NONINVASIVE & INVASIVE ARTERIAL PRESSURE MONITORING: NONINVASIVE & INVASIVE ARTERIAL PRESSURE MONITORING DR. RAFIA TABASSUM ARTERIAL BLOOD PRESSURE MONITORING: ARTERIAL BLOOD PRESSURE MONITORING Blood pressure is the vital sign that describes the driving force for perfusion of all tissues and is the major determinant of left ventricular afterload. Accurate, reliable, and timely measurement of arterial blood pressure (ABP) is crucial for the responsible care of critically ill patients and those undergoing surgical procedures. ABP can be measured accurately with invasive and noninvasive methods, but both are subject to artifacts that could lead to inappropriate therapy and patient injury unless correctly identified and interpreted. Non-invasive Blood Pressure (NIBP) Monitoring: Non-invasive Blood Pressure (NIBP) Monitoring Indications The use of any anesthetic, no matter how "trivial," is an absolute indication for arterial blood pressure measurement , taken every 3–5 min : Contraindications Avoid blood pressure cuff in extremities with vascular abnormalities (eg, dialysis shunts) or with intravenous lines. Complications of NIBP monitoring: Complications of NIBP monitoring The cuff, while measuring pressure, is preventing blood flow to the extremity. Patients have been reported to get compartment syndromes and neuropathies from overuse of the cuff. There is a change of 0.7 mm Hg in pressure for each centimeter the cuff is above (lower readings) or below (higher readings) the heart------Improper positioning may lead to inappropriate medical management. The cuff measures oscillations; any disturbance of the cuff or patient’s arm during the reading can introduce error into the results. Techniques: Techniques Palpation Doppler Probe Auscultation Oscillometry Arterial Tonometry Palpation : Palpation Systolic blood pressure can be determined by ; (1) L ocating a palpable peripheral pulse (2) I nflating a blood pressure cuff proximal to the pulse until flow is occluded (3) R eleasing cuff pressure by 2 or 3 mm Hg per heartbeat (4) M easuring the cuff pressure at which pulsations are again palpable. This method tends to underestimate systolic pressure, however, because of the insensitivity of touch and the delay between flow under the cuff and distal pulsations. Palpation does not provide a diastolic or MAP. The equipment required is simple and inexpensive. Doppler Probe : Doppler Probe U seful in obese patients, pediatric patients, and patients in shock The Doppler effect is the shift in the frequency of sound waves when their source moves relative to the observer. For example, the pitch of a train's whistle increases as a train approaches and decreases as it departs. Similarly, the reflection of sound waves off a moving object causes a frequency shift. A Doppler probe transmits an ultrasonic signal that is reflected by underlying tissue. As red blood cells move through an artery, a Doppler frequency shift will be detected by the probe. The difference between transmitted and received frequency causes the characteristic swishing sound, which indicates blood flow. Doppler Probe Cont:: Doppler Probe Cont: Because air reflects ultrasound, a coupling gel (but not corrosive electrode jelly) is applied between the probe and the skin. Positioning the probe directly above an artery is crucial, since the beam must pass through the vessel wall. Interference from probe movement or electrocautery is an annoying distraction. Note that only systolic pressures can be reliably determined with the Doppler technique. Doppler Probe Cont:: Doppler Probe Cont: A variation of Doppler technology uses a P iezoelectric crystal to detect lateral arterial wall movement to the intermittent opening and closing of vessels between systolic and diastolic pressure. This instrument thus detects both systolic and diastolic pressures . Auscultation : Auscultation Inflation of a blood pressure cuff to a pressure between systolic and diastolic pressures will partially collapse an underlying artery, producing turbulent flow and the characteristic Korotkoff sounds. These sounds are audible through a stethoscope placed under—or just distal to—the distal third of the blood pressure cuff. The clinician measures pressure with an aneroid or mercury manometer. Auscultation cont: : Auscultation cont: Occasionally, Korotkoff sounds cannot be heard through part of the range from systolic to diastolic pressure. This auscultatory gap is most common in hypertensive