Table 1: Comparison of non-invasive and invasive BP monitoring [20,21].
| Non-Invasive (Oscillometric) | Invasive (Arterial Line) | |
| Indications | • American Society of Anesthesiology minimal standard: every 5 min while under general anesthesia |
• continuous monitoring • unstable patients • strict BP control • controlled hypotension • anticipated volatility in BP • obtaining frequent labs • suspected NIBP inaccuracy |
| Mechanism | • cuff inflated above systolic BP and incrementally deflated while amplitudes of cuff pressure oscillations measured by pressure transducer • systolic BP obtained when amplitude of oscillations increases by 25-50% of maximum • MAP is point of maximum oscillations • diastolic BP obtained when amplitude of oscillations decrease by 80% or disappear • time versus pressure graph constructed whereby systolic and diastolic BPs are mathematically-derived |
• fluctuations of vascular pressure cause pulsation of saline column • displacement of electromanometer diaphragm • mechanical to electrical transduction • Fourier analysis for waveform construction |
| Factors that may affect accuracy | • patient movement/external pressure • obesity • poor perfusion • poor pulsatility (e.g. mechanical cardiac support) • extremes of BP • bradycardia or irregular pulse • improperly selected cuff size • arterial stiffness • regional vascular disease |
• excessive tubing length, compliance, or caliber • improper zeroing • wrong transducer leveling (transducer should be at the level of the heart -aiming 5 cm behind the sternum in a supine patient) • over- or under-dampening of the pressure tracing • arterial spasm or thrombosis • extremes of BP |
| Complications | • delayed therapeutic intervention • soft tissue injury • phlebitis • neuropathy • compartment syndrome • intravenous line infiltration/occlusion • interference with pulse oximetry |
• thrombosis • embolism • infection • neuropathy from hematoma/compression • vascular injury • limb loss |
| Troubleshooting | • ensure proper cuff size/circumference • ensure regular pulse • consider arterial stiffness • inspect for device malfunction (air leak, tubing kink, external pressure) • reassess at different extremities |
• assess waveform quality • re-zero/calibrate • level transducer with heart • remove catheter/tubing kinks • ensure pressure bag full • consider arterial stenosis/spasm • remove air bubbles/clot • correct hypothermia |