Understanding Pre-Oxygenation and Desaturation Time

Pre-oxygenation is a crucial step in ensuring adequate oxygenation during medical procedures. In a 70-kg man, the functional residual capacity (FRC) is approximately 2.5 liters. If pre-oxygenated adequately, all of the FRC will be 100% oxygen, and oxygen extraction per minute is 250 ml. This means that the patient will have 10 minutes of adequate oxygenation in the absence of ventilation before desaturation occurs.

However, if only 21% (room air) was in the FRC, the patient would only have 2 minutes before they started to desaturate. It is important to note that the oxygen extraction rate is 250 ml per minute in a 70-kg male. A person who has been adequately preoxygenated will not start to desaturate until double this time period, which is 10 minutes.

Therefore, if the patient is oxygenated on room air only, the correct answer for desaturation time would be 2.5 minutes. However, if the patient has been adequately preoxygenated, they will desaturate well before 25 minutes is reached. Understanding pre-oxygenation and desaturation time is crucial in ensuring patient safety during medical procedures.

Measuring Cardiac Output and Pressures with a Pulmonary Artery Catheter

A pulmonary artery catheter can provide direct and derived measurements for assessing cardiac function. Direct measurements include right atrial pressure, right ventricular pressure, pulmonary artery pressure, pulmonary artery wedge pressure, core temperature, and mixed venous saturation. The catheter can also be used to calculate cardiac output using the method of thermodilution. This involves a proximal port with a heater and a distal thermistor that senses changes in temperature.

Pulmonary artery wedge pressure is a direct measurement that can be obtained with the catheter, reflecting left atrial filling. However, it may not always accurately reflect the pressure in the left atrium due to various factors. Right ventricular pressure is another direct measurement that can be obtained.

Central venous saturation is a direct measure in some machines with a built-in saturation measurement probe, while in others, samples can be taken via the distal port and measured using a gas machine. Overall, a pulmonary artery catheter can provide valuable information for monitoring cardiac output and pressures in critically ill patients.

An Overview of Common Anaesthetic Induction Agents and Their Advantages and Adverse Effects

Propofol, ketamine, fentanyl, methohexital, and thiopental are commonly used anaesthetic induction agents. Propofol is the preferred choice for most people due to its rapid onset and offset of action, antiemetic and anticonvulsive effects, bronchodilation, and decreased cerebral metabolic oxygen demand and cerebral blood pressure. Ketamine is ideal for hypotensive patients due to its rapid onset, analgesic effects, bronchodilation, and maintenance of airway reflexes and respiratory drive. Fentanyl is an opiate analgesic used for pain relief, while methohexital is used for electroconvulsive therapy. Thiopental is a barbiturate used in general and regional anaesthesia. However, each agent has its own set of advantages and adverse effects that must be considered before use. For example, ketamine can increase intracranial pressure, cardiac output, and blood pressure, while thiopental can cause bronchoconstriction and is contraindicated in asthmatics and patients with a history of porphyria. It is important to carefully evaluate each patient’s medical history and condition before selecting an appropriate anaesthetic induction agent.

Medication Management in Perioperative Period

Phenelzine and tranylcypromine are monoamine oxidase inhibitors that need to be discontinued at least two weeks before elective surgery due to their potential life-threatening interactions with pethidine and indirect sympathomimetics. Additionally, they can prolong the action of suxamethonium by decreasing the concentration of plasma cholinesterase. Carbamazepine, an anticonvulsant, should be continued throughout the perioperative period. Gliclazide, a short-acting oral hypoglycemic, can be taken if the surgery’s anticipated duration is short.

Lisinopril, an angiotensin-converting enzyme inhibitor (ACEi), and digoxin, a cardiac glycoside, should be continued pre-operatively. Morphine sulfate tablets should also be continued pre-operatively, and a morphine infusion (PCA) should be considered for postoperative analgesia. Pyridostigmine is used in the management of myasthenia gravis and should be continued before minor surgery. However, if perioperative muscle relaxation is required, omitting one or more doses of pyridostigmine would allow a reduction in the dose of the muscle relaxant. Proper medication management in the perioperative period is crucial to ensure patient safety and optimal surgical outcomes.

