Patient Monitoring Stuart Nurre, MS, R.R.T.

Oxygenation Goal of respiratory therapy is return the patient to a normal oxygenation status, while minimizing risks. Normal oxygen saturations are > 90% Normal PaO 2 is mmHg. Factors that interfere with oxygenation are many. Hypoxia is when delivery falls below the level of cellular needs.

Causes of hypoxia – Hypoxemia  PIO 2 Hypoventilation Low V/Q Shunt Diffusion defect – Hemoglobin Deficiency – Low Blood Flow – Dysoxia

Oxygen Content [Hgb x SaO 2 x 1.34] + [PaO 2 x 0.003] Normal value is 20 vol% Oxygen Delivery CaO 2 x Cardiac output (C.O. = SV x HR)

Ventilation/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  CO 2 and a  O 2. Ventilation/Perfusion is , there is usually a  CO 2 and a  O 2. Areas of  V/Q are referred to as DEADSPACE. Areas of  V/Q are referred to as LOW or SHUNT.

Oximetry 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). – O 2 Hb > 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 SaO 2 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 – SaO 2 levels Oximetry vs CO-Oximetry

Capnography Capnometry is a number produced to estimate the arterial PaCO 2. Capnography is a graphical representation of the level of PaCO 2. Most capnograms work by using infra-red spectroscopy. The amount of light absorbed is related to the amount of CO 2 present.

There are two main types of capnograms used in most ICUs. – Mainstream – Sidestream The number displayed is referred to as the ETCO 2. (P ETCO 2 ) This number should be 4- 6 mmHg below the PaCO 2. 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.