Capnography can be very confusing if you don’t know what to look for. In the previous article we discussed the physiology behind Capnography. In this article we will discuss a few of the most common traces that you will encounter, the features and some of the causes.
Please remember this is not an exhaustive list and much more can be learned from in depth study of Capnography but for the vast majority of the anaesthetic procedures in first opinion practice this should provide a useful guide.
Firstly, in Figure 1, a reminder of the parts of a trace.
Notes: For the purposes of this article:
- The End Tidal Carbon dioxide level is referred to as PECO2 in millimetres of Mercury (mmHg), although on some machines this may be displayed at EtCO2 (technically this is only when it is measured as a %).
- The Inspired Level of Carbon dioxide is referred to as PiCO2 in millimetres of Mercury (mmHg), although on some machines this may be displayed at FiCO2 or ICO2 (technically this is only when it is measured as a %).
- We have display a three breath capnogram to demonstrate the importance of looking for trends.
- There is a green shaded area on each diagram representing “normal” PECO2.
NB. Normal PECO2 was measured in conscious spontaneously breathing animals as 35-45mm Hg, but during anaesthesia there may be wider variation with the patient still clinically normal.
The Normal Trace
- The PECO2 level should be between 35 and 45 mmHg (these values are for the conscious, spontaneously breathing animal and not influenced by the use of anaesthetic and analgesic medications).
- The PiCO2 level, should be at or very close to Zero.
- Both inspiratory (Phase II) and expiratory (Phase 0) phases are steep, and the alveolar plateau (Phase III) slopes gently up, If it looks like a line of Elephants holding tails that what you are aiming for.
If you identify an abnormality in a single breath trace wait for the next ones. Subtle things (e.g. movement of the patient) can cause one or two abnormal looking complexes, as long as they return to ‘your line of elephants holding tails’ there should be no need for immediate concern.
The Scary Trace
In this case, the carbon dioxide being sampled is in rapid decline.
- PECO2 values rapidly decrease.
- Respiratory rate may or may not be reduced.
- Shape of capnogram is abnormal.
- Cardiovascular collapse - the capnogram is a very good and early indicator of cardiovascular collapse. This trace is not just a malfunction of ventilation but also a failure of delivery of CO2 laden blood to the lungs (a serious decrease in cardiac output), ventilation may appear more normal than you would expect.
- Patient disconnection or partial disconnection from the breathing system (Patient is “nice and pink”, capillary refill time, heart rate etc are within normal parameters)
- Pulmonary embolism (rare).
What to do
- Check the patient.
- If the patient is cyanotic with abnormal parameters. Institute CPR immediately, a good resource is the RECOVER guidelines3.
- If the patient is of normal colour and capillary refill time and heart rate are within normal parameters, check and reconnect the breathing system.
On a more positive note, once resuscitation has been successful you should see the following:
Return of Spontaneous Circulation (ROSC)
- Low level peaks coinciding with manual ventilation (the aim with CPR is to keep PECO2 over 15mm Hg 1, 3 to increase the chances of return to spontaneous circulation) followed by
- more normal trace with PECO2 levels at or above normal.
- As spontaneous cardiac function returns, CO2 is then delivered to the lungs again and the capnogram will return to more normal levels (sometimes PECO2 may be high than normal).
What to do
- If PECO2 levels are not normal, cardiac output may still be low. Efforts should be made enhance to enhance cardiac output, this may include fluid therapy and/or administering positive inotropes/vasopressors.
- Manual ventilation should be continued until spontaneous effort is resumed.
- Remember to check for the cause of the crash and resolve it e.g. excessive anaesthesic depth, hypovolaemia, cardiac or pulmonary compromise.
The Misleading Trace
- Phase II and III are normal.
- Phase 0 has the appearance of descending stairs.
- Phase I is normal.
- This trace is considered to be normal2. It may appear and disappear in the same patient. It is possibly caused by the beating of the heart affecting gas movement in the airways during inspiration.
- It is more common in deep chested dogs and those animals with slower respiration such as bigger breeds.
What to do
- Continue monitoring
In the following examples we will review how rate of respiration may affect the capnogram.
In this case, a rise in respiratory rate (given that the total amount of CO2 from the animal is the same) results in a reduction to the PECO2.
- Normal shape trace.
- PiCO2 still at or near zero.
- PECO2 trending down below 35 mm Hg.
- Phase I may be shorter.
- Increased respiration rate.
- Patient is of normal mucous membrane colour and oxygen saturation.
- The anaesthesia is too light (common) or
- insufficient pain relief (common).
- May also be accompanied by an increase in heart rate.
- IMPORTANT: This trace may also be seen due to a decrease in Cardiac output or arterial blood pressure (V/Q mismatch). Always check that the patient is not cyanotic or hypotensive. See the section below on reduced cardiac output (C.O.).
What to do
- Check the anaesthetic depth and adjust if necessary.
- Check timings and potency of analgesics and administer further analgesia if required.
