ANAESTHESIOLOGY
popular belief , butorphanol is NOT an effective primary analgesic ( especially in dogs ) and does not last very long . Morphine , hydromorphone , methadone , fentanyl , oxymorphone and other full mu agonist opioids are much more effective analgesics .
2 . What is the time-frame between premedication , induction and maintenance anaesthesia for surgery ?
Is there enough time allotted for the premedications to work before induction and surgery ? For example , buprenorphine , although an effective partial mu agonist , takes 20 to 30 minutes for full analgesic effects . Starting surgery before analgesic onset will result in elevated levels of nociception , contributing to patient arousal .
3 . How are patients being monitored ?
In response to an increase in sympathetic tone , heart rate will increase during light periods of anaesthesia ; however , judging anaesthetic depth solely based on heart rate is inaccurate . Because heart rate can be affected by so many other variables , it is a poor indication of anaesthetic depth . For example , a patient entering a deeper plane of anaesthesia ( level 3 to 4 ) will commonly become tachycardic because of increased sympathetic tone from decreased ventilation and elevated partial pressure of carbon dioxide ( pCO2 ). The person monitoring the patient notices the tachycardia and mistakenly turns up the inhalant anesthetic , thinking the patient is arousing . Gradually the patient will reach deeper levels of anaesthesia , which can become serious . Besides heart rate , other monitoring parameters include arterial blood pressures , end-tidal pCO2 , jaw tone , eye position , response to a toe pinch , respiratory rate and character , mucous membrane color and capillary refill time .
Other than tachypnea , no other parameters included in the question help judge the patient ’ s level of anaesthesia . However , a 100-lb dog with a heart of 100 to 110 beats / minute is inconsistent with a surgical plane of anaesthesia ( keeping in mind heart rate is a poor indication of anaesthetic depth ). Any external stimuli — nociception , position changes — will cause the patient to quickly arouse . It is important to realise that anaesthesia is not the same as analgesia . Many anaesthetic drugs ( e . g . isoflurane , sevoflurane , propofol ) have little to no analgesic properties . In addition to monitoring a patient ’ s cardiovascular status , respiratory status and depth of anaesthesia , it is imperative to monitor the level of the patient ’ s nociception . Movement during anaesthesia could be an indication of inadequate anaesthesia ( light plane ) or reflexes ( nociception ). When used alone , a high dose of inhalant anesthetic drug is required in order to eliminate all muscle movement reflexes due to surgical stimuli during general anaesthesia ( this is called “ MAC no movement ”). Administering analgesic drugs before and during surgery ( intermittent or continuous rate infusions ) decreases the necessity for high vaporizer outputs .
4 . What breathing systems are being used ? Are the breathing systems being checked for leaks ? Are the patients intubated ?
There are two major types of breathing systems used on conventional anaesthetic machines for small animal patients : rebreathing and non-rebreathing . The rebreathing systems can be further divided into adult and pediatric circle systems . The choice of breathing system is based on patient size : patients weighing < 5 kg require a non-rebreathing system , those between 5 and 10 kg need a pediatric rebreathing system , and those > 10 kg require an adult rebreathing system .
Because oxygen flow rates and carbon dioxide removal differ between non-rebreathing and rebreathing systems , an important question needs to be clarified . Has the author noticed the difference only in large dogs ( adult rebreathing ) or small patients ( non-rebreathing ), or both ? For example , a patient on a non-rebreathing system with an inadequate flow rate will rebreathe carbon dioxide ( due to dead space ventilation within the expiratory tube ), which , in turn , will cause the patient to initially become tachypnoeic and tachycardic .
Are the patient breathing systems ( both rebreathing and non-rebreathing ) checked for leaks ? Leaks within the breathing system can cause the anaesthetic gases ( oxygen , carbon dioxide and anaesthetic vapor ) to leak out , resulting in an inadequate supply to the patient , or room air to enter in , resulting in dilution .
Are the patients intubated or are face masks used ? Because face masks frequently do not provide a tight seal against the patient ’ s snout , anaesthetic gases can leak out and room air can leak into the breathing system . Intubating the patient allows for controlled ventilation , which is helpful in controlling the depth of anaesthesia . Combining the use of a mask for inhalant anaesthesia and an active scavenger system would almost confirm air being pulled into the breathing system through the mask . This would result in a significant amount of anaesthetic gas loss and / or dilution .
5 . What kind of ventilation is being used ?
Patients allowed to spontaneously ventilate will normally have cyclical anaesthetic depth levels . A lightly anaesthetised patient will have a higher minute ventilation , thus breathing more anaesthetic gases into the lungs . Gradually the patient will breathe themselves into a deeper plane of anaesthesia , resulting in a decreased minute ventilation and decreased uptake of anaesthetic vapor . Eventually the patient ' s anaesthetic vet360
Issue 03 | JULY 2018 | 12