catheter using Seldinger technique after a peripheral catheter has been placed in the jugular vein.
When intravenous catheterization is not possible, which is often the case in neonatal and small pediatric patients, intraosseous (IO) catheterization provides a rapid, safe method for delivery of fluid therapy and medications. This is because the capillary network within the marrow cavity is in direct communication with the nutrient and emissary veins that drain into the central circulation. Crystalloids, colloids, blood products and medications, including those for cardiopulmonary resuscitation, can be administered via the IO route and can be absorbed rapidly enough to be effective for the treatment of hypovolemic shock and cardiopulmonary arrest [91–96]. Sites commonly used for IO catheterization include the trochanteric fossa of the femur, proximal tibia, tibial tuberosity, wing of the ileum, the ischium and greater tubercle of the humerus [88, 92], with the trochanteric fossa and tibia used most commonly. Contraindications of placement of an IO catheter include fracture of the bone intended for cannulation, pneumatic bones in birds, and evidence of infection near the intended catheter site. Bone growth is not impacted by IO catheterization [95]. IO catheterization can be achieved with a variety of techniques, including standard hypodermic needles and spinal needles, IO infusion needles, a spring‐loaded penetration injection gun (Vet B.I.G Bone Injection Gun (15‐G), WaisMed Ltd, Houston, TX) and an automatic rotary insertion drill (EZ‐IO (15‐G Pediatric Needle Set), Vidacare Corporation, San Antonio, TX.). In a cat cadaveric study comparing these devices, the injection gun was found to be faster and easier to use, but there were no differences detected between insertion site (humerus or tibia), complications or success between the injection gun, rotary drill or manual IO catheter [96].
Direct arterial blood pressure measurement should be considered in any hemodynamically unstable patient. It allows for continuous, accurate pressure determination in the face of hypotension, hypertension, and arrhythmias. Indwelling arterial catheters can also be used to obtain blood samples, particularly for arterial blood gas analysis. Arteries generally accessible for percutaneous placement of an arterial catheter include the dorsal metatarsal artery (most commonly used), the coccygeal artery in the tail, the auricular artery in the dorsal pinna, the femoral artery, and the radial artery [88,97–99]. Maintenance of arterial catheters in all locations, except for the dorsal metatarsal artery, is difficult in mobile patients and is generally reserved for use in sedated or anesthetized patients. Femoral, dorsal metatarsal, and coccygeal artery catheters can also become contaminated with urine and/or feces, so consideration of these issues is important prior to catheter placement. Cats tend to have poor collateral circulation. It is not recommended to leave arterial catheters in cats for longer than six to eight hours because of concern for ischemic injury to the tissues distal to the catheter [97]. Contraindications for arterial catheterization include lack of close monitoring capabilities, thrombocytopenia, thrombocytopathia, and coagulopathy.
Once the course of the artery is determined by palpation and the site is aseptically prepared, an over‐the‐needle catheter is used to puncture the artery from an angle of 15–30 degrees above the vessel. Pulsatile blood flow will be observed in the hub of the catheter upon successful arterial cannulization. If percutaneous placement of an arterial catheter is not possible, access to the dorsal metatarsal or femoral artery can be achieved with a surgical cut‐down or ultrasound guidance [98]. Care must be taken to avoid damaging the artery, femoral vein, or sciatic nerve during the initial skin incision and approach to the femoral artery. The catheter is secured with tape and/or sutures depending on the catheter type placed and placement method use. Once the arterial catheter is in place, it can be used to collect arterial blood samples after an adequate pre‐sample of blood is taken (generally 3–6 ml of blood into syringes with small amounts of heparinized saline). Clear labeling of an arterial catheter is imperative to ensure that only heparinized saline is injected into the artery. Blood collected from the artery as a pre‐sample and medications should never be injected into the arterial catheter. The catheter can also be connected to non‐compliant tubing with heparinized saline and a pressure transducer for continuous arterial blood pressure monitoring.
Analgesia, Sedation and Anxiolytics
The need for sedation and anxiolytics in the stressed or scared veterinary patient and timely analgesics for those in pain cannot be overstated. This is particularly true in patients needing emergency surgery, as many conditions requiring emergency surgical intervention create significant discomfort or pain. Anxiety and stress are present in many patients with respiratory compromise, especially those with upper airway obstruction, and should be addressed immediately to provide relief for the patient and more accurate patient assessment. As with other body systems, frequent reassessment is necessary to ensure adequate analgesia and patient wellbeing. Pain can be challenging to assess accurately in hospitalized feline patients, as they tend to be quieter and more reserved than canine patients [100, 101].
If a patient is assessed to be in pain, analgesics should be administered as soon as possible. It is not appropriate to withhold analgesia because of concerns about creating cardiovascular or respiratory instability or masking changes in patient status. When titrated doses of cardiovascular sparing analgesics and anxiolytics are used, primary cardiovascular or respiratory depression should not result. Instead, if hypotension or respiratory changes are seen after drug administration, it is more likely that the patient's cardiopulmonary instability was uncovered by relief of pain‐induced tachycardia, tachypnea, and catecholamine release with secondary vasoconstriction [100].
Multimodal analgesia is preferred in many patients, especially those with marked pain, as the complexity of pain pathways often renders single agent therapy ineffective, irrespective of dose escalation [101]. The addition of local analgesics is often beneficial to decrease the systemic dose. The addition of anxiolytics may also be helpful to decrease the stress and agitation associated with hospitalization, recumbency, and activity restriction present in many surgical patients, and therefore decrease systemic analgesic dosing.
Opioids are often the first choice of analgesia in critical veterinary patients as they have rapid onset of action, can be titrated to an individual's needs, are reversible, and are cardiopulmonary sparing medications. They can be used alone or in combination with other analgesics and anxiolytics and administered as a bolus or constant rate infusion (CRI). Common adverse effects include initial excitatory phase, nausea, vomiting, bradycardia, decreased gastrointestinal motility, and respiratory depression at high doses. Cats tend to be more prone to developing an excitatory period, so the dose of opioids in cats is generally half the canine dose. Rapid intravenous administration of morphine or meperidine can cause histamine release, vasodilation, and hypotension. If adverse effects result from opioid administration, naloxone is a pure antagonist for opioid reversal. Naloxone will reverse both the positive and negative effects of opioids, which may result in pain, excitement, and agitation. Fentanyl transdermal patches are effective ways to provide potent analgesia in an outpatient setting, but they can become displaced or ingested, and have the potential for misuse and abuse by owners, including ingestion by small children.
Lidocaine is effective as a local anesthetic and epidural analgesic. It can also be administered as an intravenous CRI, generally combined with an opioid such as morphine or fentanyl, with or without the addition of a ketamine CRI. When given intravenously, it should be used cautiously and titrated carefully in cardiovascularly unstable patients, as it can cause cardiac arrhythmias, tachycardia, and seizures. This is especially true in cats, and some debate exists as to whether this medication should be given intravenously to cats for analgesia.
N‐methyl‐d‐aspartate (NMDA) antagonists such as ketamine are very effective, owing to their multiple sites of action and effects, including analgesia, neuroprotection, and sedation. Ketamine has limited cardiopulmonary depression but can increase cardiac output and myocardial oxygen consumption, and it can cause muscle tremor activity. Controversy exists over its use in patients with head trauma as there is concern that ketamine contributes to increased ICP. It should be used with caution in patients with hypertension and cardiovascular disease. In cats, it is renally excreted so consideration for renal function with use and dose
