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Mechanical Ventilator/Patient Monitoring Part II RET 2264C Prof. J.M. Newberry Dr. J.B. Elsberry Special Thanks to Sean Chambers , J. Cairo and Jeff Davis Physiological Monitoring • Physiological data are useful in managing patients on ventilatory support in respect to oxygenation, ventilation, ventilatory mechanics (Use Loops), and hemodynamic monitoring… Hemodynamics Physics of Blood flow in the circulation Circulatory System Circulation Schematic Left Side of Heart Pulmonary Vein A Aorta V Aortic Valve Mitral Valve Tissues Lungs Tricuspid Valve Pulmonary Valve V Pulmonary Artery A Right Side of Heart Sup. & Inf. Vena Cava Heart Valves • Atrioventricular (A-V) valves - separate Atria from Ventricles • Bicuspid (Mitral) - Left Side • Tricuspid - Right Side • Semi-Lunar Valves - separate ventricles from Arteries Opening, Closing of Valves - Depends on Pressure differences between blood in adjacent areas Heart Sounds • ‘Lubb’ (1st sound) - Closure of A-V valves • ‘Dupp’ (2nd sound) - Closure of S-L valves Caused by Turbulence on closing. Anything extra ’Murmur’ (swishing of blood) Could be due to: Increases • Stenosis of Valves (calcification) Pressure on • Valves not closing properly heart (Incompetence, Insufficiency) Blood Vessels • Arteries • Capillaries • Veins Systemic Pathway: Left Ventricle of Heart Venules Aorta Arteries Arterioles Capillaries Veins Right Atrium of Heart Blood • Composition: – Approx 45% by Vol. Solid Components » Red Blood Cells (12m x 2 m) » White Cells » Platelets – Approx 55% Liquid (plasma) » 91.5% of which is water » 7% plasma proteins » 1.5% other solutes Blood Functions • Transportation of blood gases, nutrients, wastes • Homeostasis (regulation) of pH, Body Temp, water content • Protection • Viscosity of Blood = 3 3.5 times of water • Blood acts as a non-newtonian fluid in smaller vessels (including capillaries) Cardiac Output • Flow of blood is usually measured in l/min • Total amount of blood flowing through the circulation = Cardiac Output (CO) Cardiac Ouput = Stroke Vol. x Heart Rate or SV1 + SV2 + …+ SVn = C.O. l/min Influenced by Blood Pressure & Resistance Force of blood against vessel wall with water retention with dehydration, hemorrage •Blood viscosity •Vessel Length •Vessel Elasticity •Vasconstriction / Vasodilation Overall • Greater Pressure Greater Blood Differences Flow • Greater Resistance Lesser Blood Flow HEART (PUMP) REGULATION CARDIOVASCULAR SYSTEM VESSELS (DISTRIBUTION SYSTEM) AUTOREGULATION NEURAL HORMONAL RENAL-BODY FLUID CONTROL SYSTEM Blood Pressure Driving force for blood flow is pressure created by ventricular contraction Elastic arterial walls expand and recoil continuous blood flow ARTERIES (LOW COMPLIANCE) HEART DIASTOLE VEINS CAPACITY VESSELS 80 mmHg 120 mmHg SYSTOLE CAPILLARIES Blood pressure is highest in the arteries (Aorta!) and falls continuously . . . Systolic pressure in Aorta: 120 mm Hg Diastolic pressure in Aorta: 80 mm Hg Diastolic pressure in ventricle: ?? mm Hg Ventricular pressure difficult to measure arterial blood pressure assumed to indicate driving pressure for blood flow Arterial pressure is pulsatile useful to have single value for driving pressure: Mean Arterial Pressure MAP = diastolic P + 1/3 pulse pressure Pulse Pressure = systolic pressure - ?? = measure of amplitude of blood pressure wave MAP influenced by • Cardiac output • Peripheral resistance MAP CO x Rarterioles • Blood volume – fairly constant due to homeostatic mechanisms (kidneys!!) BP too low: • Driving force for blood flow unable to overcome gravity O2 supply to brain Symptoms? BP too high: • Weakening of arterial walls - Aneurysm Risk of rupture & hemorrhage Cerebral hemorrhage: ? Rupture of major artery: Principles of Estimated Sphygmomanometry Cuff inflated until brachial artery compressed and blood