Waveform capnography is a commonly used monitor in the operating room, and is increasingly seen in non-operating room environments too! The capnographic waveform can be described as having several phases:

• Phase 0 (inspiratory baseline) represents the inspiratory phase of the respiratory cycle.
• Phase 1 is the initial part of expiration, when dead space gases are being exhaled. Since the exhaled gas in this phase did not take part in gas exchange, the PCO2 is 0.
• Phase 2 (expiratory upstroke) involves exhaled gases from alveoli reaching the detector. There is a sharp rise in PCO2 during this phase.
• Phase 3 is a (more or less) flat plateau showing continued exhalation of alveolar gas. The last, maximal part of this phase is the end-tidal point (ETCO2), which is usually 35-40 mmHg. ETCO2 tends to be 2-5 mmHg lower than PaCO2, though this difference can be increased/decreased under a variety of conditions, such as ventilation-perfusion mismatch.

The shape of the capnograph waveform can tell you a lot!

For example:

1. A slanting upslope can represent airway obstruction (e.g., chronic obstructed pulmonary disease, bronchospasm, blocked endotracheal tube).
2. In patients paralyzed with a neuromuscular blocker, as the paralytic wears off they may try to breathe asynchronously against the ventilator, producing a notch called a ‘curare cleft.’
3. Quantitative capnography during resuscitation can be very useful. During CPR, there should be a visible waveform during high quality chest compressions; its absence may indicate accidental esophageal intubation
4. A sudden loss is bad, as it means that the tube is fully obstructed or disconnected or that there has been a sudden loss of circulation
5. You can also just simply tell is someone is hypo- or hyperventilating
   

• Dorsch JA, Dorsch SE. 2007. Gas monitoring. In: Understanding anesthesia equipment (Dorsch and Dorsch, Eds.) Lippincott Williams & Wilkins, Philadelphia PA.
• Kodali BS. 2013. Capnography outside the operating rooms. Anesthesiology; 118:192.