Special Situations Flashcards
Explain obesity hypoventilation syndrome:
In short (oversimplified by quite a bit), obstructive sleep apnea combined with restrictive lung disease (due to obesity) leads to hypoxia and hypercarbia. The hypoxia leads to hypoxic pulmonary vasoconstriction and elevated pulmonary artery pressures. The hypercarbia also increases pulmonary artery pressures. This manifests as right heart strain, dilatation, and hypertrophy (cor pulmonale). Since the left heart is “unaffected” (actually the dilated right ventricular septum bows into the LV cavity), hepatomegaly and peripheral oedema are seen, not pulmonary oedema (classically) (answer A). Chronic hypoxia leads to polycythaemia (answer E). Chronic hypercarbia leads to a decreased CNS CO2 responsiveness, so that minute volume has an abnormally minimal response to elevated CO2 levels (answer B). Chronically elevated CO2 levels lead to bicarbonate retention by the kidneys (answer D), normalizing the pH, and removing the little increased drive to breath there was to start with. This process creates a continual feedback loop, resulting in worsening symptoms.
FRC and RV in obesity:
Functional residual capacity (FRC) decreases in obesity due to a decrease in expiratory reserve volume not so much a change in residual volume (see respiratory question 5).
Why do obese people have eccentric LVH?
This is accomplished by increasing left ventricular end-diastolic volumes (LVEDV) to increase stroke volume (answer D). The increased volume work leads to eccentric LVH
Why is sniffing position so important for getting an airway?
When patients are not in proper sniffing position, the glottis appears more anterior than it otherwise would.
Glottis level in kids vs adults:
Neonates have a glottis at the C4 level which “drops” to the C6 level by adolescent ages, and therefore was not the cause for this finding
Which of the following reasons increase the risk of aspiration in obese individuals:
A. Gastric volume greater than 25 ml
B. Gastric pH less than 2.5
C. Increased abdominal pressure
D. Greater incidence of concurrent gallbladder disease
E. Greater incidence of concurrent gastric cancer
A 45 year old woman presents to the preoperative clinic for vaginal hysterectomy. She has a BMI of 45, normal vital signs except for a sat of 89% on room air, and normal labs (chem-7, CBC, and coags), except for a creatinine of 1.3 and INR of 1.9. She takes no medications. Which of the following would be the most appropriate next step:
A. Check transaminases and bilirubin B. Renal ultrasound C. Abdominal CT D. Refer to haematologist E. Refer back to surgeon for further workup
A: Check transaminases and bilirubin
With the available (sparse) history provided, non-alcoholic steatohepatitis (NASH) is most likely. NASH, from an anesthesiologists perspective, is not significantly different than any other cause of chronic liver disease. NASH is associated with obesity, metabolic syndrome, insulin resistance, and diabetes.
A 50 year old man with a BMI of 40 had a uvulopalatopharyngoplasty (UPPP)18 months ago and is presenting for total knee athroplasty. Which of the following are MOST LIKELY:
A. The patient is no longer at increased risk of postoperative hypercarbic respiratory failure
B. The patient is probably cured of sleep apnea
C. It would be unlikely that an appropriate seal can be made with an LMA
D. The patient is likely difficult to mask ventilate
The pathophysiology of sleep apnea (as described partially in question 1) involves upper airway obstruction at multiple points in the airway not always addressed by UPPP. Common points of obstruction are nasal passages, soft palate, tonsils, uvula, posterior tongue (most common offender), and adenoids. Even after UPPP, or more modern variants of the surgery, obese patients find another way to obstruct (answer A & B). Any patient with obstructive sleep apnea (OSA) should be assumed to be a more difficult intubation, and more importantly, difficult ventilation (answer D). The UPPP surgery does not preclude the ability of an LMA to form a good seal should positive pressure ventilation need to be used
T/F:
When the fat mass of an individual increases, both the lipid soluble Vd and the water soluble Vd increase, but obviously the Vd for lipid soluble drugs increases more. What does this even mean?
True
What does this all mean? In obese individuals, hydrophilic drugs and remifentanil should be dosed no more than 20% over the patient’s ideal body weight (supposedly). Lipophilic drugs, on the other hand, should be dosed at least more than 20% above ideal body weight (no clear recommendations given, but [ideal body weight + 40% of excess weight] is commonly quoted for BMI < 50).
For Propofol-TBW and IBW which one for induction, which one for maintenance?
IBW for induction
TBW for maintenance
Cardiac change in the elderly: SVR, systolic function:
In general, the elderly have decreased adrenergic receptor sensitivity (answer D) and an increased vagal tone, leading to a decreased heart rate (on average) (answer A). Increased arterial stiffness (decreased compliance) leads to increased systemic vascular resistance (SVR) and afterload (answer B). This leads to left ventricular hypertrophy (LVH) and diastolic dysfunction, not decreased systolic function (answer C).
In other words, older people develop LVH in response to increased after load/ SVR to preserve their systolic function at the expense of diastolic function. With LVH, the ventricle decreases in lucitropy and compliance, therefore operating at a higher end diastolic pressure at a given volume (diastolic dysfunction).
E to A ratio morphology: what does it represent? Where is it measured?
Diastolic dysfunction
This is obtained via pulse wave Doppler at the mitral valve. Essentially, the majority (70-80%) of flow should come during early (E) diastole and the rest during atrial (A) contraction. When this ratio of velocities reverses (A > E) it is an indication of mild diastolic dysfunction. As the diastolic dysfunction worsens, the E/A ratios can become more difficult to interpret. Additionally in cases when the E/A ratios are “fused” the data is not interpretable. In this case, tissue Doppler is used to study the velocities of the mitral annulus, leading to an E prime, or e’. An elevated E/e’ (over 15 or so) is further evidence of diastolic dysfunction.
What is TAPSE?
TAPSE (tricuspid annular plane systolic excursion) is a good measure of right heart function
What happens to all of these things in the elderly? A. Closing capacity B. Residual volume C. Dead space D. Arterial oxygen tension E. VQ mismatch
The elderly lung tends to loose elastic recoil with age, leading them to become overly filled (essentially emphysematous). This process increases residual volume (the volume left in the lungs after complete expiration, answer B). The decreased elasticity also leads to increased collapse of small airways, increasing closing capacity (to well within normal tidal volumes) (answer A) and therefore increasing VQ mismatching (answer E). All of this leads to a decreased PaO2 (answer D). Dead space, ventilation without perfusion, also increases (answer C).
These are the physiologic underpinnings of the various formulas that predict paO2 at a given age (and there are plenty of them). An easy one to remember is paO2 = 110-(Age X 0.4)
PaO2 and age prediction formula?
An easy one to remember is paO2 = 110-(Age X 0.4)
