Term
| What are the 3 major categories of respiratory failure? |
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Definition
1) Oxygenation failure
2) Ventilation failure
3) Oxygenation-ventilation failure |
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Term
| Why do you see an increased A-a gradient in oxygenation failure? |
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Definition
Can be caused by any lung disease that alters V/Q ratios or diffusion.
1) PaO2 and arterial hemoglobin saturation (SaO2) fall
2) Lung disease does not alter components of alveolar gas equation, so PAO2 does not change.
A-a gradient therefore increases, creating ventilation-perfusion mismatching. |
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Term
| Why is PaCO2 normal in oxygenation failure, but PaO2 drops? |
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Definition
Remember, increased Ve mediated by central chemoreceptors in ventrolateral medulla can increase CO2 excretion.
However, breathing harder will not change the oxygen content of the air that is delivered. High V/Q units cannot compensate for Low V/Q units in terms of PaO2. |
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Term
A patient with kyphoscoliosis presents with a very low Ve.
Would they have an elevated A-a gradient? Why or why not? |
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Definition
Ventilation failure due to NM or chest wall disease causes PRIMARY drop in Ve that is unrelated to the lungs themselves.
Therefore, PaCO2 and PACO2 will drop together, and no increase in the A-a gradient will be observed.
That being said, the patient WILL be hypercapnic, since Ve is depressed [PaCO2 = VCO2/ (Ve-Vd)] |
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Term
| What are the most common causes of oxygenation-ventilation failure? |
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Definition
ARDS, cardiogenic PE, COPD and severe acute asthma are most common, but any oxygenation failure can potentially cause this.
1) You have abnormal V/Q creating dead space and
2) Ve cannot be increased to compensate.
Patient will be hypercapnic and hypoxemic. |
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Term
| What are the 4 essential components of respiratory failure management? |
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Definition
1) Establish airway
2) Maintain oxygenation (supplemental O2 to keep SaO2 >90%)
3) Maintain ventilation to keep safe arterial pH (ventilation possible)
4) Treat underlying cause. |
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Term
| What 3 factors determine the FO2 of gas reaching the patient's lungs during oxygen therapy? |
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Definition
1) FO2 being delivered (Group 1=1, Group 2= something lower)
2) Flow rate of gas
3) Patients spontaneous inspiratory flow rate.
**If spontaneous inspiratory rate exceeds flow rate of delivered gas, patient will be getting too much "room" oxygen (lower the actual FIO2 delivered)** |
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Term
| Why are shunts often refractory to to oxygen therapy? |
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Definition
Large number of totally unventilated (rather than poorly ventilated) alveoli
**In V/Q imbalance, poorly perfused alveoli can still be jacked up in FIO2 is high enough** |
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Term
| What are the main indications for mechanical ventilation? |
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Definition
1) Significant respiratory acidosis resulting from acute increase in PaCO2
2) Impending ventilation failure
3) Arterial hypoxemia that is refractory to supplemental O2 (shunting, for example). |
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Term
| Why is assist-control ventilation (A/C) referred to as a "pressure variable mode"? |
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Definition
It is "volume-set" since physician-selected Vt is delivered during each mechanical breath, with a set number of breaths per minute.
However, since at each point during ventilation, the pressure generated by the ventilator is used to overcome both viscous and elastic recoil forces of the patient's respiratory system, airway pressure will vary for the same set Vt between patients. |
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Term
| Why might you select to use Pressure Support Ventilation (PSV) vs. Assist-control ventilation (ACV)? |
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Definition
1) PSV is "pressure set" and "volume variable" while ACV is "volume set" and "pressure variable"
**Vt in PSV depends on selected pressure, compliance and amount of inspiratory effort
2) PSV allows patient to control Vt and the rate of inspiratory flow, which increases COMFORT
**often used to determine whether patient is in need of ventilator or not**
3) A/C gives patient no control over volume or flow rate, but requires less effort on the part of the patient (better for acute respiratory failure) |
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Term
What 3 important factors determine tidal volume in PSV mechanical ventilation?
What about in A/C ventilation? |
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Definition
1) For PSV, the most important determinant is patient EFFORT. Compliance and set pressure are also important.
2) For A/C, Vt and RR are both SET by the physician. Remember, A/C is "pressure-variable" |
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Term
| What is the utility of using mechanical ventilation for patients in oxygenation failure? |
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Definition
| Closed-system (vs. Nasal canula and aerosol mask), so O2 delivered to alveoli is equal to the FiO2 of the ventilator. |
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Term
| What are the major physiological impacts of Positive End-Expiratory Pressure (PEEP) during mechanical ventilation? |
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Definition
Usually, airway pressure is only positive during inspiration, but it can be set as positive during expiration.
1) Increases increases the volume of gas in the lungs at the end of expiration (increases FRC)
2) Increased alveolar pressure causes an increase in pleural pressure
GOOD: In patients with extensive airspace (alveolar) filling (ARDS), increased FRC prevents collapse alveoli opened during positive-pressure inflation.
BAD: May cause over-distention and rupture of alveoli (Barotrauma) or worsening alveolar edema
Drop in cardiac output. |
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Term
| What are the benefits and drawbacks of applying positive end-expiratory pressure in mechanical ventilation? |
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Definition
GOOD:
1) In patients with extensive airspace (alveolar) filling (ARDS), increased FRC prevents collapse alveoli opened during positive-pressure inflation.
2) May improve PaO2 and SaO2
BAD:
1) Over-distention and rupture of alveoli (Barotrauma) or worsening alveolar edema
2) Increases RA pressure, decreasing SVP:RA pressure gradient. Drop in CO
Tissue O2 delivery depends on SaO2, PaO2 and CO, and in this case, the amount that CO decreases will determine the overall benefit of the treatment. |
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