Fluid System + Oxygenation Monitoring

Exam 2

Pulmonary Circulation

Transports systemic venous blood (mixed venous) into contact with the alveoli for gas exchange

• Low-pressure, low-resistance system

  • Pulmonary arteries are thin-walled, have less smooth muscle, and have greater compliance

• Blood reservoir – 40% of lung weight is from the blood volume housed in pulmonary vessels, 10% circulating volume

  • Blood volume and influence PVR → if really full, increased PVR due to pressure on vessel walls

Hypoxic pulmonary vasoconstriction

• blood vessel constrict to divert blood flow to areas with more oxygen

Path of Oxygen

1. In lungs: Atp ~PO2 = 160,  alveoli PO2 = 100

• Gas is exchanged with the atmosphere – ventilation

• Gas in pulmonary arterioles is exchanged with gases in blood of the pulmonary capillaries – external respiration

2. In the blood  - transport: PO2 of arterial blood ~100 mmHg

• Most O2 is bound/carried by O2; SaO2 97-99%

• Small amount of O2 is dissolved in plasma; PaO2

• CaO2 = totals O2 content of arterial blood (heme + plasma); 17-20 mL

3. Oxygen Delivery (900-1100 mL/min)– internal respiration: PaO2 = 100mmHg, Tissue = 10-20 mmHg

• Oxygen in blood moves down its concentration gradient as it oxygenates tissues à then moves into mitochondria (2-3 mmHg)

Oxyhemoglobin disassociation curve

Describes the relationship between hemoglobin saturation and partial pressure of oxygen

• Deoxyhemoglobin = oxygen unbound to hemoglobin

• Oxyhemoglobin = oxygen bound to hemoglobin

P50 = partial pressure of oxygen when hemoglobin saturation is 50 %

Curve shifts

• Temperature: increased temp – shift right, increase P50 – less affinity

• pH: decreased pH – shift right, increase P50 – less affinity

• PCO2: increased PCo2 – shift right, increase P50 – less affinity

• 2,3-DPG – increased – shift right, increase P50 – less affinity

Bohr Effect

Describes how increased CO2 and H+ ions decreases hemoglobin’s affinity for oxygen (decreased pH)

• CO2 enters blood cell and is converted to carbonic acid

• H+ binds to hemoglobin and causes a conformational change that weakens affinity to O2

Oxygen consumption is determined by?

1. Oxygen demand – amount of O2 tissues require for metabolism
   - Affected by: metabolic rate, temperature, activity/shivering, stress, disease, medication

2. Oxygen consumption (VO2) – amount of O2 actually used by the body per minute

3. Norm: 200-250 mL O2

Supply-demand oxygen consumption mismatch outcome + compensatory mechanisms

If there is a greater demand than supply, the body will use anaerobic metabolism and produce lactic acid

• Compensatory mechanisms for reduced supply/delivery

  • Acute: increase CO, increase Tissue O2, redistribution of blood flow to vital organs

  • Chronic: increase RBC/hemoglobin (hypoxemia or high altitude)

O2 extraction ratio

O2ER = percentage of delivered oxygen that is actively consumed by the tissues

• Describes relationship between DO2- amount of oxygen supplied – and VO2 – amount of oxygen consumed

Pulse Oximetry

• Uses two light-emitting diodes (LEDs) to send red light and infrared light through a pulsating arterial vascular bed

• Photodetector measures the light absorbed/transmitted through the tissue

  • Saturated Hgb absorbs more infrared light→ 940 nm wavelength

  • Desaturated Hgb absorbs more red light → 660 nm wavelength

• More oxygenated blood = more red light transmitted, more infrared light absorbed

• Less oxygenated blood = more red light absorbed, more infrared light transmitted

Beer Lamber Law

• Describes light absorption – loss of light intensity/ light attenuation

  • Ideally, the only cause would be due to absorption of light by molecules

• Attenuation is proportional to

  • Concentration of absorbing substance

  • Distance the light travels

  • Absorption characteristics of the substance → Oxyhemoglobin and deoxyhemoglobin have different optical properties

Functional Hgb

participated normally in oxygen transport; oxy- and deoxyhemoglobin

Carboxyhemoglobin (COHb)

nonfunctional Hgb

1. Bound to carbon monoxide – has higher affinity compared to O2 so it holds on longer

2. May not be diagnosed by standard pulse oximetry

3. Systemic PO2 levels may not be affected

Methemoglobin (MetHb)

nonfunctional Hgb

1. Hemoglobin containing Fe^3+ - Ferric iron

2. Can occur with exposure to nitrates/nitrates

3. Standard system PO2 may not reliable identify

PaO2 and SaO2

Normal PaO2: 80-100 mmHg

Normal SaO2: 97-100%

Abnormal = 60-80 mmHg + 90% saturation

• below 60 mmHg and 90% saturation the drop off of oxygenation becomes significantly more severe (40 mmHg - 75%)

Identify errors in pulse oximetry that may provide a false or poor signal.