patients and can lead to an inaccurate DBP measurement. Korotkoff sounds are often difficult to auscultate during episodes of hypotension or marked peripheral vasoconstriction. In these situations, the subsonic frequencies associated with the sounds can be detected by a microphone and amplified to indicate systolic and diastolic pressures. Motion artifact and electrocautery interference limit the usefulness of this method. Oscillometry : Oscillometry Arterial pulsations cause oscillations in cuff pressure. These oscillations are small if the cuff is inflated above systolic pressure. When the cuff pressure decreases to systolic pressure, the pulsations are transmitted to the entire cuff and the oscillations markedly increase. Maximal oscillation occurs at the MAP, after which oscillations decrease. Because some oscillations are present above and below arterial blood pressure, a mercury or aneroid manometer provides a gross and unreliable measurement. Oscillometry CONT: : Oscillometry CONT: Automated blood pressure monitors electronically measure the pressures at which the oscillation amplitudes change . A microprocessor derives systolic, mean, and diastolic pressures using an algorithm. Machines that require identical consecutive pulse waves for measurement confirmation may be unreliable during arrhythmias (eg, atrial fibrillation). Oscillometric monitors should not be used on patients on cardiopulmonary bypass. Arterial Tonometry : Arterial Tonometry Arterial tonometry measures beat-to-beat arterial blood pressure by sensing the pressure required to partially flatten a superficial artery that is supported by a bony structure (eg, radial artery). A tonometer consisting of several independent pressure transducers is applied to the skin overlying the artery . The contact stress between the transducer directly over the artery and the skin reflects intraluminal pressure. Continuous pulse recordings produce a tracing very similar to an invasive arterial blood pressure waveform. Limitations to this technology include sensitivity to movement artifact and the need for frequent calibration. Clinical Considerations : Clinical Considerations The accuracy of any method of blood pressure measurement that involves a blood pressure cuff depends on proper cuff size . The cuff's bladder should extend at least halfway around the extremity, and the width of the cuff should be 20–50% greater than the diameter of the extremity . Invasive Arterial Blood Pressure Monitoring (IABP): Invasive Arterial Blood Pressure Monitoring (IABP ) INDICATIONS Continuous, real-time blood pressure monitoring Planned pharmacologic or mechanical cardiovascular manipulation Rapid BP changes are anticipated (major fluid shifts). Failure of indirect monitoring (burns). Intra-aortic balloon counterpulsation . Induced hypotension. Cardiac surgery. Major vascular surgery. INDICATIONS CONT: : INDICATIONS CONT: Repeated blood samplings a. Arterial blood gas (ABG) b. Hematocrit c. Glucose Supplementary diagnostic information from the arterial waveform---- Arterial pulse contour analysis I . Systolic pressure variation II . Pulse pressure variation Patient with end organ disease CONTRAINDICATIONS : CONTRAINDICATIONS No collateral blood flow. Local infection. Vascular insufficiency. Raynaud's phenomenon . COMPLICATIONS: COMPLICATIONS Hematoma/ Hemorrhage Pseudoaneurysm Arteriovenous fistula Infection Thrombosis Vasospasm Air emboli Intra-arterial injection Distal ischemia secondary to thrombosis, proximal emboli, or prolonged shock------Skin necrosis & Loss of digits COMPLICATIONS CONT:: COMPLICATIONS CONT: Peripheral neuropathy and damage to adjacent nerves Misinterpretation of data Cerebral air embolism secondary to retrograde flow with flushing Patients at Increased Risk of Complications 1. Severe atherosclerosis 2. Diabetes 3. Low cardiac output 4. Intense peripheral vasoconstriction: Raynaud’s disease Pseudoaneurysm of the radial artery: Pseudoaneurysm of the radial artery Components and principles of IBP monitoring: Components and principles of IBP monitoring The measuring apparatus Intra-arterial cannula Fluid filled tubing The transducer The monitor Components of an arterial monitoring system: Components of an arterial monitoring system Two 20G arterial cannulae. The lower cannula has a guidewire that can be slid into the artery through the needle to allow smooth placement of the cannula (inset).: Two 20G arterial cannulae . The lower cannula has a guidewire