Patient MonitoringPatient Monitoring

Stuart Nurre, MS, R.R.T.

OxygenationOxygenation

Goal of respiratory therapy is return the patient to a normal oxygenation status, while minimizing risks.

Normal oxygen saturations are > 90% Normal PaO2 is 80-100 mmHg.

Factors that interfere with oxygenation are many. Hypoxia is when delivery falls below the level of

cellular needs.

Causes of hypoxia– Hypoxemia

PIO2

Hypoventilation Low V/Q Shunt Diffusion defect

– Hemoglobin Deficiency– Low Blood Flow– Dysoxia

Oxygen Content

[Hgb x SaO2 x 1.34] +

[PaO2 x 0.003]

Normal value is 20 vol%Oxygen Delivery

CaO2 x Cardiac output

(C.O. = SV x HR)

Ventilation/PerfusionVentilation/Perfusion

The lung and perfusion system are not perfect. Due to gravity and lung structure the two systems do not meet equally

The apex of the lung receives more ventilation then does the base.

The apex of the lung receives less perfusion than the base.

These differences occur in healthy lungs. This difference is more pronounced in the

diseased lung.

Ventilation/Perfusion is , there is usually a CO2 and a O2.

Ventilation/Perfusion is , there is usually a CO2 and a O2 .

Areas of V/Q are referred to as DEADSPACE.

Areas of V/Q are referred to as LOW or SHUNT.

OximetryOximetry

The machine is an oximeter and the test performed is an oximetry.

Oximetry is used to asses the oxygenation of a patient. It can be invasive or non-invasive.

Visual recognition of hypoxia is difficult.– Cyanosis, tachycardia, tachypnea, SOB,

WOB, altered LOC.

The theory of oximetry is based on two principles, spectrophotometry and photoplethysmography.

Spectrophotometry uses the Beer-Lambert law. Which states that the concentration of a species is directly related to the amount of light it absorbs. Oxyhemoglobin (O2Hb) absorbs light at a different wavelength that deoxyhemoglobin (HHb).– O2Hb > 940 nm– HHB > 640 nm

Photoplethysmography measures the change in blood volume by the change in the amount of light absorbed.

Veins and tissues have a constant amount of light absorbed. But as systole occurs, more blood arrives and more light is absorbed. This allows the machine to subtract the constant amount from the dynamic amount.

All of this information is calculated in the computer and a SaO2 is displayed.

Oxyhemoglobin Dissociation Curve

Left shift

Decreases

1. H+

2. Temperature

3. 2,3 - DPG

Right Shift

Increases

1.H+

2. Temperature

3. 2,3 - DPG

Physiologic and technical Considerations of oximetry.– NOT VENTILATION– Low flow states– Dyshemoglobinemia– Dyes– Nail polish/acrylic nails– Skin pigmentation– Ambient light– SaO2 levels

Oximetry vs CO-Oximetry

CapnographyCapnography

Capnometry is a number produced to estimate the arterial PaCO2.

Capnography is a graphical representation of the level of PaCO2.

Most capnograms work by using infra-red spectroscopy. The amount of light absorbed is related to the amount of CO2 present.

There are two main types of capnograms used in most ICUs.– Mainstream – Sidestream

The number displayed is referred to as the ETCO2. (PETCO2) This number should be 4-6 mmHg below the PaCO2. This is referred to the the gradient.

When the gradient increases it is due to increased in dead space ventilation.

Deadspace ventilation is caused by– Lung disease– Pulmonary embolic events– Changes in cardiac output

The uses of capnography are:– Alveolar ventilation measurements

A potentially misleading monitor is worse than no monitor at all.

– Deadspace ventilation trending– Cardiopulmonary resuscitation.