Poor signal

• Low perfusion/vasoconstriction

• Hypotension

• Motion

• Improper sensor placement

• BP cuff cycling on the same extremity

Potential interference/inaccurate readings

• Dyshemoglobinemias

• Intravascular dyes

• Strong ambient light

• Nail products

• Severe anemia

• Skin pigmentation may contribute to measurement bias

Swan-Ganz catheter to measure mixed venous oxygenation saturation

• Using a fiber-optic Swan-Ganz catheter, incorporated into an PAC → measures the optical properties of hemoglobin and continuously estimates oxygen saturation

• NORMAL: 60-80%

  • Low SvO2: decreased CO, Hgb, SaO2, increased VO2

  • High SvO2: Increased DO2 and decreased VO2

    • Impaired tissue extraction/shunting

    • Affinity increased (holds on to O2)

    • Not always a good thing

Hemodynamic monitoring

• Describes the relationship between pressure, flow and resistance

• Assess cardiac function, vascular function, perfusion, and response to therapy

Pascal’s Principle

• Pressure applied to a enclosed fluid is transmitted undiminished to every portion of the fluid and the vessel walls of the containing vessel

• Monitoring: patient → catheter → fluid → column → transducer

• Incompressible fluid, rigid tubing, and no air

Poiseuille’s law

• Flow through a cylindrical tube is

  • directly proportional to the pressure difference and the fourth power of the radius

  • inversely proportional to the fluid’s viscosity and the tube’s length

• Flow is dependent on radius, length, and viscosity/resistance of fluid

Cross Sectional Areas of vessel

1. Capillaries – 2500 cm²

2. Venules – 250 cm²

3. Small veins - 80 cm²

4. Arterioles 40 cm²

5. Small Arteries – 20 cm²

6. Venae cava – 8 cm²

7. Aorta – 2.5 cm²

Capillaries have the largest cross-sectional area due to have billions in the body

Cross-sectional Area and velocity

• The same volume of blood must pass through all vessels

• Velocity is inversely proportional to vascular cross-sectional area

  • velocity/speed increases as the cross-sectional area decreases

• Capillaries have the slowest velocity due to the largest cross-sectional area → more time for nutrient exchange

Flow vs velocity

Flow = how much blood is being moved

Velocity = speed/how fast the blood is traveling

Reynold’s Number

a.     Compares inertial forces with viscous forces to predict flow patterns

• Inertia = fluid's desire to keep moving

• Viscosity = fluids resistance/friction

b.     Predicts whether blood flow will be smooth and orderly (Laminar)  or chaotic and swirling (Turbulent)

• Velocity and diameter are highly influential

  • in aorta: large diameter and high velocity = high Re - may be turbulent

  • In capillaries: small diameter, low velocity = low Re - laminar

Reynold’s Number Equation

Re = (p*v*d)/u

p = density of the blood (mass/volume; inertia)

v = velocity of blood flow (how fast)

d = diameter of the blood vessel (resistance)

u = viscosity of blood (how thick/sticky)

Reynold’s Number predictions

i. If Re < 2000 = Laminar

ii. If Re > 4000 = Turbulent

iii. If 2000 < Re < 4000 = Transition flow

Catheter + noncompliant tubing

• Carries pressure from the patient to the transducer

• Catheter design

  • Biocompatible, smooth, nonthrombogenic, as short as practical, low-compliance walls, small outer diameter

Pressure transducers

• Converts mechanical pressure to an electrical signal when pressure pushes on a pressure-sensitive diaphragm

• Transducer senses movement produced by pressure transmitted through the fluid column

Capacitance transducer

Uses two plates → pressure pushes on the diaphragm causes the plates to push against each other → oscillating voltage → increase in capacitance

