Anaesthesia for medical patients: how to care for patients with respiratory disease
Respiratory patients can be some of our most challenging cases to anaesthetise. If we don’t do it carefully, or we rush a patient into a procedure without stabilising them first, it can easily be fatal.
There is a lot we need to think about when it comes to anaesthetising patients with respiratory disease. This starts before we even think about anaesthetising them - with initial stabilisation - then with careful planning, preparation, and communication.
We then need to think about their monitoring requirements and management during their anaesthetic - which is actually when they’re likely to be the most stable, since we’ve got them intubated and on 100% oxygen at that point.
The area where we are most likely to run into complications is in recovery, where we’re extubating them and trying to wean them from 100% oxygen. These patients need especially careful monitoring during the recovery period.
In today’s post, we’ll be going into detail on all things respiratory anaesthesia. By the end of the post, you should feel way more confident anaesthetising respiratory patients for their investigations!
PS. If you find this useful, make sure you sign up to my weekly newsletter, Medicine Nurse Notes - this week, I’m going to be sharing a case study with everyone, putting everything we’ve discussed into practice!
Pre-anaesthetic considerations
We know that pre-anaesthetic stabilisation is important in all medical patients, but especially those with respiratory compromise.
Why is this? Well, in most cases, we take a patient with respiratory disease and then add to their disease process, by:
Administering medications that depress the respiratory system, worsening hypoventilation
Performing procedures that may compromise respiration further, by increasing airway resistance (e.g. bronchoscopy) or administering fluid directly into the respiratory tract (e.g. bronchoalveolar lavage)
This means that, to minimise the risk these procedures cause our patients, pre-anaesthetic assessment and stabilisation is absolutely vital.
Initial stabilisation
Before we even think about anaesthetising our patient, right when they first present to us, we need to stabilise them. This means:
Performing an initial triage assessment, then a more thorough examination when the patient allows
Using a ‘hands off approach’ in patients with respiratory compromise, since stress further increases their oxygen demand, and increases risk of decompensation
Administering anxiolytics or (safe) sedative agents such as butorphanol, and allowing the patient to acclimatise where possible before securing IV access
Administering oxygen therapy in a hypoxaemic patient (SpO2 <95%)
One point to mention before we go further here: do not rush a respiratory patient into X-ray. Why? Well, the information you get from your images is unlikely to make a massive difference to the way you initially treat that patient - and you could easily tip them over the edge by trying to restrain them for x-rays.
Instead, do something like a point-of-care ultrasound, which you can perform with the patient conscious and not restrained (in most cases), whilst they receive oxygen therapy. Then, once they’re stabilised, you can anaesthetise them for further imaging and procedures as needed.
Pre-anaesthetic assessment
At risk of sounding like a broken record, we need to ensure a thorough pre-anaesthetic assessment has been performed. This includes:
Assessing the patient’s demeanour/behaviour
Performing a full clinical examination and collecting vitals
Auscultating the heart, lungs and upper airways (yes, listen over the trachea and larynx too - especially in your respiratory patients!)
Evaluating any initial blood results that have been performed
Discussing with the veterinary surgeon to determine which respiratory diseases they suspect may be present, and how this might impact your anaesthetic.
Where in the respiratory tract is the patient’s disease?
Why is it important for us to know this? Well, a few reasons. Firstly, it will change the procedures we perform and how we work-up and diagnose these patients.
But secondly, and perhaps more importantly for us, it will also affect how we nurse these patients under GA.
For example:
Patients with upper respiratory tract diseases such as BOAS or laryngeal paralysis are more likely to have issues securing airway access, becoming hypoxaemic on induction/recovery as a result, and are at a higher risk of airway obstruction in recovery
Patients with lower respiratory tract diseases such as feline asthma are at risk of bronchoconstriction and hypoxaemia, so having pre-calculated doses of medications like terbutaline (a bronchodilator) and steroids on-hand is important in these patients
Patients with pulmonary parenchymal diseases such as pneumonia have impaired gaseous exchange, so are at risk of hypoxaemia throughout, and often require support to open their consolidated alveoli - so can benefit from mechanical ventilation during their anaesthetic
Patients with pleural space diseases such as pneumothorax will struggle to ventilate due to the contents accumulating within the pleural space, so draining these patients quickly after induction is an important consideration
Once we’ve stabilised and assessed our patient, and considered their suspected disease process, we can then formulate our anaesthetic plan.
Planning and preparation
There are a few things to think about when planning to anaesthetise a respiratory case. They are:
Which drugs to use
What equipment to prepare
How we’ll administer oxygen
How we’ll recover the patient
What drugs should we use?
Generally we want to use drugs that cause minimal respiratory depression. In many cases, this means using opioids with benzodiazepines, or potentially low doses of alpha-2 adrenergic agonists, depending on the location of the patient’s respiratory disease.
For induction, any rapidly acting injectable induction agent is acceptable. Propofol has some (mild) bronchodilating effects, as does ketamine, so may be preferred in patients with lower airway disease where bronchospasm is a risk. Care, however, should be taken to avoid propofol apnoea, or to ensure the patient is ventilated whilst this transient effect passes.
Inhalant anaesthesia, ideally with controlled mechanical ventilation where possible, is the safest method for maintaining anaesthesia in patients with airway disease:
Airway access is maintained with an ET tube
Volatile agents have a non-specific bronchodilating effect
They are rapidly eliminated
However, they are not suitable for all respiratory procedures - such as airway endoscopy, or tracheal stent placement, for example. Any procedure where there is a risk of environmental gas contamination should have total intravenous anaesthesia (TIVA) instead.
