Monitoring II: Oxygenation and Ventilation

What is the goal of oxygen monitoring?

Ensure adequate oxygen delivery

When does hypoxemia begin?

SaO2 < 90% or PaO2 < 60mmHg

What are the consequences of hypoxemia?

End-organ hypoxia, dysfunction, cell death, organ failure

What is SaO2?

Saturation of arterial hemoglobin with O2

What is P50?

Partial pressure where SaO2 is 50% (typically 26 or 27 mmHg)

How do you find the delivery of oxygen to tissues (DO2)?

Cardiac output (L/min) x CaO2

97% of oxygen is bound to hemoglobin, yet that value is determined by another factor which is what?

Gas oxygen that is present

At what point in partial pressure of oxygen is there a very little increase in SaO2?

80mmHg

SaO2 is a value that is difficult to directly calculate, how do we get a value for it during surgery?

SpO2 (pulse oximetry) is used to calculate SaO2

What do pulse oximeters detect?

Estimate SaO2 by detecting % hemoglobin in either oxyhemoglobin or deoxyhemoglobin state

What can SpO2 tell you?

Percent Hb saturated with O2

Existence of peripheral perfusion

Pulse rate; rhythm

Information regarding Resp and CV systems

What is required for pulse oximeters to be accurate?

Must detect pulsatility

What does deoxyhemoglobin absorb more of?

Red light cause deoxygenated blood to be less red

What does oxyhemoglobin absorb?

More IR light causing oxygenated blood to be brighter red

What should SpO2 be in a healthy patient breathing room air?

96-99%

What should SpO2 be in an anesthetized patient?

>95%

If SpO2 is <90% what are causes of hypoxemia?

Hypoventilation, low inspired oxygen concentration, V/Q mismatch, R-L shunt, diffusion impairment

What can cause a falsely low SpO2?

Any movement

When do we use pulse oximetry?

Unsure of oxygenation status (check before GA)

Every moderate-heavily sedate patient

Every anesthetized patient

Every patient recovering from general anesthesia

Why would you need to move the lingual clip during anesthesia?

The clip can occlude blood flow

What is ventilation?

Mechanical process that causes the movement of gas between atmosphere and respo system conducting airways and alveoli via respiratory muscles

How do we maintain normal levels of blood oxygen, CO2, and pH?

Ventilation

What does hyperventilation do to CO2?

Reduces it

What does hypoventilation do to CO2?

Increases it

What is Ve?

Ventilation = Tidal volume x RR

What controls ventilation?

Respiratory center in medulla controlling RR, ventilatory rhythm, breath size

What influences ventilation?

Acid-base status and partial pressure of O2 and CO2

What has the most impact on ventilation?

CO2 levels

What are sources of CO2?

Aerobic respiration and anaerobic respiration

What happens to our ability to breath while anesthetized?

We lose the ability to sense CO2 levels

What does the partial pressure of CO2 tell you?

Effective ventilation both how well CO2 is being eliminated (mechanical) and the ventilatory drive from brainstem (stimulatory)

What are the major effects of anesthetics on ventilation?

Altered static lung volumes

Respiratory depression

What does muscle relaxation due to anesthesia cause?

Decreased primary and accessory respiratory muscles function and decreased tidal volume

What does a smaller functional residual capacity (FRC) mean?

Decreases gas exchanges efficiency

Decrease time to desaturation

What is the normal PaCO2?

37mmHg

Describe what is happening here

When people have anesthetics on board, the brain needs a higher PaCO2 in order to stimulate the body to breath at the same rate when awake. This is additive between a premedication like an opioid and a volatile anesthetic like isoflurane

T/F every patient will hypoventilate?

True

What are the results of respiratory depressents?

Decreased sensitivity of central chemoreceptors to CO2

Results in ventilatory response relationship shifting right and down

Reduces RR, Vt, or both

Increases PaCO2, and PvCO2

What are some respiratory depressants?

Opioids, ketamine, propofol, alfaxalone, volatile anesthetics, benzodiazepines (except acepromazine in small animals)

What benzodiazepine does not cause a respiratory depressant effect in small animal only?

Acepromazine

For every 10 increase in PACO2, what happens to pH?

Drops by 0.7

What is capnography?

Measures CO2 from expired breaths

What does capnography directly measure?

End-Tidal gas sampled at end of expiration which reflects PCO2

What happens to End-Tidal CO2 if the heart arrests?

It decreases

What is the benefit of capnogrpahy?

Gives you information on ventilation status of the patient

Assess cardiovascular system

Assess patency of airway

When is ETCO2 highest during the waveform of ETCO2?

Phase 3 during the alveolar plateau

What are the two types of capnographs?

Mainstream and side stream

What does a mainstream capnogrpah mean?

