Pulmonary Disorders I

Think of carbon dioxide (CO2) as an acid. The rule: every time the CO2 rises by 10, the pH falls by 0.08. Normal CO2 is 40, and this patient's CO2 is 60, which is 20 points too high. If the high CO2 were the ONLY problem, the pH would be 7.24. But the actual pH is 7.18, which is even lower. That extra drop in pH means another acid problem is happening at the same time: a metabolic acidosis on top of the respiratory acidosis (Barkley, 2021, p. 369). The bicarbonate (HCO3) is 22, which is normal (normal = 22 to 26). If the body were fixing the problem on its own, the HCO3 would have climbed. Because it has not, this acidosis is uncompensated (Barkley, 2021, p. 369). Choice a is wrong because it ignores that the pH is lower than what the CO2 alone predicts (Barkley, 2021, p. 369). Choice c is wrong because the pH is acidic, not alkaline, and the CO2 is high, not low (Barkley, 2021, p. 369). Choice d is wrong because long-term compensation requires a high HCO3, and this patient's HCO3 is normal (Barkley, 2021, p. 369).

The finger probe (pulse oximeter) cannot tell the difference between hemoglobin carrying oxygen and hemoglobin carrying carbon monoxide. In carbon monoxide poisoning the probe counts both the same way, so it shows a nice normal number like 98% even though the patient is actually being starved of oxygen. That is why this patient can have a normal reading while being poisoned (Barkley, 2021, p. 371). When carbon monoxide poisoning is suspected, an ABG is the more trustworthy test of true oxygenation (Barkley, 2021, p. 371). Choice a is wrong because methylene blue dye and methemoglobinemia do the opposite: they make the reading falsely LOW, and this reading is falsely high (Barkley, 2021, p. 371). Choice b is wrong because alkalosis and hypothermia make the PaO2 come out lower than the saturation predicts, and neither explains a falsely high reading after breathing smoke (Barkley, 2021, p. 371). Choice d is wrong because poor circulation to the fingers matters in shock, but this patient is not described as being in shock (Barkley, 2021, p. 371).

Fluid in the lungs comes from one of two places. If the heart is weak and backing up fluid into the lungs, the pressure filling the left heart (the PCWP) is high. If the lungs themselves are leaky and inflamed, as in sepsis or ARDS, the pressure in the lung circulation (the PAP) is high, but the heart pressure stays normal (Barkley, 2021, p. 373). So noncardiogenic (pulmogenic) edema = a normal PCWP with a high PAP, which is choice b. Cardiogenic edema = a high PCWP with a normal PAP (Barkley, 2021, p. 373). Choice a is wrong because a high PCWP with a normal PAP describes cardiogenic edema, the heart problem, not the lung problem (Barkley, 2021, p. 373). Choice c is wrong because any high PCWP points to the left heart, so it supports cardiogenic edema no matter what the PAP is (Barkley, 2021, p. 374). Choice d is wrong because the CVP reflects right heart filling and volume status, while the PCWP reflects left heart filling. Low numbers here mean the patient is under-filled, not that the left heart is failing (Barkley, 2021, p. 374).

This patient's CO2 is too high (58), which is making his blood acidic. The only way to remove CO2 is to breathe more. So the fix is to raise the ventilator rate, and if needed, give bigger breaths. While raising the rate, watch for auto-PEEP, which means air stacking up in the lungs between breaths (Barkley, 2021, p. 365). Choice a is wrong because breathing LESS removes less CO2 and makes the problem worse (Barkley, 2021, p. 365). Choice c is wrong because PEEP is for oxygen, not CO2. PEEP holds the air sacs open to improve the oxygen level, and cranking it up can injure the lungs or lower the blood pressure (Barkley, 2021, pp. 366, 508). Choice d is wrong because the FIO2 setting is about oxygen, not CO2, and lowering it would drop this patient's oxygen level (Barkley, 2021, p. 366).

A sudden spike in airway pressure plus a sudden drop in oxygen with breath sounds lost on one side means the lung has likely collapsed (pneumothorax). The right first move is an immediate chest X-ray, and be ready to place a chest tube. Patients with ARDS on the ventilator are at high risk for this (Barkley, 2021, p. 430). Choice a is wrong because mucus in the tube can set off the alarm, but it does not cause a sudden loss of breath sounds on one side (Barkley, 2021, p. 508). Choice b is wrong because sedation helps when the patient is fighting the machine, which is not what is happening here (Barkley, 2021, p. 508). Choice d is wrong because if the tube had slipped into the right main bronchus, the LEFT lung would go quiet, not the right (Barkley, 2021, p. 366).

