6 April 2021
Brachycephalic Anaesthesia – Part 2: The peri-anaesthesia period
This article, on Brachycephalic Anaesthesia, covers the peri-anaesthesia period considerations for brachycephalic breeds and discusses pre-medication, induction and intubation and monitoring of anaesthesia. It follows the previous article on pre-anaesthetic considerations.
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In recent years there has been a shift in the benchmark to measure a successful anaesthetic. Instead of avoiding anaesthetic mortality, it has moved towards decreased anaesthetic morbidity (ACVA, 2009).
Earlier detection of hypercapnia, hypoxemia and hypotension are now common in general practice and can be attributed to decreasing anaesthesia morbidity. The veterinary team should additionally anticipate the specific anaesthesia needs of Brachycephalic Airway Syndrome (BAS) breeds, who may suffer from Brachycephalic Obstructive Airway Syndrome (BOAS), when selecting anaesthesia protocols and drugs.
Pre-medication and pre-anaesthetic drugs – Pre-medication choice
When selecting pre-anaesthetic drugs, consider the aims of pre-medication (Murrell, 2007):
- Facilitate patient handling
- Sedate and anxiolysis
- Reduce induction doses
- Provide analgesia
- Contribute to a balanced anaesthesia
- Contribute to a smooth and quiet recovery
Intravenous (IV) access prior to pre-medication is advantageous in BAS patients but is not always possible, and intramuscular (IM) pre-medication may need to be administered to allow IV catheter placement. Continued attempts to place an IV catheter can cause stress, leading to dyspnea or airway obstruction, and may also cause a catecholamine release which may mean more sedation drugs and/or higher doses are required. Higher doses of sedation drugs may not increase the degree of sedation but increase the severity of side effects and their duration (Murrell, 2007). It is important to allow the appropriate amount of time for pre-anaesthesia drugs to take effect.
Acepromazine, medetomidine and methadone can be given in combination, greatly reducing the dose of both sedation drugs (Murrell, 2007). At the authors’ hospital, 0.2 mg/kg methadone + 0.005 mg/kg acepromazine + 0.003 mg/kg medetomidine IM is used on brachycephalic patients that do not tolerate IV catheter placement. Other pre-medication drugs are shown in Table 1.
Table 1. Pre-medication options and their side effects, benefits and dosing, adapted from Adshead (2014, p. 82).
|Drug||Benefits||Side effects||Peak onset/duration of action||IM dose||IV dose|
|Acepromazine||Good anxiolytic, sedation improved when administered with an opioid||Hypotension, unreliable sedation when used alone, not reversible||35–40 min/can be given q 4–6 h||0.005–0.01 mg/kg||0.003–0.005 mg/kg|
|Medetomidine||Profound sedation, reversible, some analgesic properties, drug sparing (reduction in induction drugs needed)||Dose-dependent bradycardia (normally well tolerated)||15–20 min IM 2–3 min IV||0.003–0.01 mg/kg||0.003–0.005 mg/kg|
|Methadone||Good analgesia||If given too fast, IV can cause bradycardia and respiratory depression||30 min/can be given q 4 h||0.1–0.3 mg/kg||0.1–0.3 mg/kg|
|Buprenorphine||Moderate analgesia, mild sedation||Moderate analgesia||45 min/can be given q 6–8 h||0.01–0.02 mg/kg||0.01–0.02 mg/kg|
|Butorphanol||Mild analgesia, good sedation||Poor analgesia and should not be used for surgical patients||10–15 min/lasts for 60–90 min||0.1–0.3 mg/kg||0.1–0.3 mg/kg|
Administration of pre-medication
The muscle site chosen for IM injection affects the bioavailability of a drug due to the blood flow at the injection site (Benet, Kroetz, & Sheiner, 1996). The cervical epaxial muscles or the quadriceps muscle are well tolerated and have good blood flow, few fascial planes and a low-fat content (Self, Hughes, Kenny, & Clutton, 2009). Different routes of administration of pre-medications are shown in Table 2 with their advantages and disadvantages. In the authors’ experience, the cervical epaxial muscles are the best tolerated due to minimal restraint around the neck, meaning breathing is less restricted, as seen in Figure 1. This may also describe why the placement of an IV in the saphenous vein is well tolerated and demonstrated in Figure 2.
Figure 1. IM injection, cervical apaxial.
Figure 2. Placing of a saphenous catheter.
Table 2. The advantages and disadvantages of different routes of administration, adapted from Murrell (2007, p. 101).
|Intravenous||Reliable absorption, usually faster onset of action||Need to obtain IV access, which may stress patient|
|Intramuscular||Minimal handling required for administration, drug absorption more reliable than subcutaneous||May be painful when administered, slower onset of action than IV|
|Subcutaneous||Easy to administer||Delay of onset even more so than intramuscular, effect of administration less reliable|
Pre-medication can cause relaxation of the naso- and oropharyngeal smooth muscle, which can worsen upper airway obstruction (Clutton, 2007). Once pre-medication has been administered, the patient should be constantly observed with equipment to provide oxygen and intubation readily available.
