1 What is the difference between capnometry and capnography? Which is better?

Capnometry is defined as the numeric measurement and display of the level of CO₂. It is not nearly as valuable as capnography, in which the expired CO₂ level is graphically displayed as a function of time and concentration.

2 Describe the most common method of gas sampling and the associated problems

Sidestream devices aspirate gas (typically 50 to 250 ml/min), usually from the Y-piece of the circuit, and transport the gas via small-bore tubing to the analyzer. Sampling can also be performed by nasal cannulas; but, because of room air entrainment and dilution of the CO₂ concentration, the Y-piece provides qualitatively and quantitatively a better sample than nasal cannulas. Problems with sidestream measurement include a finite delay until the results of the gas sample are displayed and possible clogging of the tubing with condensed water vapor or mucus. Infrared absorption is the most common method of CO₂ analysis.

3 What is the importance of measuring CO₂?

End-tidal carbon dioxide (ETCO₂) monitoring has been an important factor in reducing anesthesia-related mortality and morbidity. Short of bronchoscopy, carbon dioxide monitoring is considered the best method of verifying correct endotracheal tube (ETT) placement. In addition to its value as a safety monitor, evaluation of expiratory CO₂ provides valuable information about important physiologic factors, including ventilation, cardiac output, and metabolic activity, and proper ventilator functioning. ETCO₂ levels have also been used to predict outcome of resuscitation. A prospective observation study of 150 out-of-hospital cardiac arrests found that an ETCO₂ level less than 10 mm Hg after 20 minutes of standard advanced cardiac life support was 100% predictive of failure to resuscitate.

4 Describe the capnographic waveform

The important features include baseline level, the extent and rate of rise of CO₂, and the contour of the capnograph. Capnograms may be evaluated breath by breath, or trends may be assessed as valuable clues to a patient’s physiologic status. There are four distinct phases to a capnogram (Figure 24-1). The first phase (A–B) is the initial stage of exhalation, in which the gas sampled is dead space gas, free of CO₂. At point B, there is mixing of alveolar gas with dead space gas, and the CO₂ level abruptly rises. The expiratory or alveolar plateau is represented by phase C–D, and the gas sampled is essentially alveolar. Point D is the maximal CO₂ level, the best reflection of alveolar CO₂, and is known as ETCO₂. Fresh gas is entrained as the patient inspires (phase D–E), and the trace returns to the baseline level of CO₂, approximately zero.

Figure 24-1 The capnographic waveform. A–B: Exhalation of CO₂ free gas from dead space; B–C: combination of dead space and alveolar gas; C–D: exhalation of mostly alveolar gas; D: end-tidal point (alveolar plateau); D–E: inhalation of CO₂ free gas.

5 What may cause elevation of the baseline of the capnogram?

The baseline of the capnogram may not return to zero at high respiratory rates. However, if the baseline is elevated more than approximately 2 mm Hg CO₂, the patient is receiving CO₂ during inspiration, and this is often termed rebreathing (Figure 24-2). Possible causes of rebreathing include the following:

An exhausted CO₂ absorber

Channeling of the gas within the CO₂ absorber

An incompetent unidirectional inspiratory or expiratory valve

Accidental administration of CO₂ (perhaps from a CO₂ tank used for laparoscopy)

Inadequate fresh gas flow

Figure 24-2 Rebreathing of CO₂ as demonstrated by failure of the waveform to return to a zero baseline.

6 Does ETCO₂ correlate with PaCO₂?

CO₂ is easily diffusible across the endothelial-alveolar membrane; thus ETCO₂ should provide an estimate of alveolar CO₂ partial pressures and arterial carbon dioxide partial pressures (PaCO₂). Alveolar carbon dioxide partial pressures and PaCO₂ normally differ by about 5 mm Hg, the small difference caused mostly by alveolar dead space (nonperfused areas of the lung that are ventilated.)

As maldistribution between ventilation and perfusion increases, the correlation between ETCO₂ and PaCO₂ decreases, with ETCO₂ being lower. Increased dead space results in an increased gradient and may be associated with shock, air embolism or thromboembolism, cardiac arrest, chronic lung disease, reactive airway disease, or lateral decubitus positioning. Conversely, increases in cardiac output and pulmonary blood flow will decrease the gradient.