flow stopped what kind of sound? Slowly release pressure in cuff: turbulent flow Pressure at which . . . . . . sound (= blood flow) first heard: . . . sound disappeared: • Pressure can be stated in terms of column of fluid. mm Hg 50 100 200 300 400 Pressure Units cm H2O PSI 68 136 272 408 544 0.9 1.9 3.8 5.7 7.6 ATM 0.065 0.13 0.26 0.39 0.52 Pressure = Height x Density or P = gh Density of blood = 1.035 that of water If Right Atrial pressure = 1 cm H2O in an open column of blood Pressure in feet = 140 cm H2O Rupture Incompetent venous valves Varicosities Venous Valves Actual Pressure in foot = 4-5 cm H2O Pressures in the Circulation Review • Pressures in the arteries, veins and heart chambers are the result of the pumping action of the heart • The right and left ventricles have similar waveforms but different pressures • The right and left atria also have similar waveforms with pressures that are similar but not identical How do we Continuously Monitor Left and Right Heart Pressures Hemodynamic Function Summarized in these Wiggers Diagrams from Pilbeam-Cairo 3. As blood enters the aorta, the aortic pressure begins to rise to form the systolic pulse 4. As the LV pressure falls in late systole the aortic pressure falls until the LV pressure is below the aortic diastolic press. 2. Pressure rises until the LV pressure exceeds the aortic pressure 5. Then the aortic valve closes and LV pressure falls to LA pressure The blood begins to move from the ventricle to the aorta 1. The LV pressure begins to rise after the QRS wave of the ECG •The first wave of atrial pressure (the A wave) is due to atrial contraction •The second wave of atrial pressure (the V wave) is due to ventricular contraction Normal Pressures • RV and pulmonary systolic pressures are 15-25 mm Hg • Pulmonary diastolic pressure is 5-12 mm Hg • LA pressure is difficult to measure because access to the LA is not direct AS produces a pressure gradient between the aorta and LV i.e. For blood to move rapidly through a narrowed aortic valve orifice, the pressure must be higher in the ventricle • The severity of AS is determined by the pressure drop across the aortic valve or by the aortic valve area • The high velocity of blood flow through the narrowed valve causes turbulence and a characteristic murmur AS can be diagnosed with a stethoscope Pressure Measurement • Accurate pressure measurements are essential to understanding the status of the circulation • In 1733 Steven Hales connected a long glass tube directly to the left femoral artery of a horse and measured the height of a column of blood (8 feet, 3 inches) to determine mean BP • Direct pressure measurements are made frequently in the cardiac catheterization laboratory, the ICU and the OR • A tube is inserted into an artery and connected to an electrical strain gauge that converts pressure into force that is sensed electrically • The output of the transducer is an electrical signal that is amplified and recorded on a strip chart • For correct pressure measurements the cannula and transducer must be free of air, the cannula should be stiff and short Cardiac Output (CO)Measurement • The measurement of blood flow through the circulation is usually done clinically using either the Fick method or Thermodilution • The Fick method states that the cardiac output is equal to the oxygen consumption divided by the arterial-venous oxygen difference CO = Oxygen consumption / A-V O2 MEASUREMENT OF CARDIAC OUTPUT THE FICK METHOD: VO2 = ([O2]a - [O2]v) x Flow Spirometry or nomogram (250 ml/min) VO2 Flow = [O2]a - [O2]v Pulmonary Artery Blood (15 ml%) Arterial Blood (20 ml%) CARDIAC OUTPUT PULMONARY BLOOD FLOW VENOUS RETURN PERIPHERAL BLOOD FLOW . VO2 CARDIAC OUTPUT (Q) = [O ] - [O ] 2 a 2 v = 250 ml/min 20 ml% - 15 ml% = 5 L/min . Q = HR x SV . Q SV = HR . Q CARDIAC INDEX = m2 body surface area 5 L/min 70 