Different Flow Rates of Cannulas for Infusions

Cannulas are small tubes inserted into a vein to administer medication or fluids. Different sizes and colors of cannulas are available, each with varying flow rates. The grey 16G cannula has the highest flow rate of 180 ml/min, while the blue 22G cannula has the lowest flow rate of 33 ml/min. The pink 20G cannula is the most commonly used in adults for standard infusions, with a flow rate of 54 ml/min. The green 18G cannula has a flow rate of 80 ml/min, and the orange 14G cannula has the largest size and highest flow rate of 270 ml/min. It is important to choose the appropriate cannula size and flow rate for each patient’s needs.

Neuropathic Pain and Treatment Options

Neuropathic pain is a chronic condition that affects neuronal membrane excitability and often results in allodynia, hyperalgesia, and hyperpathia. Unlike nociceptive pain, neuropathic pain is less responsive to opioids and can be more difficult to treat. To manage this type of pain, coanalgesics such as antidepressants, anticonvulsants, and antiarrhythmics are commonly prescribed. Anticonvulsants are typically administered at similar doses and schedules as in the treatment of epilepsy. The analgesic response to mexiletine can be predicted by evaluating the improvement in pain after an intravenous lidocaine infusion. Amitriptyline, on the other hand, has a faster analgesic response than its antidepressant effect, with pain relief occurring within a week of treatment. Overall, managing neuropathic pain requires a multifaceted approach that may involve a combination of medications and other therapies.

Oculogyric Crisis

Oculogyric crisis is a type of acute dystonic reaction that is commonly associated with the use of neuroleptics and anti-emetic medications like metoclopramide. Unfortunately, the clinical spectrum of this condition is not well understood, which often leads to misdiagnosis as a psychogenic disorder. Symptoms of oculogyric crisis can occur suddenly or over several hours and may include restlessness, agitation, malaise, and a fixed stare. The most characteristic symptom is the upward deviation of the eyes, which may be sustained or accompanied by other eye movements like convergence or lateral deviation. Other associated symptoms may include neck flexion, mouth opening, tongue protrusion, and ocular pain. Fortunately, the symptoms of oculogyric crisis can be rapidly resolved with the use of medications like procyclidine.

Cannabinoids and Their Effects

Cannabinoids are compounds found in the resin of cannabis sativa, with 9-tetrahydrocannabinol (9-THC) being the most important active component. However, the oral bioavailability of THC, whether in pure form or as part of marijuana, is low and highly variable, with effects taking anywhere from 0.5 to 3 hours to occur. Smoking marijuana also does not significantly increase bioavailability, with rates rarely exceeding 10-20%.

Interestingly, the analgesic effects of cannabinoids can be blocked by naloxone and other opioid receptor antagonists. Additionally, synthetic cannabinoids have been found to reduce inflammation caused by arachidonic acid by inhibiting eicosanoid production. Overall, the effects and limitations of cannabinoids is important for both medical and recreational use.

Diagnosis of Subarachnoid Haemorrhage

Subarachnoid haemorrhage can be diagnosed with a high degree of accuracy through an urgent CT scan, which can confirm the condition in 95% of patients. In most cases, a lumbar puncture is not necessary unless the patient’s medical history suggests the need for one and the CT scan results are normal. If a bloody tap is suspected, the number of red blood cells should decrease with each successive sample. If an LP is performed six hours after the onset of symptoms, the supernatant fluid should be examined for xanthochromia after centrifugation.

Managing a ‘Can’t Intubate’ Scenario in a Patient with Neck of Femur Fracture

In a ‘can’t intubate’ scenario with a patient who has a neck of femur fracture, the priority is delivering oxygen. If attempts at intubation have been difficult and the patient is desaturating, the Difficult Airway Society recommends attempting ventilation with a laryngeal mask airway (LMA) as a temporary measure. If this fails and the patient continues to desaturate, an emergency cricothyrotomy is advised for urgent oxygen delivery. Using a videolaryngoscope to secure a definitive airway is not recommended if oxygenation is a priority. Similarly, attempting a fibreoptic intubation or using a long blade for further attempts at intubation can cause damage and swelling in the airway, leading to laryngospasm. Waking the patient up is also not advisable in this emergency situation, as neck of femur fractures have a high mortality rate if left untreated.