Inductance Transducer

Uses a coil → pressure causes diaphragm to move and change position of coil → changes inductance of system

Resistance Type Transducer

• Uses a Strain gauge bonded to the back of the diaphragm

• Pressure pushes on the diaphragm, causing it to bend/move → the strain gauge is stretched or compressed → resistance changes → electrical signal changes

• Wheatstone bridge: changes in resistance are easily converted to a change in voltage

  • Makes small changes easy to detect

Diaphragm moves → strain gauge resistance changes → Wheatstone bridge detects it → voltage changes → pressure displayed

Amplifier

strengthens the electrical signal coming from the transducer

Flush system + pressure bag + stopcocks

• Flush: continuously provides a small amount of fluid through the pressure tubing

  • Prevents clotting; keeps catheter open

• Pressure bag: applies pressure on the flush solution so that it is higher than patient pressure (prevents backflow)

• Stopcocks: controls flow direction; used to connect the transducer to the patient, collect samples, zero the transducer

Monitor/signal processor

• Processes the electrical signal and converts it to pressure measurements

Important functions

1. Zero: establishes atmospheric pressure as 0 mmHg → balancing the Wheatstone bridge

2. Scale: changes how the waveform is displayed

3. Alarms: identify pressure outside selected limits

Scale + Gain of transducer monitor

scale = changes how the waveform is displayed

gain = changes display sensitivity or amplification

Natural Frequency

• Frequency at which any structure prefers to oscillate/vibrate naturally after it has been disturbed

  • Longer and more compliant = lower natural frequency

  • Shorter and stiffer = higher natural frequency

    • Monitoring system should have a much higher natural frequency than the patient’s waveform so that it won’t sink in and cause the waveform to be amplified/exagerated (longer, more compliant takes more time and can cause the waveforms to get distorted)

  
  

What can disturb pressure wave forms

• Catheter diameter and length

  • Longer tubing = diminished pressure

  • High compliance = absorbs changes in pressure

  • Connection/stockcocks = distortion

• Air bubbles

Resonance

• Resonance happens when an outside force is applied at or very close to a system’s natural frequency

• Can amplify the waveform signal

Damping

The loss of energy from an oscillating system

• some is needed

Transducer waveform Damping Type

Overdamped

• low systolic BP and High diastolic BP

• narrow pulse pressure

• Causes: air bubbles, kinks/poor connection, small-lumen catheter, blood backed into the pressure tubing

Damping of Transducer waveform

Underdamped

• High Systolic BP and Low Diastolic BP

• Wide pulse pressure

• Causes: resonance, hyperdynamic signals, tachycardia, and tubing vibration

type of transducer waveform

Optimal Transducer Waveform

Square-Wave/Fast Flush Test

• Evaluates the dynamic response of the transducer monitoring system

• Process

  • Activate the fast-flush device → release → observe waveform response

• diagnose damping from transducer

Square-Wave/Fast flush test

adequate damping

Square-Wave/Fast flush test

Underdamping

Square-Wave/Fast flush test

Overdamping

Patient factors that influence damping

• higher-frequency signals occur with tachycardia or sepsis

• Aortic regurgitation (blood leaks backward into the LV) or hyperdynamic circulation

Zeroing transducer

• calibrate the system to atmospheric pressure to program it as 0 mmHg

• Process: close transducer off to the system, open the Wheatstone bridge to air, press zero on the monitor system

Leveling Transducer

• location of transducer when zeroing + monitoring pressure to prevent hydrostatic error

  • If too low, the transducer diaphragm is pushed on by the weight of the fluid

• Phlebostatic axis: in line with the patient’s heart/RA

2 mmHg for every 1 unit of vertical difference from the phlebostatic axis

Lag time

longer tubing the longer the lag time, because of inertia, fluid-filled column within system should be transmitted simultaneously and with equal force

Oscillations

numbers of cycles per second

Fidelity

• Ability to reproduce the patient’s actual pressure waveform

• How faithfully the system reproduces the patient’s pressure waveform

  • High fidelity = preserves timing, shape, and amplitude

Dynamic response

• How accurately the system follows rapidly changing pressure

• Response must be quick without exaggerating or blunting the waveform

Ringing

repeated oscillations caused by inadequate damping

Catheter whip

motion artifact from catheter movement

Hydrodynamic pressure

Pressure produced by fluid in motion

Hydrostatic Pressure

Pressure exerted by a stationary fluid column