Due to the increased risk for gastroesophageal reflux and subsequent regurgitation in BAS-affected dogs, omeprazole can be administered orally 4 hours prior to anaesthesia to reduce gastric fluid volume and decrease the risk of reflux (Panti et al., 2009). Omeprazole is a proton pump inhibitor that suppresses stomach acid secretion by inhibiting the H+/K+-ATPase system found within gastric parietal cells (Adshead, 2014). Frequent transferring of the patient between different areas and a light plane of anaesthesia can also increase the risk of regurgitation.
It may be advantageous to use a breathing system where positive pressure ventilation (PPV) can be performed; a circle system, a paediatric Ayers T-piece or a Bain. The reasons for and benefits of being able to provide PPV are:
- Under anaesthesia, especially when positioned in dorsal recumbency, the tidal volume (TV) may be reduced due to pressure on the diaphragm. The minute volume (MV) is a calculation of TV (10–15 ml/kg) multiplied by the respiratory rate. Hypercapnia from an inadequate MV can cause respiratory acidosis.
- Anatomical abnormalities (see part 1: Scales & Clancy, 2019) means a smaller endotracheal tube (ETT) is often required, which increases resistance to gas flow and increases the work of breathing. This can increase the incidence of hypoxia, myocardial workload (Grubb, 2016) and hypercapnia (end-tidal CO2 > 45 mmHg). Hypercapnia commonly occurs during general anaesthesia due to depression of respiratory centres in the brain causing hypoventilation.
Pre-oxygenation should be performed prior to anaesthesia induction to delay the onset of hypoxemia if there is post-induction apnea or a delay in placing an ETT due to obstruction from soft tissue structures.
Pre-oxygenation increases the fraction of inspired oxygen (FiO2) of the functional reserve capacity (FRC) in the lungs as shown in Figure 3, replacing the room air with a more oxygen-rich concentration. If there is airway obstruction, the higher FiO2 in the FRC gained from pre-oxygenation will still diffuse into circulation and provide oxygen to tissues.
Figure 3. Chart showing functional lung reserve capacity.
In a normal healthy patient breathing room air (21% oxygen, or FiO2 0.21), the partial pressure of oxygen (PaO2) dissolved in the blood is 80–100 mmHg, which correlates to a SaO2/SpO2 of >95%. As the PaO2 is estimated as 5 × FiO2, breathing a higher concentration of oxygen than room air can shift the PaO2 to the right-hand side of the oxyhaemoglobin dissociation curve (ODC), prolonging the time of desaturation and hypoxemia. Pre-oxygenation for 3 minutes can delay the onset of hypoxemia by 3–4 minutes. The ODC is shown in Figure 4.
Figure 4. Oxygen dissociation curve.
Table 3 lists some pre-oxygenation options and the FiO2 aims. While beneficial, pre-oxygenation should be avoided in cases where it causes stress to the patient as the benefits are outweighed.
Table 3. Pre-oxygenation FiO2 concentrations, adapted from Grimm et al. (2015, p. 36).
|Percentage of oxygen (O2)/FiO2||Flow rate of 100% O2|
|Flow by||Up to 40%/0.4||0.5–5 l/min, deliver within 2 cm of the nose|
|Face mask||Up to 60%/0.6||2–8 l/min, tight-fitting but careful around eyes|
|Oxygen cage||Up to 50%/0.5||Variable|
The induction of anaesthesia should be with an injectable agent that is rapid in effect and allows quick endotracheal intubation with a quick recovery and no long-lasting effects. A masked or chamber induction should be avoided due to the stress on the patient and the potential of soft tissue obstruction. Masked induction has also been associated with increased mortality (Brodbelt, Pfeiffer, Young, & Wood, 2008).
Anaesthesia induction agents can cause respiratory depression and sometimes apnea, which may lead to desaturation and hypoxia in a patient where endotracheal intubation may be delayed, especially if they have not been “pre-oxygenated” as described above. Keeping the head elevated while the patient is in sternal recumbency during the induction of anaesthesia may be beneficial to reduce the risk of aspiration of stomach content because BAS-affected dogs have an increased risk of gastroesophageal reflux.
Most BOAS-affected dogs have some degree of laryngeal collapse due to their conformation (see part 1: Scales & Clancy, 2019) and there is no advantage between alfaxalone or propofol on laryngeal movement (Norgate et al., 2018). A consideration when using alfaxalone as an induction agent with minimal sedation is that the patient may paddle, have minor muscle twitching or other types of movements in the recovery period when handled or disturbed (Alfaxan.co.uk, 2017).