that can be slid into the artery through the needle to allow smooth placement of the cannula (inset). The measuring apparatus: The measuring apparatus The measuring apparatus consists of an arterial cannula (20G in adults and 22G in children) connected to tubing containing a continuous column of saline which conducts the pressure wave to the transducer. The arterial line is also connected to a flushing system consisting of a 500ml bag of saline pressurised to 300 mmHg via a flushing device. The flush system provides a slow but continual flushing of the system at a rate of approximately 4-5ml per hour. A rapid flush can be delivered by manually opening the flush valve. There is also usually a 3-way tap to allow for arterial blood sampling and the ejection of air from the system if necessary. The three-way tap must also be clearly labelled as arterial, to minimise the risk of inadvertent intra-arterial injection of drugs. The Transducer : The Transducer A transducer is any device that converts one form of energy to another – for example, the larynx is a type of physiological transducer (air flow is converted to sound). The output of transducers is usually in the form of electrical energy. Most transducers have frequencies of several hundred Hz (> 200 Hz for disposable transducers). In the case of intra-arterial monitoring the transducer consists of a flexible diaphragm with an electric current applied across it. As pressure is applied to the diaphragm it stretches and its resistance changes, altering the electrical output from the system. The transducers used are differential pressure transducers and so must be calibrated relative to atmospheric pressure before use. PHYSICAL PRINCIPLES: PHYSICAL PRINCIPLES Sine Waves A wave is a disturbance that travels through a medium, transferring energy but not matter. One of the simplest waveforms is the sine wave These may be thought of as the path of a point travelling round a circle at a constant speed and are defined by the function y = sin x. Sine Waves Description : Sine Waves Description Amplitude – their maximal displacement from zero Frequency which is the number of cycles per second (expressed as Hertz or Hz) Wavelength, which is the distance between two points on the wave which have the same value (e.g. two crests or troughs) Phase, which is the displacement of one wave as compared with another – expressed as degrees from 0 to 360 Fourier Analysis : Fourier Analysis The arterial waveform is clearly not a simple sine wave as described above, but it can be broken down into a series of many component sine waves. The arterial pressure wave consists of a fundamental wave (the pulse rate) and a series of harmonic waves. These are smaller waves whose frequencies are multiples of the fundamental frequency (e.g. if the fundamental frequency is 1Hz, you would see harmonic waves with frequencies of 2Hz, 3Hz, 4Hz and so on.). The process of analysing a complex waveform in terms of its constituent sine waves is called Fourier Analysis. Two sine waves of differing frequency, amplitude and phase: Two sine waves of differing frequency, amplitude and phase The sum of the two sine waves above PowerPoint Presentation: In the IABP system, the complex waveform is broken down by a microprocessor into its component sine waves, then reconstructed from the fundamental and eight or more harmonic waves of higher frequency to give an accurate representation of the original waveform. Strain gauge The strain gauge principle: stretching a wire or silicone crystal changes its electrical resistance: Strain gauge T he strain gauge principle: stretching a wire or silicone crystal changes its electrical resistance The arterial pulse pressure is transmitted via the column of fluid in the tubing to a flexible diaphragm, displacing it. This displacement can then be measured in a number of different ways. The commonest method is with a strain gauge. Strain gauges are based on the principle that the electrical resistance of wire or silicone increases with increasing stretch. The flexible diagram is attached to wire or silicone strain gauges and then incorporated into a Wheatstone bridge circuit in such a way that with movement of the diaphragm the gauges are stretched or compressed, altering their resistance. The Wheatstone Bridge : The Wheatstone Bridge The Wheatstone bridge is a circuit designed to measure unknown electrical resistance. Classically, these were arranged with three resistors of known