • to distinguish oxyhemoglobin (oxyHb) and deoxyhemoglobin (deoxyHb)

  11. Typical pulse oximeter sensing configuration on a finger Probes for fingers and ear lobes are commonly used

  12. Two light-emitting diodes Emit light of the following wavelengths • 660 nm (red) • Absorbed more by dexoyHb • 940 nm (infrared) • Absorbed more by oxyHb • Ratio of absorbencies is calibrated against direct measurements of arterial oxygen saturation (SaO2) • Numeric value of the red-to-infrared ratio (R/IR) is converted to SpO2

  13. Bias +/- precision of SpO2 varies with levels of actual SaO2 • Accuracy varies widely because of different algorithms employed • Prudent to assume that SpO2 is over-estimating SaO2, and to take some action whenever SpO2 falls below 93% Jubran & Tobin. Chest 1990

  14. van Oostrom JH and Melker RJ. Anesth Analg 2004;98:1354 –8

  15. Question • Carbon monoxide (>= 1 answers) • a) shifts the oxygen dissociation curve to the leftb) lowers the PaO2c) lowers the arterial oxygen contentd) is elevated in the blood of cigarette smokers • Correct responses are a, c and d. • Carbon monoxide does not lower the PaO2

  16. CO poisoning > 200x affinity for Hb than O2

  17. Carboxyhemoglobin • COHb absorption spectrum is at 940nm (IR), i.e. similar to that of oxyHb • Therefore, read by the oximeter as if it were oxyHb (does not affect the reading) • For example • SpO2 95% can represent true SaO2 of 85% +10% COHb

  18. Question • Blood gas checked in room air: PaO2 is 77 mm Hg (10.3 kPa) • Below are three values for arterial oxygen saturation from this patient: • 95%, calculated from PaO2 value • 98%, from pulse oximeter • 85%, from co-oximeter • Question: • Which value is the most reliable? This patient actually had 10% carboxy-hemoglobin and 2% methemoglobin

  19. Answer • Co-oximeter directly measure oxyhemoglobin, carboxyhemoglobin and methemoglobin, using four wavelengths of light • Calculated SaO2 is only reliable if nothing else but oxygen is binding to hemoglobin, which you can't know from just the PaO2.

  20. Methemoglobinemia • MetHb depresses the SpO2 reading non-linearly • Absorbs R light (660 nm) similar to that by deoxyHb • Absorbs IR light (940 nm) to a greater extent than of both oxyHb and deoxyHb • Net result: MetHb causes the SpO2 to migrate toward 85%, further increases in [metHb] do not lower the SpO2 any further • For example: • True high SaO2 (>85%) condition: MetHb results in a falsely low SpO2 • True low SaO2 (<85%) condition: MetHb results in a falsely high SpO2

  21. Etiology • Fe within Hb is oxidized from the ferrous (Fe2+) state to the ferric (Fe3+) state • Effects: • formation of methemoglobin • inability to transport oxygen and carbon dioxide • brownish discoloration of the blood

  22. Causes of MetHb • Acquired: • Exposure to Nitrites, Aniline dyes, Silver nitrate, Nitroprusside, Antimalarials; • Local anesthetics (Benzocaine, prilocaine, and lidocaine, particularly when applied to mucosa, such as during bronchoscopy, or after repeated cutaneous exposure to eutectic mixture of lidocaine-prilocaine (EMLA(R) cream) over a short period of time; • Nitric and nitrous oxides; • Vegetables (eg, spinach, beets, carrots) inadequately cooked or contaminated with bacteria • Hereditary: deficiency of NADH cytochrome b5 reductase or NADPH-flavin reductase or the presence of hemoglobin M.

  23. The patient had been wrongly given sodium nitrite instead of the herbal NatriiSulfas (chiefly NaPO4) as a laxative for clearing heat and decreasing edema.

  24. Low perfusion-resistant pulse oximeter • Masimo Signal Extraction Technology (Masimo SET) • a unique method of measuring signals accurately and reliably in the presence of patient motion and low perfusion • employs 5 parallel processing engines

  25. Radical Signal Extraction Technology (SET) pulse oximeter, Masimo Corp. (Irvine, CA)

  26. Rad-57 portable pulse CO-oximeter, manufactured and entered by Masimo Corp. (Irvine, CA). • The Rad-57 is the world’s first pulse CO-oximeter. • Masimo Rainbow SET® technology analyzes 7+ wavelengths of light to accurately measure carboxyhemoglobin (SpCO®) andmethemoglobin (SpMet™) percent levels in the blood noninvasively and continuously

  27. Rainbow technology uses 7+ wavelengths of light to continuously and noninvasively measure carboxyhemoglobin (SpCO®) and methemoglobin (SpMet™) Perfusion Index (PI) with trending capability indicates arterial pulse signal strength and may be used as a diagnostic tool during low perfusion. Pleth Variability Index (PVI): captures vital thoracic pressure changes that may compromise normal cardiac function affecting systemic circulation Accurate on cyanotic patients. Signal IQ® waveform for signal identification and quality indication during excessive motion and low signal to noise situations.

  28. Transcutaneous monitoring • Measures O2 and CO2 diffusing through the skin • Mainly used in infants (NICU, PICU) • Relis on the oxygen content of capillary blood • agrees well with arterial blood pO2 when tissue perfusion is adequate, but not in states of hypoperfusion • measured by heating skin locally to dilate capillaries • The heat emitted by the electrode may cause areas of redness on the skin. Hence, the site of placement of the sensor needs to be changed regularly.

