Advanced life support: why CO2 and ECG is so important
In our previous posts, we’ve looked at preparing for cardiopulmonary resuscitation and how to perform basic life support. In addition to BLS, advanced life support is used to effectively monitor the patient, secure appropriate IV access, and administer medications to improve patient outcomes.
Advanced life support
Advanced life support is made up of 3 areas:
Obtaining and securing intravenous access
Attaching monitoring equipment and using monitoring results to guide further decision making
Administering drugs (and/or electrical defibrillation) as required
Monitoring is a key aspect of advanced life support. There are only two parameters we need to monitor during cardiopulmonary resuscitation; these are end-tidal carbon dioxide (ETCO2) levels and the patient’s heart rhythm (ECG trace). Pulse oximetry and non-invasive blood pressure monitoring are not useful during CPR but are useful in monitoring a post-arrest patient, or critical patients in general.
Using capnography to assess cardiopulmonary resuscitation
Capnography, or end-tidal CO2 monitoring, is not just vital in our anaesthetised patients. This tells us an enormous amount of information during CPR, including estimating the patient’s cardiac output, assessing the quality of our chest compressions, and signalling when a patient may have a return of spontaneous circulation (ROSC).
Assessing cardiac output
Carbon dioxide is a waste product created by various cells and tissues within the body. The CO2 from these cells enters the bloodstream, where it is transported within red blood cells back to the heart, then via the pulmonary artery to the lungs for gaseous exchange to take place. During gaseous exchange, the CO2 diffuses out of the alveolar capillaries, across the alveoli, and is removed from the body as the patient exhales.
In order to generate an end-tidal CO2 reading, the blood flow from the heart to the lungs must be sufficient. If there is not sufficient blood flow to the lungs, the patient will have a low end-tidal CO2 reading - so we can assess our patient’s cardiac output using a capnograph.
Assessing chest compressions
During cardiopulmonary resuscitation, we are generating our patient’s cardiac output through chest compressions. We can therefore use the patient’s end-tidal CO2 reading to assess the quality of our chest compressions.
The magic number we want to aim for is 15mmHg. If you are getting CO2 readings below 15, this could mean that:
The patient is being hyperventilated (respiratory rate >10 breaths/minute)
The chest compression technique needs to be reviewed - ensure that the rate is not too fast or too slow, that the compressor’s posture is correct, and that the thorax is being compressed by ⅓ to ½ of the width with each compression, whilst allowing full recoil each time.
Detecting ROSC
If a sudden increase in end-tidal CO2 is seen, with readings of above 30mmHg, this can indicate a return of spontaneous circulation. If this is seen, the patient should be checked thoroughly for the presence of spontaneous pulses or a heartbeat.
Electrocardiography in cardiopulmonary resuscitation
Another important monitoring tool in advanced life support is electrocardiography (ECG). This provides us with vital information on the patient’s cardiac rhythm, which we can then use to guide medication choices and determine whether defibrillation is required.
Setting up the ECG
An ECG should be placed as soon as possible after basic life support begins. This can be done using electrode pads or clips, generally placed on the paws or limbs. As a reminder, our ECG leads are placed in the following order:
Red lead - right forelimb
Yellow lead - left forelimb
Green lead - left hindlimb
Black lead (if applicable) - right hindlimb
The ECG monitor should be set to lead II.
It is vitally important that surgical spirit or alcohol-containing solutions are not placed on patients undergoing CPR. This is because alcohol is a significant fire risk in a defibrillated patient. If you need to use anything to improve the contact between the patient and the electrodes, a small amount of electrode gel (this is like ultrasound gel but is conductive, so should be used for CPR patients) should be used.
Assessing the ECG
The ECG trace should be assessed at the end of each 2-minute cycle of basic life support, rather than during the cycle. This is because our chest compressions will affect the ECG trace, resulting in unreliable readings.
At the end of each cycle, a 3-4 second pause in chest compressions is used to assess the patient, examine the ECG trace, and agree the rhythm seen between the whole CPR team. Basic life support is then resumed with a new person administering chest compressions.
There are 4 ECG rhythms seen during CPR - these can be divided into shockable and non-shockable rhythms.
The Non-Shockable Rhythms
Non-shockable rhythms are those which cannot be converted to a normal rhythm using electrical defibrillation. The two non-shockable rhythms are asystole and pulseless electrical activity.
Asystole is a flat (or near-flat) line associated with no electrical activity in the heart.
Pulseless electrical activity is a trace which should be associated with a pulse but is not. It appears as regular, repeating complexes which might be sinus, but might take other forms. It is a slower rhythm (rate <200 beats per minute).
The Shockable Rhythms
Shockable rhythms are those which are responsive to electrical defibrillation. This temporarily stops the electrical activity in the heart, in order for normal pathways to be restored. My next post will cover defibrillation in much more detail, so make sure you give it a read!
The two shockable rhythms seen during cardiopulmonary resuscitation are pulseless ventricular tachycardia and ventricular fibrillation.
Pulseless ventricular tachycardia is a rapid rhythm with a rate of over 200 beats per minute. The complexes are regular and repeating, as seen below.
Ventricular fibrillation is seen as an erratic, wavy line. There are no discernable complexes and a heart rate cannot be measured. This rhythm is associated with chaotic, irregular ‘fluttering’ activity of the cardiac muscle, resulting in a lack of proper cardiac contraction.
As you can see, monitoring is a hugely important part of advanced life support and provides the team with an enormous amount of information. So next time you have an arrest, or perform a CPR drill in practice, make sure you get that monitoring equipment on at an early stage!
Do you use an ECG and capnography in your cardiac arrest patients? I’d love to know your thoughts in the comments below!
References
Fletcher, D. et al. 2012. RECOVER evidence and knowledge gap analysis on veterinary CPR part 7: Clinical guidelines. Journal of Veterinary Emergency and Critical Care, 22 (S1), S102-S131.
