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Advanced Anaesthetic
WK 12
| Question | Answer |
|---|---|
| what does blood gas analysis measure 3 primary parameters - | - Oxygenation (PO2) - Ventilation (PCO2) - Acid base status =/- electrolytes, blood glucose, blood lactate, ionised calcium, % saturation of oxygen, base excess, bicarbonate concentration |
| Why would you measure Blood Gases (BGs) | Aids in the diagnosis, treatment and managment of many disease processes... metabolic or respiratory dysfunction used alone or in conjunction with other blood tests =/- diagnostics |
| BGA results are also indicators of how the patient is compensating for | a metabolic or respiratory disease/dysfunction. |
| sample collection site venous or arterial samples can be used venous sample = | Easier to access Can be painful Aseptic Technique Directly from vein (ideally jugular) or a central venous line |
| Arterial Sample = | Difficult to access, especially in a conscious patient Can be very painful Strict aseptic technique Directly from artery or central arterial line May require local anaesthesia or EMLA for arterial puncture |
| venous sampling | see images can be done by single blood draw (off the needle) central line (in situ) |
| Arterial sampling | see images can be done by single blood draw (off the needle central line (in situ) |
| sample collection | Anaerobic technique Airtight syringe to prevent inadvertent air entry. or Direct collection from a central line discard first few drops of blood. |
| sample collection equipment | Heparinised syringe Lithium heparin blood gas syringe (ready-made) or a lithium heparin coated syringe. If not using a heparinised syringe, analyse the sample immediately. |
| Blood Gas Analysis BGA Machine | The Woodley EPOC machine is a very common analyser used in veterinary practice Others are available for example Idexx Vetstat |
| Normal parameters | PO₂: 5 × inspired % O₂ PCO₂: 35–45 mmHg pH: 7.35–7.45 HCO₃⁻: 22–36 mmol/L Base excess: –4 to +4 Anion gap: Dogs 12–25 mmol/L; Cats 13–27 mmol/L |
| respiratory or metabolic ? | Respiratory alkalosis: pH >7.45; PCO₂ ↓; HCO₃⁻ normal/↓ (comp) Respiratory acidosis: pH <7.35; PCO₂ ↑; HCO₃⁻ normal/↑ (comp) Metabolic alkalosis: pH >7.45; PCO₂ normal/↑ (comp); HCO₃⁻ ↑ Metabolic acidosis: pH <7.35; PCO₂ normal/↓ (comp); HCO₃⁻ ↓ |
| Oxygenation | Would expect P02 to be roughly 5x inspired oxygen severe at <69mmHg does not tell us oxygen content need Haematocrit (HCT) or Haemoglobin (Hg) to calc oxygen content often used in combination with pulse oximetry |
| Ventilation 'gold standard' | > 45mmHg PCO₂ = hypoventilation. < 35mmHg PCO₂ = hyperventilation. Can be used alongside capnography |
| the rvn role everything excluding diagnosis = | Obtaining & processing blood sample managing peripheral & central lines Using & managing BGA equipment. Interpreting & recording results Responding to resultsVS instruction Monitoring patient progression/deterioration Obtaining & processing samples |
| Blood pressure (BP) Analysis 2 methods = | direct indirect |
| Arterial Blood Pressure - Recap | Cardiovascular status (+ tissue perfusion) Systolic arterial pressure (SAP) = 90-120mmHg (contraction) Diastolic arterial pressure (DAP) = 55-90mmHg (relaxation) Mean arterial pressure (MAP) = 60-100mmHg. For adequate organ perfusion, MAP = >60mmHg |
| Blood Pressure method - Indirect | Doppler vs Oscillometric Use a pneumatic cuff… Correct size cuff must be used! Correct placement over peripheral artery (see arrow on cuff). Easy and non-invasive. Less accurate than direct BP monitoring. |
| Doppler blood pressure Doppler Ultrasonography | SAP in dogs, MAP in cats. Inexpensive. Audible pulse. Cuff can be left in-situ (not inflated!). |
