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Case 2.4 – Monitoring: Standard & Invasive

Category: Clinical Skills | Discipline: Anaesthesia | Setting: Operating Theatre

Case

You have on your routine day-stay list for knee arthroscopy a morbidly-obese 40 year-old lady. She weighs 135kg and is 155cm tall. Unfortunately, there is no BP cuff that is both narrow enough for her quite short arms, and long enough to adequately appose the Velcro on the cuff; it keeps on "popping off" every time the automated BP machine inflates.

You ask the nurse to assemble the equipment you need to insert an intra-arterial line to allow you to measure the BP.

Questions

1. What is the minimum routine monitoring that is required for a patient undergoing general anaesthesia? What other (optional, or additional) monitoring tools are available?

See ANZCA PS18. Basically:

Minimum Required Monitoring:

  • SpO₂ (pulse oximetry)
  • ETCO₂ (end-tidal carbon dioxide)
  • ECG
  • NIBP (non-invasive blood pressure)
  • Oxygen concentration
  • Volatile agent concentration
  • Respiratory mechanics

Additional Monitoring Available:

  • Temperature
  • Neurological (BIS or Entropy)
  • Invasive arterial pressure
  • Central venous pressure
  • Pulmonary artery pressure
  • Cardiac output monitoring
  • Neuromuscular function (train-of-four)
2. What are the indications for invasive arterial pressure monitoring? Describe how this is performed.

A. Absolute Indications:

  • Non-pulsatile arterial flow eg Cardiopulmonary Bypass; Left Ventricular Assist Device

B. Relative Indications:

  • i. Likely major BP swings or haemodynamic instability. Need for vasoactive drug infusion.
  • ii. Inability to measure reliably non-invasively eg the patient in this case, or Atrial fibrillation, or very slow heart rate
  • iii. Very prolonged surgery where there is a small risk of radial nerve damage from repeated cuff inflation
  • iv. Surgery on both arms (can often use the leg unless obese)
  • v. Need for repeated blood sampling especially arterial blood gases

How it is performed:

Needs:

  • A catheter in an artery (usually radial or femoral)
  • Fluid-filled column (non-compliant tubing filled with saline)
  • A transducer attached to a fast-flush device (transducer converts one form of energy to another, in this case mechanical to an electrical signal; the fast-flush device is a bag of saline in a pressure bag inflated to greater than the systolic pressure)
  • A means of processing and displaying the signal (a monitor)

Requirements for accurate pressure reading:

  • A short cannula
  • Rigid tubing as short as possible between the patient and the transducer as free of air bubbles as possible
  • A transducer with as high a natural resonant frequency as possible (in practice, this is determined by the manufacturer)
  • Proper Zeroing ie the monitor needs to know what is the atmospheric pressure above which the BP is being measured
  • The transducer must be at an appropriate level compared to the patient eg heart-height or circle of Willis height
3. What other invasive haemodynamic pressures can be monitored in the OR? What is Central venous pressure, what can affect it? Describe the waveform seen in a normal person.

Other pressures measured include:

  • Central Venous (CVP)
  • Pulmonary Artery pressure and PA occlusion pressure ("Wedge pressure")
  • Intracranial (ICP)
  • Occasionally others like coronary sinus, internal carotid artery back-pressure

CVP (Central Venous Pressure):

CVP is the next most commonly-measured. It is the pressure in the right atrium or vena cava and is measured via a catheter inserted into a great vein (usually internal jugular or subclavian). It is a reflection of the volume in the circulation as a whole.

It can be affected by:

  • 1. Intravascular volume (as 90% of volume is in the venous system)
  • 2. Venous tone (ie venoconstriction will increase CVP)
  • 3. Intra-thoracic pressure (so will be affected by positive-pressure ventilation)
  • 4. Right heart function (Tricuspid valve and right ventricle)

Note that it is not affected by left-ventricular function and so is not a good indicator of LV loading conditions.

There is no one "normal" CVP, rather a range that provides the best loading conditions in an individual. If BP is low, a trial of intravenous volume and re-assessment of CVP after that volume load is often useful.

General guide: if CVP <10, room for volume; CVP>12, probably "full" but may be worth a trial of volume (particularly with regard to those variables above that can affect CVP).

Bottom line is not to treat the numbers, but to use the numbers as one more piece of information to enable an informed management decision.

CVP is often used, however, as a surrogate for LA pressure – there are many, many fallacies in this argument – however, using CVP as a guide to filling the heart after cardiac and major surgery is often very helpful.

CVP waveform:

Normally a, c and v waves and x and y descents.

  • A wave from atrial contraction
  • C wave from TV closure
  • V wave from atrial filling in systole
  • X descent from movement of TV during RV systole
  • Y descent from rapid emptying of RA when TV opens

Abnormal patterns can be used to diagnose cardiac abnormalities eg V-wave (superimposed over c and v) with tricuspid regurgitation; large a wave with complete heart block.

4. What are some indications for, and complications of, central venous catheterisation?

Indications for central Venous line placement:

  • a. No other venous access possible
  • b. To monitor CVP eg major surgery or major blood or fluid shifts possible
  • c. To allow infusion of irritant substances eg catecholamines; potassium solutions; vancomycin; TPN
  • d. For long-term venous access
  • e. For temporary or permanent haemodialysis access
  • f. To allow pulmonary artery catheterisation
  • g. To allow transvenous cardiac pacing

Complications of central lines:

  • a. Damage to other structures – artery (carotid, subclavian); pleura and lung; thoracic duct; vagus; recurrent laryngeal nerve; pericardial tamponade
  • b. Arrhythmias either from guide-wire insertion or the catheter itself
  • c. Infection. Minimise by use of: full aseptic insertion technique + antibiotic impregnated catheter + dedicated central-line team + dressing change only when indicated not routinely + access line only when necessary + prep with 2% Chlorhexidine in alcohol (not in operating rooms). Usually S aureus but can be anything also fungi.
  • d. Thrombosis
  • e. Air embolus. Ensure lines only opened when patient supine
  • f. Catheter embolus (Not with modern catheter-over-guidewire systems)
  • g. Misinterpretation of the data
  • h. Patient harm due to inattention while inserting the line