Sensor is present at the level of the Y piece

What is the downside of mainstream capnographs?

It is large and bulky

What is the most accurate capnograph type?

Mainstream

What is a tolerated EtCO2 in healthy patients?

55-60 mmHg

When is it dangerous for EtCO2 to be 55-60?

If they already are acidotic or there brain mass messing something else up

What do you do if the patient is hypoventilating?

Turn down inhalant or give IPPV (intermittent positive pressure ventilation)

What is required for IPPV?

Requires endotracheal intubation w/ inflated cuff

What are indications for IPPV?

Hypoventilation / apnea

Management of anesthetic depth

Disrupted thoracic wall/diaphragm, loss of pleural pressure

Neuromuscular blocking agents

How do you do an IPPV?

Close pop-off valve

Squeeze re-breathing bag over 1-1.5 seconds

Release pop-off valve

Repeat as needed to maintain appropriate EtCO2

T/F giving an extra breath once a minute does not help?

True

What is peak inspiratory pressure (PIP)?

Maximum pressure achieved within anesthetic circuit while administering a positive pressure breath. This estimates the pressure in thoracic cavity

What value is very important to monitor when giving an IPPV?

PIP (peak inspiratory pressure)

What happens when you give IPPV?

You introduce positive pressure to the thoracic cavity. This causes external compression of low-pressure venous vessels causing reduced venous return (preload) during inspiration reducing CO an and BP

What are the types of pulse ox sensors?

Reflectance and transmittance

How does the transmittance work?

Red light and infrared light shine through the tissue and are read on the other side. Wavelength absorption of each estimates the SpO2

How do reflectance probes work?

Shine red light and infrared light into tissue and it reflects back on the SAM side. Wavelength absorption estimates the SpO2

What happens during systole?

Peripheral blood levels increase causing more oxygenated blood in the system

What happens to the venous blood during a cardiac cycle?

Stays relatively constant

What are the most common cause of pulse ox inaccuracies?

Clip induced tissue compression

Vasoconstriction/poor peripheral perfusion

Movement

Skin pigment

What causes vasoconstriction/poor peripheral perfusion leading to a false pulse ox?

Hypothermia, iatrogenic drugs like dex or vasopressors

What is functional residual capacity?

Volume of gas left over after a normal tidal volume expiration

What are the systemic effects of hypoventilation?

Acid-base imbalance (acidosis)

Cardiovascular dilation and negative inotropy

Higher CNS depression and increased cerebral blood flow

Respiratory system

What are the acid-base effects of hypoventilation?

Respiratory acidosis which leads to increased K levels

What are the cardiovascular effects of hypoventilation?

Peripheral vasodilation and negative inotropy

Sympathetic nervous system stimulation

Increase catecholamines (arrythmia risk)

What are the CNS effects of hypoventiliation?

Depression at higher levels

Increased cerebral blood flow

Cerebral edema if prolonged elevation

What are the respiratory effects of hypoventilation?

Pulmonary vascular constriction

What is really bad for patients with a brain tumor?

Hypoventilation because cerebral blood flow and pressure increases

What is CO2 levels during phase 1?

None

What are CO2 levels during phase 2?

Beginning of expiration so increases

What does peak CO2 at the end of phase III correlate with?

PaCO2

Bronchospasm and rebreathing/air trapping (shark fin)

• Dead space has not emptied before next inspiration

• Increasing level of baseline PeCO2 due to air trapping

Emphysema

• Arterial CO2 represented by early peak, not end tidal, due to hypercompliance and poor gas exchange

• Also seen with a pneumothorax with air leak

Sudden loss of waveform

• Critical event needing emergency intervention

• ET tube disconnected, dislodged, kinked, or obstructed

Mechanical airway obstruction

• Fixed mechanical obstruction affecting both inspiration and expiration

• alpha and beta angle are both above 90 degrees

Cardiogenic oscillation

• Pulsation from heart to parenchyma causing small volume changes

• Sign of cardiomegaly

Downtrending ETCO2

• Decrease waveform due to

  • Shock/low CO 

  • Pulmonary embolism

  • Hyperventilation

What is the VT for IPPV in dogs and cats?

8-12

What is the RR for dogs on IPPV?

8-20

What is the RR for cats with IPPV?

10-20

What is the inspiratory time for dogs and cats for IPPV?

1-1.5 seconds

What is the peak inspiratory pressure for dogs on IPPV?

8-12

What is the peak inspiratory pressure for cats on IPPV?

5-8

What is the EtCO2 for dogs and cats on IPPV?

45-55

What should the peak inspiratory pressure be for puppies and kittens?

5-8

What should peak inspiratory   pressure be for adult horses?

20-30