Maintenance of general anaesthesia may be with a volatile agent in oxygen or using a total intravenous anaesthesia technique. Only gaseous maintenance will be discussed further.
Inhalant anaesthesia using a volatile agent in oxygen allows for quick metabolism in the lungs, an easily titratable anaesthesia depth and a fast recovery. If available, sevoflurane may be the preferred volatile agent of choice due to its low blood gas solubility resulting in a quicker recovery from the termination of anaesthesia and then extubation (Lozano et al., 2009), which is an advantage for BOAS dogs.
An oxygen flow rate that supports the patient’s tidal volume and patency of the breathing system should be used. Fresh gas calculations are detailed in Table 4 and can be titrated down to a flow rate just before rebreathing is seen on the capnograph.
Table 4. Fresh gas flow calculations adapted from Welsh (2013, p. 109).
|Tidal volume (TV)||10–15 ml/kg|
|Minute volume (MV)||TV × respiration rate, or 200 ml/kg/min|
|Circuit factor (CF)||T-piece and Bain = 2.5|
Lack = 1
|Fresh gas flow (FGF) for non-rebreathing systems||MV × CV = ml/min|
|FGF for circle system||First 10 min = 100 ml/kg|
After 10 min = 10 ml/kg
(minimum of 0.5 l/min)
Due to anatomical differences already discussed in the previous article, BAS breeds can be notoriously difficult to intubate (Clancy & Hoy, 2016).
Routinely, at least three ETTs should be prepared; however, for these patients, a wider variety should be made readily available at the time of induction. These patients tend to need smaller than expected ETTs; sizes ranging from 3.5 to 6 mm should be prepared for French Bulldogs and Pugs.
A laryngoscope should be used to visualise the airway. If the patient has a long soft palate, even the use of a laryngoscope will not allow full visualisation, so another laryngoscope blade or a tongue depressor can be used by a second person to gently move the soft palate out of the way as seen in Figure 5. The following may also help:
Figure 5. Visulisation with two laryngoscopes.
Figure 6. Bougies and urinary catheter attached to syringe.
|Use of a stylet (a thin plastic wire) inserted into the ETT which can help make smaller ETTs more rigid||Figure 6|
|Attach a urinary catheter to a 2.5-ml syringe with the plunger removed, and plug into a 7-mm ETT connector. Attach capnography to confirm placement, remove the syringe and thread the ETT over the urinary catheter||Figure 6|
|A bougie can be used to guide an ETT into the airway. It is more rigid than a urinary catheter so may be easier to place; however. they tend not to be readily available in practice||Figure 6|
To confirm correct placement of ETT
- Attach capnography to the ETT and observe an end-tidal carbon dioxide (ETCO2) trace, keeping in mind the ETCO2 may appear low until the ETT tube is cuffed. This is considered the gold standard way of confirming placement (Hall et al., 2001).
- If a clear PVC ETT is used, condensation may be visible on the tube caused by the patient’s breath.
- A breath may sometimes be felt coming from the ETT when the patient expires. Hair should never be put in front of the tube to confirm intubation as it may be inhaled by the patient (Clancy & Hoy, 2016), and pushing on the patient’s thorax should not be performed.
Once ETT placement has been confirmed, a leak test should be performed and the cuff inflated appropriately; attach the breathing system and turn on the oxygen, close the APL valve and provide the patient with a breath while an assistant listens for oxygen escaping around the ETT. If oxygen is heard escaping around the ETT, the cuff should be inflated until it is no longer heard. Now the volatile agent can be administered. Due to the increased risk of regurgitation for these patients, always inflate the ETT a little to form a tight seal.
The shorter facial conformation of these patients means most standard ETTs will be too long and lead to increased dead space. The ETT should be cut to size which, can be done prior to intubation by measuring from the patient’s thoracic inlet to the incisors as seen in Figure 7.
Figure 7. Measure of ETT to thoracic inlet.
Positioning should always be on a soft and padded surface with the joints in a natural position where possible. A slightly tilted recumbency can reduce the pressure on the diaphragm of the abdominal contents, which encourages an adequate FRC and TV. If tilted >30° it can reduce blood flow to the brain (Mosing, 2016). Care must be taken not to overextend the head and neck to prevent any pressure on the laryngeal structures.
When repositioning the patient, in the authors’ experience, it is advisable to turn from a dorsal–sternal or sternal–dorsal slowly and in two movements, resting in lateral recumbency in between. Their high vagal tone may induce a vagal response (bradycardia and hypotension) if performed rapidly. Atropine can be administered in this instance. The ability of the patient to manage arterial blood pressure changes due to posture changes are also diminished under anaesthesia. Always disconnect the breathing system from the ETT when repositioning (Grubb, 2010).
