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DIY Monitoring Systems
Project jfish Anyone interested in DIY monitoring projects should take a look at Daniel Jolley's jfish Project. It's a must see! There are already instructions and support documents for building a peripheral nerve stimulator. Future plans include anaesthesia monitoring workstations using a laptop computer and a standalone compact pulse oximeter. There are also discussion lists to join. We highly recommended Daniel's web site and his project.
Electrocardiography Plans for a PC-based ECG "machine" can be found at Mike Hayman's ECG on a shoestring web site.
Central Venous Pressure (CVP) CVP is measured via a catheter with the tip in an intrathoracic vein inserted via the jugular, subclavian or femoral veins into the superior or inferior vena cava or the right atrium (although the latter is to be avoided if possible). The catheter is connected, via a 3-way tap, to a saline infusion with a manometer in between. If a manometer and/or a 3-way tap is/are not available, a ‘butterfly’ cannula can be inserted into the rubber injection port of the giving set. The butterfly tubing will need to be clamped or otherwise closed between readings. Measurement of CVP in the supine patient is made with the IV catheter and the manometer connected together (with the drip switched ‘out’ using the tap) and the manometer zero at the level of the right atrium (mid-axillary line - marked on the chest). Between readings the manometer is ‘out’. References:
Stethoscope - Oesophageal (commercial also available) A glove finger is tied over the end of a feeding tube (or other suitable plastic tubing) which is attached to the stethoscope (chest piece removed) by tubing, the length of which determines the range of movement by the user, eg anaesthetist, while continuing to monitor heart and respiratory sounds. (Placement is via oro-pharyngeal insertion into the oesophagus, while listening, to ensure optimal location before securing the tubing.) For paediatric use a small feeding tube and the smallest glove finger should be selected. Note: changes in the intensity of the HS (heart sounds) usually indicate a change in BP (blood pressure). If and when it is possible to measure BP with a sphygmomanometer while listening to HS in this way, an ability to relate HS to an approximate BP may be developed - a useful skill in times of need! Patricia Coyle FRCA, FANZCA (and many others!)
Invasive Mean Arterial Pressure The following technique allows mean arterial pressure to be measured in the absence of an electronic transducer. The transducer is replaced by either an aneroid blood pressure gauge or a mercury column gauge. Please refer to the diagram below (from Zorab). A 3-way tap is connected to the gauge by a 0.5 - 1 metre length of tubing. A syringe is used to inject heparinised saline (1 unit heparin/1ml IV fluid) up this tubing until the gauge reads 200 mmHg. The tap is then closed. The heparinised saline is used to flush the air out of another length of tubing connected to the 3-way tap, and this tubing is connected to a 20 gauge cannula located in an artery. The cannula is flushed with heparinised saline and the tap turned to connect the cannula to the gauge during measurement. The syringe of heparinised saline is now replaced by a heparinised saline drip. The manometer is located with the meniscus at about heart level during measurement. A slow but continuous flow of the heparinised saline is necessary, otherwise clotting will occur. This can be achieved by pressurising a bag of the heparinised saline (eg by wrapping a BP cuff around the bag and partially inflating it) and adjusting the drip rate. References:
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