What to monitor and why! The essentials: Cardiovascular, respiratory, and temperature (Proceedings)


What to monitor and why! The essentials: Cardiovascular, respiratory, and temperature (Proceedings)

Apr 01, 2008


A good anesthetist MUST have an understanding of normal physiology- as this it is altered by general anesthesia. An understanding of pathophysiology (when a disease is present) is also essential is be able to monitor effectively. If a disease is life threatening, multiple monitoring tools are available to help aid the human senses. Without continuous monitoring complications are often not detected until much advanced and possibly irreversible. If a death does occur, it would be difficult to know what led to death, without the help of careful monitoring. Anesthetics are not safe; the anesthetist is what makes them safe. It is also pertinent to record everything you do. In order to monitor effectively, the veterinarian, technician, or other responsible person should check the patient's status at least every 5 minutes during anesthesia and recovery! Some common complications encountered in anesthesia are hypotension, hypoventilation, hypoxemia, hypothermia & pain.

Respiratory System Monitoring (Tools)

Respiratory rate (RR)

This will vary with species. Also chest excursions- indication of VT (tidal volume), Respirometry- VT and VE (minute ventilation), Capnography- end tidal CO2 (carbon dioxide), Pulse oximetry- SpO2 (saturated percent of oxygen) & Blood gas analysis.

Tidal Volume (VT)

The volume of gas entering and leaving the patient's respiratory system during the inspiratory and expiratory phase time (normally 10-20ml/kg) - it can be measured by respirometry. When choosing a breathing bag size, you should calculate >5 times tidal volume to decrease airway resistance.

Rate Monitors and Apnea Alarms

A thermistor is placed in patient airway (between ET tube and breathing system).This changes in resistance as warmed gases pass over thermistor; which get converted to electrical signal. Machine beeps with each expiration and can be set to alarm if no breath for pre-set period of time.


Measures tidal volume & minute ventilation. They are computerized or digital.

Respiratory Monitoring and Physiology

Respiratory Function- Maintenance of adequate respiratory function is a prime requirement for safe anesthesia. Excessive increases in PaCO2 (arterial carbon dioxide pressure) or moderate hypoxemia can contribute to poor organ function and can lead to anesthetic fatalities. Physiologically, cellular metabolism leads to constant production of carbon dioxide. CO2 is produced in the tissues and diffused into venous blood which then reaches the right side of the heart then lungs via pulmonary circulation. CO2 is eliminated mostly by the lungs and to a smaller degree, the kidneys. The respiratory centre in brain stem is sensitive to changes in CO2 in blood. Ventilation should increase in response to increases in CO2. If PaCO2 doubles, ventilation should double also (conscious patient). A normal PaCO2 is maintained between 35-45 mmHg (millimetres of mercury). A measurement of CO2 levels in arterial blood gives the most reliable indication of adequacy of ventilation. We must also ensure adequate oxygen concentration in the patient's arterial blood to avoid a hypoxemic episode. There are three common methods to measure oxygenation. A simple way is to observe of mucous membrane color, which is not a very sensitive method. A more accurate method is pulse oximetry (noninvasive estimation of hemoglobin saturation). The best is arterial blood gas analysis.


Monitoring ETCO2 (end tidal carbon dioxide). This is a non-invasive continuous measurement of carbon dioxide from the patient. It provides an indication of adequacy of ventilation (hypoventilation (ETCO2 >45mmHg) or hyperventilation (ETCO2 <35mmHg). There are characteristic traces that are produced for specific situations. It's a useful monitor for assessing ventilator settings. Also, can determine accuracy of intubation (ETT obstruction) as well as circuit malfunction & disconnection. It can be used as a cardiovascular monitor as CO2 is produced when there is cardiac output. Another method of measuring CO2 is through capnometry. This method will provide you with an ETCO2 number, not a waveform.