In addition, volatile agents cause dose-dependent respiratory depression and therefore a decrease in alveolar ventilation. To minimise this, use balanced anaesthetic techniques, utilising analgesia to reduce volatile agent doses.
What else do we need?
Other items to keep in your respiratory anaesthesia toolkit include:
An emergency airway kit, including a laryngoscope, smaller ET tubes, stylets and rigid urinary catheters for difficult intubations +/- tracheostomy equipment
Suction, in case of excessive airway secretions or regurgitation/aspiration
Equipment for pre-oxygenation
Pre-calculated doses of emergency medications
Once everything is ready and you’ve got your anaesthetic protocol, it’s time to pre-oxygenate your patient and anaesthetise them.
Monitoring and maintenance
There’s a lot to think about when monitoring the anaesthetised respiratory patient. In addition to all of our usual anaesthetic considerations, we need to monitor oxygenation and ventilation especially closely.
Oxygenation
Oxygenation is defined as the ability for oxygen to enter the bloodstream and reach our tissues. Oxygenation, then, is not how well our patient is able to breathe oxygen in, but how well that oxygen can diffuse across the alveoli, from the lungs, into the bloodstream.
We can assess oxygenation in two ways:
Pulse oximetry, which measures the percentage of haemoglobin molecules saturated with oxygen, and
Arterial blood gas analysis, which measures the amount of oxygen present in arterial blood.
Pulse oximetry is a readily available, easy-to-use method of monitoring oxygenation. Hypoxaemia is defined as an SpO2 of less than 95%; severe hypoxaemia is defined as a SpO2 of less than 90%.
Pulse oximetry is, however, less reliable in a hypotensive patient, vasoconstricted patient, or when the probe is placed on an area of high movement (e.g. on the tongue during a bronchoscopy procedure). Despite this, it is a vital monitoring tool.
Arterial blood gas is the gold standard method of monitoring oxygenation, but requires either direct arterial sampling (which is technically challenging) or the placement of an arterial catheter. For this reason, it is generally used for especially high-risk respiratory patients/procedures only.
Ventilation
Unlike oxygenation, ventilation is defined as the delivery of oxygen-rich air to the alveoli - it is the physical act of breathing. We monitor ventilation by assessing carbon dioxide levels, either in exhaled respiratory gases (end-tidal CO2/ETCO2) or in arterial blood (PaCO2).
Higher CO2 levels indicate hypoventilation - either because the patient has respiratory depression, or because there is a physical abnormality impairing ventilation (e.g. in pleural space disease). We also see hypoventilation as a consequence of severe hypoxaemia, as the patient develops respiratory fatigue. This is why getting oxygen supplementation on board as soon as hypoxaemia is noted is especially important.
Under anaesthesia, hypoventilation is managed by:
Correcting the underlying cause if one is present; e.g. performing thoracocentesis in a patient with pleural space disease
Administering intermittent positive-pressure ventilation where indicated to reduce CO2 levels
Placing the patient on a mechanical ventilator in severe cases, to control ventilation and maintain a normal ETCO2 level.
Recovery
Recovery is perhaps the most high-risk area of anaesthesia for our respiratory patients.
These patients should be recovered slowly and calmly. Patients who were especially stressed prior to anaesthesia may benefit from some sedation (e.g. a low dose of acepromazine or dexmedetomidine, or some rectal trazodone) in the post-operative period, to avoid stress or excitation increasing oxygen demand in recovery.
Patients who were hypoxaemic during anaesthesia, or required oxygen supplementation prior to anaesthesia, should recover in oxygen and have their oxygen supplementation continued as required in recovery.
It’s also worth noting that any respiratory patient can develop hypoxaemia on recovery, so after extubation, monitor them closely with flow-by or mask oxygen available. For as long as it will be tolerated, keep an eye on their SpO2 level and add supplemental oxygen as required.
Keep monitoring your respiratory patients closely in recovery, keeping an eye on vital parameters, SpO2, and respiratory pattern and effort as they continue to recover.
Lastly, we know that our respiratory patients can deteriorate at any time in recovery, so keep an emergency kit with any case where this is a particular risk. This could include:
Emergency re-intubation equipment +/- tracheostomy equipment in patients with airway disease
Thoracocentesis equipment in patients with pleural space disease
Emergency doses of bronchodilators pre-calculated in patients at risk of bronchoconstriction
So there you have it! Breathe easier when anaesthetising respiratory patients by:
Performing careful pre-anaesthetic assessments
Thinking about the area of the respiratory system affected and how this impacts your patient
Pre-oxygenating and being prepared for any deterioration
Monitoring oxygenation and ventilation closely
Recovering your patient slowly, with supplemental oxygen and close monitoring
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References
Barletta, M. 2017. Anaesthesia for patients with respiratory diseases [Online] FetchDVM360. Available from: https://www.fetchdvm360.com/wp-content/uploads/2017/08/CVCKC-2017-full-proceedings_rev.pdf
Dodam, JR. 2010. Anaesthesia for patients with respiratory disease [Online] DVM360. Available from: https://www.dvm360.com/view/anesthesia-patients-with-respiratory-disease-proceedings
Grubb, T. 2010. Anaesthesia for the patient with respiratory compromise [Online] DVM360. Available from: https://www.dvm360.com/view/anesthesia-patient-with-respiratory-compromise-proceedings
Johnson, RA. 2015 Anaesthetic considerations for upper and lower respiratory disease. In: Smith, LJ. ed. Questions and answers in small animal anaesthesia. Iowa: Wiley-Blackwell.
Merrill, L. 2012. Small animal internal medicine for veterinary technicians and nurses. Iowa: Wiley-Blackwell.