beats/min = 0.0714 L or 71.4 ml = = 5 L/min 1.6 m2 = 3.1 L/min/m2 Oxygen Delivery to the Tissues It’s simple but very important . DO2 = CaO2 x Qc • The measurement is done by determining the oxygen consumption using respiratory gas measurements and the O2 content of arterial and mixed venous blood • The mixed venous blood sample is obtained from a PA with a catheter • The arterial sample can be drawn from any artery Indications & Contraindications for Arterial Cannulation Arterial Cannulation • Continuous BP, frequent ABG’s • Complications—hemorrhage, infection, ischemia (embolus, thrombus, arterial spasm) Indications & Contraindications for Venous Cannulation Central Venous Catheter: • Fluid admin., nutritional support, CVP measurements/monitoring • Complications: pneumothorax, embolus & thrombus formation, infection Pulmonary Artery Catheter: • PCWP measurements, Cardiac Output and mixed venous blood gases • Complications: pneumothorax, arrhythmias, embolus & thrombus formation, infection and cardiovascular injury Hemodynamic Pressures that Can be Measured Directly • Heart Rate: 60-90/min • Systemic (Arterial) BP: 90-140/60-90 torr • CVP 2-6 torr • PAP: 15-35/5-15 torr • PCWP: 5-12 torr . • C.O. (Q): 4-8 l/min. - • PvO2 : 40 torr - • PaO2 : 80-100 torr Next PA Monitoring Angles 45° 30° 0° • Supine • O to 60 degrees from horizontal Components of Hemodynamic Pressure Monitoring • The invasive catheter and the high pressure tubing connecting the patient to the transducer • The transducer • The flush system • The bedside monitor IDENTIFY PHLEBOSTATIC AXIS Intersection of the 4th ICS and ½ the anterior- posterior diameter of the chest AACN PAP Measurement Practice Alert McHale DL, Carlson KK. AACN Procedure Manual for Critical Care 4th ed WB Saunders: Philadelphia, Pa 2001 (479) With permission from Elsevier LEVELING – Eliminates effects of hydrostatic forces on the observed hemodynamic pressures – Ensure air-fluid interface of the transducer is leveled before zeroing and/or obtaining pressure readings – Phlebostatic axis: • Level of left atrium • 4th ICS & ½ AP diameter • Mark the chest with washable felt pen AACN PAP Measurement Practice Alert UNDER DAMPED SYSTEM OVER DAMPED SYSTEM – Sluggish, artificially rounded & blunted appearance – SBP erroneously low; DBP erroneously high – Causes: large air bubbles in system, ostial lesion, compliant tubing, loose/open connections, low fluid level in flush bag AACN PAP Measurement Practice Alert Reprinted from Darovic GO. Hemodynamic Monitoring: Invasive and Noninvasive Clinical Application 2nd ed. Philadelphia,Pa: WB Saunders Co;1995;161-162 Used with permission from Elsevier AO Pressure Damped AO Pressure PAP DOCUMENTATION • Measure at end expiration • Measure pressures from a graphic tracing • Measure pulmonary capillary wedge pressure at end-expiration using the mean of the a wave – a wave indicates atrial contraction and falls within the P – QRS interval of the corresponding ECG complex AACN PAP Measurement Practice Alert RESPIRATORY COMPONENT Changes in intrathoracic pressure during respiration change PAP readings AACN PAP Measurement Practice Alert Record and trend pressure readings at end expiration Used with permission of PACEP Collaborative AACN PAP Measurement Practice Alert AACN PAP Measurement Practice Alert Used with permission of PACEP Collaborative Accurate Measurement • • • • Regular zero check Open transducer to room air Hit balance button on monitor Zero in Ft. Myers does not = zero in Denver Effect of air in line Effect of long line Troubleshooting • What can we do to reduce damping thus ensuring the best frequency response? – Short, stiff tubing – No blood, contrast or air in line Correctable Sources of Artifact Review • Loss of frequency response – Air, blood, contrast • Improper zero – Phlebostatic axis • Improper calibration – Hg if valve case