| Oscillometric BP | Also uses a pneumatic cuff Cyclic measurement. More automated process. SAP, MAP, DAP, pulse rate. Can take repeated measurements at timed intervals, |
| BP Methods - Direct Gold standard → direct from artery. Placement of an arterial catheter required where from ? | Dorsal pedal artery. Femoral artery. Coccygeal artery. Auricular artery (larger patients). |
| How does it work? | Arterial catheter connected to… 1. Tubing primed with heparinised saline. 2. Transducer (zeroed and calibrated at the level of the right atrium). 3. Monitor cable. |
| how does it work During heartbeat, saline moves with arterial pulsations | → produces an electrical signal → blood pressure (numerical reading). |
| wave form | see image SAP DAP Dicrotic Notch |
| Wave form cont | see image |
| wave form dampening dampening can be caused by | - air in the connecting lines - length of tubing - blood clot formation - small catheter size |
| hypotension MAP <60mmHg. Can be caused by: | Drugs e.g. isoflurane. Vasodilation. Hypovolaemia. Reduced cardiac output. Many more! |
| hypotension what do we do | Check equipment Palpate pulse quality Assess anaesthetic depth Inform VS: Reduce volatile agent? IVFT? Positive inotropes? |
| hypertension MAP 100>mmHg. | Can be caused by: Light plane of anaesthesia. Pain. Drugs e.g. ketamine. Other disease. Many more! |
| hypertension what do we do | Palpate pulse quality Assess anaesthetic depth Inform VS: Deepen anaesthetic plane? Analgesia? Vasodilators? |
| Electrocardiography (ECG) | Measures the heart’s electrical impulses Shows rate, rhythm, and conduction patterns Helps diagnose arrhythmias, chamber enlargement, electrolyte disturbances, and anaesthetic complications |
| ECG how it works | The heart generates electrical signals from the SA node These travel through the atria, AV node, bundle branches, and Purkinje fibres The ECG machine detects these signals through skin electrodes and converts them into waveforms (P, QRS, T) |
| What the ECG Shows | P wave: atrial depolarisation QRS complex: ventricular depolarisation T wave: ventricular repolarisation Intervals (PR, QT) show conduction speed Segments show electrical stability |
| The basic guide to interpretation | What is the heart rate? normal, brady/tachycardia What is the rhythm? regular, irregular Is there a QRS complex for every P wave Is there a P wave for every QRS complex is there consistent & reasonably related What is morphology of the QRS complex? |
| see diagrams of | PQRST ranges |
| What Capnography Does | Capnography measures the CO₂ concentration in exhaled air, giving a continuous picture of ventilation. It shows how effectively the patient is breathing, exchanging gas, and circulating blood. |
| Capnography How It Works | CO₂ is produced by cellular metabolism Carried in blood to the lungs Exhaled during ventilation The capnograph detects this CO₂ and displays it as: ETCO₂ value (end‑tidal CO₂) Waveform (capnogram) |
| why capnography is used | Monitor anaesthetised patients Detect apnoea or hypoventilation early Identify airway obstruction or disconnection Assess CPR effectiveness (ETCO₂ rises when compressions improve circulation) Evaluate metabolic status (e.g., hypermetabolic states) |
| normal capnography values | ETCO₂: 35–45 mmHg (dogs/cats) Waveform: smooth square‑shaped plateau |
| capnography What the Waveform Shows | Phase I: baseline (no CO₂) Phase II: rapid rise (mixing of dead space + alveolar gas) Phase III: alveolar plateau Phase 0: inspiration (CO₂ drops to zero) |
| capnography common abnormalities | High ETCO₂: hypoventilation, rebreathing, equipment failure Low ETCO₂: hyperventilation, disconnection, cardiac arrest Shark‑fin shape: airway obstruction (e.g., bronchospasm) |
Created by:
lucy.fox