- Check colour and arterial blood pressure.
Similarly, a fall in respiratory rate will result in a rise in PECO2 and steps should be taken to correct both the high PECO2 (by giving a couple of breaths) and the lower respiration rate. In a patient that is excessively deep, the higher PECO2 may be what is stimulating them to breathe. Lowering the PECO2 without addressing the excessive depth of anaesthesia may cause apnoea.
- Capnogram of normal or near normal shape.
- PiCO2 at or near zero.
- PECO2 trending upwards above 55-60 mmHg.
- Respiration rate is decreased.
- More severe respiratory depression will be associated with a decrease in heart rate.
- Excessive depth of anaesthesia is the most common cause.
- If there is no decrease in respiration rate, the cause may be positional, especially during prolonged anaesthesia. E.g. due to lung atelectasis.
What to do
- Manually ventilate once or twice per minute. This should help to reduce the PECO2 to more normal levels. Care should be taken not to over ventilate and not to use excessive pressure.
- Check the depth of anaesthesia. Decrease the level of inhalational agent, reverse a sedative agent previously given or even reverse an opioid if the hypoventilation is severe and prolonged (recommended only as a last resort due to also antagonising analgesic component).
Note that in both the above examples, the PiCO2 is at or near zero. If the PECO2 is high and the PiCO2 is above zero, then you are looking at a case of rebreathing.
- PECO2 is above normal and rising.
- PiCO2 is above normal and rising.
- May be accompanied by a rise in respiratory rate due to the stimulatory effect of the high levels of CO2 on the respiratory centre.
- Excessive dead space.
- Fresh gas flow is insufficient (especially T piece and Bain).
- Exhausted soda lime (rebreathing circuit).
- Insufficient time of exhaled breath in soda lime (large dog with a small canister).
What to do
- Change the soda lime canister (fresh soda lime or a bigger canister).
- Increase fresh gas flow.
- Re-intubate with a shorter endotracheal tube.
Reduced Cardiac Output
- The capnogram shape is near normal.
- PiCO2 is at or near zero.
- PECO2 is below normal.
- Respiratory rate is near normal.
- Reduced cardiac output (patient may appear pale or cyanotic, BP is low) or a
- Endotracheal tube that is letting gas past (you may smell inhalational agent!!) the tube is either too small or the cuff is not inflated properly. The CO2 is being produced but is not reaching the detector.
- Reduced minute volume (e.g. from a tension pneumothorax).
- Hypothermia- CO2 is not being produced by the animal.
- Fresh gas flow too high
- Patient has a small tidal volume (e.g. small animals, rabbits or small cats) and capnograph machine is sidestream – remember sidestream capnographs take out 100-150 ml per min so there may not be enough volume to measure properly. Fresh gas flow may also be a component of this due to dilution of the patient’s tidal volume.
What to do
- Monitor Blood Pressure and take steps to correct if below normal by administering fluids or positive inotropes/vasopressors.
- Check fit of Endotracheal tube, inflation of cuff and connections in breathing system.
- Urgently address minute volume deficiency if suspected.
- Monitor temperature.
More rarely hyperthermia may cause more CO2 to be produced and result in high PECO2 readings with a normal or increased respiration rate.
When analysing the capnogram it is important to remember which phase if the capnogram is which and that inspiration is active, while expiration is generally passive. The below capnogram is commonly seen with an obstruction (folded tube, mucus).
- Slanting and prolonged Phase II.
- Increased slope of phase III.
- Obstructed ETT.
- Bronchospasm (e.g. anaphylaxis, asthma).
- Expiratory valve malfunction.
- Pressure relief valve (to scavange) is closed.
What to do
- Check inflation of bag, open valve if over-inflated.
- Check (physically and by listening) ETT and breathing system (including valves).
- Consider administering a bronchodilator.
On the other hand, increased inspiratory resistance would look like this:
- Slanting and prolonged phase 0.
- Abnormal phase III.
- Pressure on the chest e.g. a vet or nurses hand on the chest (or even a stethoscope in small animals), it can also be seen where there is something else impeding inspiration, e.g.
- Abdominal contents compressing the thoracic cavity.
- Diaphragmatic hernia.
- Malfunctioning inspiratory valve.
- Too low a Fresh Gas Flow.
What to do
- Remove whatever is causing the pressure on the chest.
- Check breathing system with particular reference to the inspiratory limb.
- Check reservoir bag and Increase Fresh Gas Flow if noticed to have deflated.
Finally, another commonly seen trace is caused by muscle fasciculations in the patient. Common causes include certain drugs (e.g. propofol or ketamine), lack of adequate pain relief or depth of anaesthesia (e.g. immediately following induction or prior to recovery).
- PECO2 is normal.
- PiCO2 is normal.
- Capnogram is normal shaped but does not appear smooth.
- Muscle twitching affecting smooth breathing.
What to do
- Check premedication and induction protocol including analgesia.
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