  29. Continuous blood gas monitoring • A fiber optic sensor with three sensing elements for monitoring pH, PCO2, and PO2 plus a thermocouple for measuring temperature. • Length: 30 cm; diameter <0.5 mm, dead space 6/10,000 of a milliliter • A Y connector on the sensor allows simultaneous continuous blood pressure monitoring and enables intermittent withdrawal of blood samples and infusions. • Performance in the clinical setting was not as satisfactory, especially for PO2 values. (Ganter M and Zollinger A. Br J Anaesth 2003) Diametrics Paratrend 7+ sensor for adults

  30. Capnography

  31. Capnography • Measurement of CO2 in expired gas • Analyzers utilize • Infrared (employed in most intensive care units) • mass or Raman spectra technology • photoacoustic spectra technology

  32. Mainstream or sidestream analyzers • Mainstream analyzer • sampling window inside the ventilator circuit for CO2 measurement • Sidestream analyzer • aspirates gas from the ventilator circuit, and the analysis occurs away from the ventilator circuit

  33. Mainstream CO2 sensor • During exhalation, exhaled gas passes directly over the sensor • Designed primarily for intubated patients • Difficult to use in nonintubated patients because • it requires a mouthpiece or mask that patients in respiratory distress find uncomfortable • In addition, when a mask is used, any supplemental oxygen must be delivered at a flow rate of greater than 6 Lpm to ensure the patient does not rebreathe CO2 that can accumulate in the mask at lower oxygen flow rates. • extra weight of the sensor dragging on the endotracheal tube

  34. Sidestream • In sidestream capnography, a sample of exhaled gas is aspirated from the patient’s airway interface into the monitor, which houses the sensor. • Designed for use in both intubated and nonintubated patients. • Prone to obstruction with moisture. • Microstream technology is a unique low-flow (50 cc/min) system that permits precise measurement of CO2 levels without problems of dilution or moisture accumulation.

  35. Normal capnogram Expiration: • A or phase 1: Carbon dioxide cleared from the anatomic dead space • B or phase 2: dead space and alveolar carbon dioxide • C or phase 3: alveolar plateau • D or phase 4: end-tidal carbon dioxide tension (PETCO2): normal range of ETCO2 is 35 – 45 mm Hg. Inspiration • Inhaled PCO2 is zero (the amount in inhaled air is negligible)

  36. DISLODGED ETT: Loss of waveform, Loss of ETCO2 reading CPR: “Square box” waveform; baseline CO2 = 0; ETCO2 = 10-15 mm Hg (possibly higher) with adequate CPRManagement: Change rescuers if ETCO2 drops < 10 ROSC: As in CPR, but ETCO2 rises above 10-15 mm HgManagement: Check for pulse

  37. “ SHARKFIN” (Slanting and prolonged phase 2 and increased slope of phase 3 ) with/without prolonged expiration = Bronchospasm (asthma, COPD, allergic rxn) Esophageal intubation: Small CO2 spikes Hypoventilation: low RR, gradually increases ETCO2 values > 45 mmHg with normal base line RISING BASELINE = Patient is rebreathing CO2:Rebreathing producing gradual elevation of base line and ETCO2 values Management: Check equipment for adequate oxygen inflow, allow intubated patient more time to exhale

  38. Onset of hyperventilation: results in gradual lowering of ETCO2 values. Sustained hyperventilation: high RR; shortened waveform; baseline ETCO2 = 0; ETCO2 < 35 mm Hg PATIENT BREATHING AROUND ET TUBE: angled, sloping downstroke on waveformAdult: Broken cuff or tube is too smallPediatric: tube is too small

  39. Common indications • INTUBATED APPLICATIONS:• Verification of ETT placement• ETT surveillance during transport• CPR: compression efficacy, early sign of ROSC, survival predictor • NON-INTUBATED APPLICATIONS:• Bronchospasm: asthma, COPD, anaphylaxis• Hypoventilation: drugs, stroke, CHF, post-ictal• Shock & circulatory compromise• Hyperventilation syndrome: biofeedback monitor

  40. Estimation of PaCO2 • In health: PetCO2<= PACO2 <= PaCO2 • In healthy subjects, PaCO2 - PetCO2 is 1 to 5 mmHg • Correlation coefficients between PetCO2 and PaCO2 are 0.69 to 0.92 • In patients with lung disease and ventilation-perfusion (V-Q) imbalance • PaCO2 - PetCO2 is much higher because of increased physiologic dead space • Once the difference between the two values is established, and providing the patient remains clinically stable, the PetCO2 may be followed in lieu of the PaCO2.

  41. Limitations • Composition of the respiratory gas mixture • High concentrations of either or both oxygen or nitrous oxide may affect the capnogram • When a gas that the mass spectrometer cannot detect (such as helium) is present, the reported values of CO2 are incorrectly elevated in proportion to the concentration of helium present. • High breathing frequencies • exceed the response capabilities of the capnograph. • The presence of Freon (used as a propellant in metered dose inhalers) • artificially increase the CO2 reading of mass spectrometers (ie, to show an apparent increase in [CO2]) • A similar effect has not yet been demonstrated with Raman or infrared spectrometers. • Contamination of the monitor or sampling system by secretions or condensate, a sample tube of excessive length, a sampling rate that is too high, or obstruction of the sampling chamber can lead to unreliable results.

  42. Respiratory Neuromuscular function Airway Occlusion Pressure Breathing Pattern Maximal Inspiratory Airway Pressure (MIP) Maximal Expiratory Pressure (MEP) Transdiaphragmatic pressure

  43. Airway Occlusion Pressure • = Negative pressure generated 0.1 sec following onset of inspiration against an briefly and surreptitiously occluded airway • Measured by inserting a shutter in the ventilator's inspiratory line as near as possible to the patient, and recording airway pressure tracing at the Y piece by maintaining occlusion only for the first 200 to 300ms of inspiration • Why first 100 ms? • chosen because a normal subject requires at least 150 ms to sense the occlusion and react against it

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