resistance and one of variable resistance (the strain gauge). (R2/R1=R3/Rx) When the ratio of the resistors on the known side of the circuit (R2/R1) equals the ratio on the other side of the circuit (R3/Rx) the bridge is balanced, no current will flow and no potential difference will be measured by the galvanometer (VG). The Wheatstone Bridge Cont: : The Wheatstone Bridge Cont: When the resistance of the strain gauge (Rx) changes due to pressure applied to the attached diaphragm, the two sides of the bridge become unbalanced and a current flows. The resulting potential difference is measured by the galvanometer and is proportional to the magnitude of the pressure applied. The Wheatstone Bridge Cont: : The Wheatstone Bridge Cont: Newer Wheatstone bridge setups use strain gauges in all four positions. The diaphragm is attached in such a way that when pressure is applied to it, gauges on one side of the Wheatstone bridge become compressed, reducing their resistance, whilst the gauges on the other side are stretched, increasing their resistance. The bridge then becomes unbalanced and the potential difference generated is proportional to the pressure applied. This setup of four strain gauges has the advantage that it is four times more sensitive than a single gauge Wheatstone bridge. It also compensates for any temperature change as all of the strain gauges are affected equally. Zeroing: Zeroing For a pressure transducer to read accurately, atmospheric pressure must be discounted from the pressure measurement. This is done by exposing the transducer to atmospheric pressure and calibrating the pressure reading to zero. Note that at this point, the level of the transducer is not important. A transducer should be zeroed several times per day to eliminate any baseline drift. Levelling: Levelling The pressure transducer must be set at the appropriate level in relation to the patient in order to measure blood pressure correctly. This is usually taken to be level with the patient’s heart, at the 4th intercostal space, in the mid- axillary line. Failure to do this results in an error due to hydrostatic pressure (the pressure exerted by a column of fluid – in this case, blood) being measured in addition to blood pressure. This can be significant – every 10cm error in levelling will result in a 7.4 mmHg error in the pressure measured; a transducer too low over reads, a transducer too high under rea ds. Damping: Damping Anything that reduces energy in an oscillating system will reduce the amplitude of the oscillations. This is termed damping. Some degree of damping is required in all systems (critical damping), but if excessive (over damping) or insufficient (under damping) the output will be adversely effected. In an IABP measuring system, most damping is from friction in the fluid pathway. There are however, a number of other factors that will cause over damping including: • Three way taps • Bubbles and clots • Vasospasm • Narrow, long or compliant tubing • Kinks in the cannula or tubing Damping: Damping Anything that reduces energy in an oscillating system will reduce the amplitude of the oscillations. This is termed damping. Some degree of damping is required in all systems (critical damping), but if excessive (over damping) or insufficient (under damping) the output will be adversely effected. In an IABP measuring system, most damping is from friction in the fluid pathway. There are however, a number of other factors that will cause over damping including: • Three way taps • Bubbles and clots • Vasospasm • Narrow, long or compliant tubing • Kinks in the cannula or tubing Damping Cont:: Damping Cont: These may be a major source of error, causing an under-reading of systolic blood pressure (SBP) and over reading of diastolic blood pressure (DBP) although the mean blood pressure is relatively unaffected. Critical damping is defined as the minimal amount of damping required to prevent any overshoot. The damping co-efficient in a critically damped system is 1. The Monitor : The Monitor It is not necessary for the anaesthetist to have an in-depth understanding of the internal workings of the monitor. Modern monitors amplify the input signal; amplification makes the signal stronger. They also filter the ‘noise’ from the signal – unwanted background signal is removed with an electronic filter - and display the arterial waveform in ‘real time’ on a screen. They also give a digital display of systolic, diastolic and mean blood