This patient has BOTH problems at once: carbon dioxide is too high (68) AND oxygen is too low (58). He is awake and breathing on his own. The best first step for a COPD flare like this is BiPAP, a mask that delivers two levels of pressure and helps push the CO2 out. BiPAP is the choice when CO2 is high and oxygen is low together (Barkley, 2021, p. 468). Choice b is wrong because CPAP is the choice when only oxygen is low and the CO2 is normal, like fluid in the lungs from heart failure (Barkley, 2021, p. 468). Choice c is wrong because high-flow nasal oxygen gives extra oxygen, but it does not actively help blow off the CO2 the way BiPAP does (Barkley, 2021, p. 468). Choice d is wrong because a breathing tube is saved for patients who are too sleepy to protect their airway or whose breathing is failing. This patient is alert and cooperative (Barkley, 2021, p. 468).

Her symptoms (trouble more than 2 days per week but not every day, waking at night 3 to 4 times a month, rescue inhaler several days a week) fit step 2, mild persistent asthma. At this step, the recommended daily medicine is a low-dose inhaled corticosteroid, which calms the inflammation in the airways (Barkley, 2021, p. 414). Choice a is wrong because step 1 (intermittent) means symptoms on 2 days per week or fewer, waking at night 2 times per month or fewer, and rescue inhaler use on 2 days per week or fewer. Only that mild a pattern is treated with a rescue inhaler alone (Barkley, 2021, p. 413). Choice c is wrong because montelukast is a backup option for step 2, but it is weaker than the inhaled steroid (Barkley, 2021, p. 414). Choice d is wrong because a medium-dose inhaled steroid plus a LABA is the step 4 treatment for severe persistent asthma, which is much worse than this patient's pattern (Barkley, 2021, p. 415).

An FEV1/FVC below 70% means the airways are blocked (obstruction), and an FEV1 of 55% of predicted is GOLD stage 2, moderate COPD (Barkley, 2021, pp. 399, 404). Now figure out the group. He had 2 flare-ups and 1 hospital stay in the past year, so he is HIGH RISK. His CAT score (12) and mMRC grade (2) mean he has a lot of symptoms. High risk + many symptoms = Group D. Group D treatment is a LAMA, or a LAMA plus a LABA, or an inhaled steroid plus a LABA (Barkley, 2021, pp. 404, 408). Choice a is wrong because Group A patients have 0 to 1 flare-up per year, no hospital stays, and few symptoms, and they only need a quick-relief inhaler (Barkley, 2021, pp. 404, 408). Choice b is wrong because Group B patients have many symptoms but are still low risk (0 to 1 flare-up per year), and they are treated with a LABA or a LAMA (Barkley, 2021, pp. 404, 408). Choice d is wrong because Group C patients are high risk but have few symptoms (CAT under 10 or mMRC 0 to 1), which does not match this patient (Barkley, 2021, pp. 404, 408).

Her lungs are so sick that even on 80% oxygen her PaO2 is only 60. Divide 60 by 0.80 and you get 75, and any ratio below 200 means ARDS. For ARDS the ventilator strategy is to protect the lungs: small breaths (about 6 ml/kg) so the stiff lungs are not over-stretched, plus PEEP to keep the air sacs open, and close watching of the airway pressures (Barkley, 2021, pp. 428-429). Choice a is wrong because a 12 ml/kg breath is roughly double the safe size. Adult breaths are set at 6 to 8 ml/kg of ideal body weight, with a goal of keeping the plateau pressure at or below 30 cm H2O. Big breaths can injure the lungs (Barkley, 2021, p. 507). Choice b is wrong because oxygen above 50% for more than 24 hours can damage the lungs, which is called oxygen toxicity (Barkley, 2021, p. 507). Choice c is wrong because CPAP is one constant pressure with no machine breaths, and it requires a patient who can breathe on their own. This patient is intubated because she cannot (Barkley, 2021, p. 508).

The key number is FEV1/FVC = 0.60. Anything below 70% means the patient has trouble getting air OUT of the lungs: that is obstruction. On top of that, his TLC (125%) and RV (150%) are both above 120% of normal, which means air is trapped and the lungs are over-inflated (air trapping and hyperinflation) (Barkley, 2021, pp. 390, 393). Choice b is wrong because restrictive disease is the opposite: small, stiff lungs with low volumes and a low TLC, while flow out is normal or even fast (Barkley, 2021, pp. 390, 393). Choice c is wrong because the FVC alone is not what decides this. The ratio is below 70%, and an FEV1 of 48% of predicted is severely reduced (Barkley, 2021, p. 393). Choice d is wrong because muscle weakness is found by testing breathing muscle strength (maximal inspiratory and expiratory pressures), not by this pattern of trapped air (Barkley, 2021, p. 391).