When increases in dead space ventilation are suspected, the gradient between ETCO₂ and PaCO₂ can be determined by arterial blood gas analysis. With the gradient factored in, tracking of ETCO₂ is still a useful, indirect measure of arterial CO₂ content. Of note, shunt has minimal effect on arterial-alveolar CO₂ gradients.

7 Is it possible to see exhaled CO₂ after accidental intubation of the esophagus?

Carbonated beverages or medications (e.g., Alka-Seltzer) return CO₂ after esophageal intubation. However, the usual CO₂ values and waveform would not be expected, and the CO₂ would likely exhaust quickly. Some CO₂ can also reach the stomach if mask ventilation is suboptimal.

8 What might result in sudden loss of capnographic waveform?

A sudden loss of capnographic waveform (Figure 24-3) may be caused by the following:

Esophageal intubation

Ventilator disconnection or malfunction

Capnographic disconnection of malfunction

Obstructed ETT

Catastrophic physiologic disturbance such as cardiac arrest or massive pulmonary embolus

Figure 24-3 A sudden drop of ETCO₂ to near zero may indicate a loss of ventilation or catastrophic decreases in cardiac output.

9 What process might lead to decreases in ETCO₂?

Rapid decreases in CO₂ waveforms may be associated with the following:

Hypotension

Hypovolemia

Decreased cardiac output

Lesser degrees of pulmonary embolism

Dislodgement of a correctly placed ETT

Other, and usually less dramatic, decreases in ETCO₂ may be caused by the following:

Incomplete exhaled gas sampling

Airway leaks (including endotracheal cuff leaks)

Partial circuit disconnections

Partial airway obstruction

Hyperventilation

Hypothermia

Increasing dead space

Decreased metabolic activity (e.g., after neuromuscular blockade) (Figure 24-4)

Figure 24-4 A gradual lowering of ETCO₂ indicates decreasing CO₂ production or decreasing pulmonary perfusion.

10 What processes may increase ETCO₂?

ETCO₂ values may rise gradually secondary to the following (Figure 24-5):

Hypoventilation

Increasing body temperature

Increased metabolic activity (e.g., fever, sepsis, malignant hyperthermia)

Partial airway obstruction

Bronchial intubation

Rebreathing

Exogenous CO₂ absorption (such as during laparoscopy) and venous CO₂ embolism

Exhausted CO₂ absorber

Inadequate fresh gas flow

Faulty ventilator or anesthesia circuit valves

Transient increases in ETCO₂ (may be noted after intravenous bicarbonate administration, release of extremity tourniquets, or removal of vascular cross-clamps)

Figure 24-5 A rising ETCO₂ is associated with hypoventilation, increasing CO₂ production, and absorption of CO₂ from an exogenous source such as CO₂ laparoscopy.

11 What processes can change the usual configuration of the waveform?

Asthma and chronic obstructive pulmonary diseases cause a delayed upslope and steep alveolar plateau (Figure 24-6).

Figure 24-6 A steep slope suggests obstructive lung disease.

A commonly seen abnormal capnogram results when the patient makes spontaneous respiratory efforts and inhales before the next mechanical inspiration. This characteristic cleft in the alveolar plateau is a useful clinical sign indicating that the patient has started to breathe (Figure 24-7). Finally, a cuff leak may result in variable early decreases in the normal waveform configuration (Figure 24-8). Waveforms that are irregular and unlike neighboring waveforms may be caused by manipulation against the diaphragm by the surgeons, pressure of a surgeon against the chest wall, percussing against an abdomen distended by CO₂, or spontaneous breathing out of phase with mechanical ventilation.

Figure 24-7 A cleft in the alveolar plateau usually indicates partial recovery from neuromuscular blockade. Surgical manipulation against the inferior surface of the diaphragm or weight on the chest may produce similar though usually other irregular waveforms as seen in Figure 24-8.

Figure 24-8 A sudden drop of ETCO₂ to a low but nonzero value is seen with incomplete sampling of patient exhalation, system leaks, or partial airway obstruction.