Respiratory alternans: cycling of high frequency and volume with low frequency and volume to apnea.
May indicate brainstem injury, stroke, heart failure, or high altitude.
Respiratory examination in neuromuscular disease
Commonly used to measure respiratory rates and approximate tidal volume.
Utilize ECG leads and measure changes in impedance generated by the change in distance between leads as a consequence of the thoraco‐abdominal motions of breathing.
Leads should be placed at points of maximal change in abdominal contour.
Fail to detect obstructive apnea.
Detect tachypnea more accurately and may falsely report bradypnea.
Monitoring of oxygenation: pulse oximetry
Routine for critically ill patients.
Non‐invasive, transcutaneous measurement of the oxygen saturation of hemoglobin in arterial blood by spectrophotometry and optical plethysmography.
Oximeters distinguish between oxyhemoglobin and reduced hemoglobin on the basis of their different absorption of light (oxyhemoglobin absorbs less red and more infrared light than reduced hemoglobin).
Probes are attached to digits, nose, ear lobes, or forehead (where the vascular density is much higher than other areas).
The response time of the ear lobe measurement is faster than from the finger probes by approximately 6 seconds. Blood flow is measured from the supraorbital artery in which blood flow is abundant and less likely to be affected by vasoconstriction.
Requires adequate perfusion.
Closely correlates with direct arterial measurement in a well perfused patient when the oxygen saturation is in range of 70–100%.
Essential method to monitor arterial oxygen saturation during transport of critically ill patients.
Low flow states (Raynaud’s, shock), irregular heart rates.
Motion artifact, hand tremors, Parkinson’s disease.
Nail deformities, hyperpigmentation, nail polish.
Caveats and technical difficulties:
Methemoglobin, carboxyhemoglobin, sulfhemoglobinemia: should assess with co‐oximeter measurement on whole blood.
Hypothermia may cause poor quality signal (<35°C) or loss of signal detection (<26.5°C) due to vasoconstriction.
Normal values do not exclude tissue anoxia.
Normal values do not reflect adequate arterial oxygen content.
Strong electromagnetic waves affect the sensor readings. MRI safe system should be used. Severe burns associated with pulse oximetry have occurred in patients undergoing MRI.
Ambient light in the room may alter the photodetectors’ sensitivity (e.g. in the operating room) and may cause falsely low or falsely high values depending on the light wavelengths (Table 20.1).
High venous pressures (e.g. compartment syndrome, tourniquet).
Table 20.1Potential causes of false high and low values in pulse oximetry.
False low values
False high values
Tourniquet or manometer cuff
Nail polish, acrylic nails, nail deformity
Elevated glycohemoglobin A1c (rarely)
Hyperpigmentation Methylene blue
Using pulse oximetry in the ICU
In a normal adult, the result of oxygen saturation obtained from an ABG must correlate with the SpO2 obtained by the pulse oximetry probe. An oxygen saturation gap is present when there is more than a 5% difference.
Pulse oximetry should not be used as a primary monitoring modality in the following situations:
In hypervolemia and shock.
For detecting worsening lung function in patients on a high concentration of oxygen.
For monitoring during induced or acquired hypothermia.
Monitoring of ventilation: capnography
Capnography is the measurement of exhaled CO2 concentration over time.
Capnography uses infrared absorbance to determine exhaled CO2 values.
Capnography monitors samples of expired CO2 by using mainstream or sidestream techniques. The mainstream technique measures end‐tidal carbon dioxide (ETCO2) directly from the patient’s respiratory circuit (sensor is located at the hub of the endotracheal tube) and is used in intubated patients. The sidestream technique measures ETCO2 using a nasal cannula (sample gas is analyzed by a sensor inside the monitor) and is used in both non‐intubated and intubated patients.
Capnography reflects ventilation, perfusion, and metabolism and provides valuable information about the effectiveness of CO2 elimination, CO2 transport, and CO2 production.
Colorimetric capnography: filter color changes from purple to yellow and detects carbon dioxide and confirms tracheal intubation.
Quantitative waveform capnography offers continuous, non‐invasive measurement and graphic display of ETCO2 (Figure 20.1).
Qualitative capnography provides a range of ETCO2 values (e.g. 0–10 mmHg or >35 mmHg).
Acute clinical situation monitoring
Confirmation of endotracheal tube placement.
Qualitative assessment of cardiac output during CPR (ROSC).
Qualitative assessment of airway obstruction in asthma (COPD).
Routine monitoring applications for capnography
Monitoring of adequacy of ventilation and V/Q relationships.
Monitoring mechanical ventilation: identification of leak or disconnection.
Maintenance of endotracheal tube position (e.g. during transport).
Monitoring sedation in a non‐intubated patient (e.g. procedural sedation).
Maintenance of optimal ventilation for hypocapnia in neurosurgery.