For brachycephalic anaesthesia, the authors suggest the use of capnography, pulse oximetry, blood pressure and temperature monitoring. It can also be advantageous and considered the gold standard to have arterial blood pressure monitoring, arterial blood gas analysis and echocardiogram (ECG); normal values and common issues with these monitoring parameters are shown in Table 5.
Table 5. Normal parameters and common issues for capnography, pulse oximetry, blood pressure and temperature with additional values for blood gas analysis and ECG.
|Monitoring equipment||Measures||Normal readings||Common issues detected|
|Pulse oximeter||Oxygen saturation of haemoglobin and heart rate||>95% (may be as low as 93% in some brachycephalic patients with chronic airway issues)||HypoxiaCan show bradycardia|
|Blood pressure||Blood pressure||Systolic: 120 mmHgDiastolic: 40 mmHgMean: 60–70 mmHg||Hypotension|
|Capnography||End tidal carbon dioxide (ETCO2)||35–45 mmHg||Hypercapnia (ETCO2 <45mmHg)Hyperventilation (ETCO2 <35 mmHg)Rebreathing of CO2Airway obstruction (shark fin trace)|
|Thermometer||Body temperature||37–38 °C||•Hyperthermia|
|Arterial blood gas analysis||pHpCO2pO2Base excess, sometimes electrolytes||7.350–7.47032.0–43.0 mmHg80.0–105 mmHg||•Respiratory acidosis|
|Electrocardiogram (ECG)||Electrical conductivity of the heart in trace form||•Bradycardia|
•Atrial ventricular node block (AV block)
Blood pressure monitoring
Invasive arterial blood pressure monitoring is the gold standard; however, it is not practical in a general practice setting. Oscillometric blood pressure monitoring can provide systolic, diastolic and mean blood pressure readings; however, they are not considered to be as accurate as a Doppler (Schauvliege, 2016). Therefore, a Doppler is preferred, if available. There has been debate as to whether readings produced on a Doppler are closer to mean or systolic blood pressures (McMillan, 2016), so whichever the user decides to choose, the readings should be read as a trend under anaesthesia rather than thought of in solitude (McMillan, 2016).
Due to confirmation of brachycephalic breed’s legs, it can be difficult to find appropriately sized blood pressure cuffs that fit evenly over 40% of the circumference of the leg (Schauvliege, 2016). They can become loose once inflated and can lead to inaccurate readings, as seen in Figure 8. It is therefore the authors’ suggestion that the cuff be placed below the hock as seen in Figure 9.
Figure 8. Loose-fitting blood pressure cuff on a hind limb.
Figure 9. Correct sizing of blood pressure cuff below the hock.
Capnography can provide information on ventilation, gaseous exchange and cardiac output. Brachycephalic patients usually have a high PaCO2, seen as a high ETCO2 which can cause hyperventilation leading to rebreathing, and an increase in the fraction of inspired carbon dioxide (FiCO2), Figure 10. If seen, the oxygen flow rate should be increased and the patient may need to be ventilated. A shark fin trace can sometimes be seen due to mucus occluding the ETT Figure 11. If this is seen the ETT should be suctioned or replaced.
Figure 10. Capnography showing rebreathing. Capnography trace never returns to baseline of zero.
Figure 11. Capnography showing an occluded ETT.
The use of ECG monitors under anaesthesia can detect issues with the normal electroconductivity of the heart, while also providing a heart rate. As mentioned previously, brachycephalic patients can have an increased vagal tone that can be stimulated by intubation and changes in recumbency. If this occurs bradycardia will be seen on ECG, and second-degree atrial–ventricular (AV) block may occur, which is represented as P waves that do not produce a QRS/T on the ECG as seen in Figure 12.
Figure 12. Showing AV block on ECG. A P-wave can be seen with no corresponding QRS complex.
This second article has highlighted some of the specific anaesthesia monitoring considerations for brachycephalic patients. The next article to follow is on the post-anaesthetic period.
Additional information – Notes on contributors
Courtney Scales, DipVN, NCert(Anaesth) RVN
Courtney is originally from New Zealand, where she qualified as a Veterinary Nurse in 2010 and worked in a number of small animal clinics in Auckland. An anaesthesia passion took her to a large referral hospital in Australia in 2015. She has been working at the Royal Veterinary College since 2017. Email: firstname.lastname@example.org.
Niamh J. Clancy, Dip AVN (Small Animal), DipHE, CVN, Dip VN, PGCert VetEd, FHEA, RVN
Niamh graduated from University College Dublin with her Diploma in Veterinary Nursing in 2011. She then moved to the UK and worked in hospitals across London while studying for her Advanced Diploma, which she obtained from Myerscough College in 2015. She currently divides her time as an anaesthesia nurse at the Queen Mother Hospital for Animals and as a Clinical Educator in Anaesthesia for the Veterinary Nursing School at the Royal Veterinary College. She recently obtained her Certificate in Veterinary Education. Email: email@example.com.