pressure. Artery selection: Artery selection Radial : Preferred because of its superficial location and collateral flow. Must check ipsilateral ulnar artery flow before cannulation- Allen's test . Ulnar : Deeper, more tortuous course. Must check ipsilateral radial artery flow before cannulation. Brachial : Large and easily identifiable, but close to the elbow making catheters more prone to kinking and to thrombosis. Femoral : Prone to pseudo-aneurysm and atheroma formation, incidence of infection and arterial thrombosis. Dorsalis pedis and posterior tibial : Most distorted waveforms. Modified Allen's tests can be performed to document adequate collateral flow around these arteries. Axillary : Nerve damage can result from a hematoma or traumatic cannulation. Air or thrombi can quickly gain access to the cerebral circulation during retrograde flushing of the left axillary artery. Allen’s test. Ask the patient to make a fist, use your thumbs to occlude the patient’s radial and ulnar arteries. Ask the patient to unclench their fist – the palm will remain pale (A), whilst the blood supply is still occluded. When you remove the thumb that is occluding the ulnar artery, the palm will flush red if the ulnar artery is functional (B).: Allen’s test . Ask the patient to make a fist, use your thumbs to occlude the patient’s radial and ulnar arteries. Ask the patient to unclench their fist – the palm will remain pale (A), whilst the blood supply is still occluded. When you remove the thumb that is occluding the ulnar artery, the palm will flush red if the ulnar artery is functional (B). A B Insertion of a radial arterial line: Insertion of a radial arterial line This should be performed as an aseptic technique. The wrist should be cleaned with alcoholic chlorhexidine solution prior to cannulation and in conscious patients the skin should be infiltrated with 1% plain lignocaine. The arm should be abducted in the anatomical position and the wrist should be hyper-extended to aid cannulation. This is most conveniently done by an assistant. If an assistant is not available use tape to secure the patients hand fingers extended over a bag of fluid. A technique for securing the patient’s wrist extended using adhesive tape and a fluid bag.: A technique for securing the patient’s wrist extended using adhesive tape and a fluid bag. Cannulation of the radial artery: Cannulation of the radial artery A: Proper positioning and palpation of the artery are crucial. After skin preparation, local anesthetic is infiltrated with a 25-gauge needle. B: A 20- or 22-gauge catheter is advanced through the skin at a 45° angle. C: Flashback of blood signals entry into the artery, and the catheter-needle assembly is lowered to a 30° angle and advanced 1–2 mm to ensure an intraluminal catheter position. D: The catheter is advanced over the needle, which is withdrawn. E: Proximal pressure with middle and ring fingers prevents blood loss, while the arterial tubing Luer-lock connector is secured to the intraarterial catheter. If location of the artery is difficult an alternative method involves positioning your thumb so that the radial pulse is running directly under the centre of your thumb. Then advance the cannula at 30 degrees under the centre point of your thumb: If location of the artery is difficult an alternative method involves positioning your thumb so that the radial pulse is running directly under the centre of your thumb. Then advance the cannula at 30 degrees under the centre point of your thumb Pathologic arterial waveforms: Pathologic arterial waveforms A. Pulsus alternans —alternating higher and lower systolic peaks, commonly associated with large pericardial effusion or severely depressed ventricular function--- Systolic left ventricular failure . B. Pulsus bisferiens —characterized by a double systolic peak, low diastolic pressure, and a wide pulse pressure, usually indicating severe aortic regurgitation. C. Pulsus tardus —slurred upstroke with a delayed systolic peak, characteristic of severe aortic stenosis . PowerPoint Presentation: D. Spike-and-dome configuration characteristic of hypertrophic obstructive cardiomyopathy , with a normal systolic upstroke but wide, delayed dicrotic notch and prolonged ejection phase. E. Pulsus paradoxus —cycles of increasing and decreasing systolic blood pressure correlating to respiratory cycle ( arrows point to spontaneous breaths). In this example, the degree of variation is approximately 35–40 mm Hg.