Respiratory Physiology and Pathophysiology Flashcards
Describe the Bohr effect
The Bohr effect refers to a shift of the Hb-O2 dissociation curve caused by changes in CO2 or pH.
In the systemic capillaries, the Pco2 is higher than in the arterial blood (and the pH correspondingly lower) because of local
CO2 production. These circumstances shift the Hb-O2 dissociation curve to the right, which increases the offloading of O2 to the tissues.
The opposite occurs in the pulmonary capillaries; here, the Pco2 is lower (and the pH correspondingly higher) because of CO2 elimination, and the dissociation curve is shifted to the left to facilitate O2 binding to Hb
Describe the Haldane effect
The Haldane effect is the ability of deoxygenated hemoglobin (a protein composed of an amino group) to carry more carbon dioxide (CO2) than in the oxygenated state.
Two mechanisms explain this:
1) First increased Pao2 decreases the ability to form carbamino compounds (CO2 bound to
terminal amino groups in Hb molecules) —reducing the amount of CO2 bound to Hb —thereby raising the amount of dissolved CO2;
2) Second, the amino acid histidine, which
has an imidazole group that is an effective H+ buffer at physiologic pH, is an important linking molecule between heme groups and the Hb chains. Increasing the partial pressure of
oxygen (PO2) increases the amount of O2 bound to Hb; this changes the conformation of the Hb molecule, which in turn alters the heme-linked histidine and reduces its H+ buffering capacity. Therefore, more H+ is free (not buffered) and
binds to HCO3–, releasing stored CO2.
Fresh gas enters the lung by cyclic breathing at a rate and depth (tidal volume, VT) determined by metabolic demand, usually 7 to 8 L/min.
While most inspired gas reaches the alveoli, some (… mL) of each VT remains in the
airways and cannot participate in gas exchange. Such dead space (VD) constitutes approximately one third of each VT.
Anatomic VD is …, and physiologic VD is …
100-150
the fraction of the VT that remains in the “conducting” airways
any part of a VT that does not participate in gas exchange
The volume remaining above Residula Volume where expiration below Functional Residual Capacity closes some airways is termed …, and this volume added to the RV is termed …
closing volume (CV),
the closing capacity (CC; i.e., the total capacity of
the lung at which closing can occur)
How airway closure is affected by age? i
In youth, the closure does not occur until expiration is at or near RV, whereas with older age, it occurs earlier in expiration (i.e., at higher lung volumes).
This occurs because PPL is on average more “positive” (i.e., atmospheric, equal to PAW) as age increases. Closing can occur at or above FRC in individuals aged 65 to 70 years,50 such that dependent regions will undergo closure during normal expiration. This may be the major reason why oxygenation decreases with age
COPD … the lung volume at which closure occurs, possibly exacerbated by airway edema and increased bronchial tone
increases
Describe the West zones
This system is based on the principle that perfusion to an alveolus depends on the pressures in the pulmonary artery (PPA), pulmonary vein (PPV), and alveolus
(PALV).
In the apex (zone I), the key issue is that pulmonary arterial pressure is less than alveolar pressure; therefore no perfusion occurs.
PALV > PPA > PPV
Below the apex in zone II, PPV is less than alveolar pressure, and the veins are collapsed except during flow, as in a “vascular waterfall.” Although PALV is always greater than PPV, perfusion occurs when PPA exceeds PALV (i.e., intermittently, during systole)
PPA > PALV > PPV
Below this zone is zone III, in which there are
two important differences: PPA and PPV both always exceed PALV. As a result, there is perfusion throughout systole and diastole (and inspiration and expiration)
PPA > PPV > PALV
*** It was subsequently recognized that there is also a decrease in perfusion in the lung base, or zone IV, that is thought to occur because of the
effects of gravity compressing the lung at the bases—and the blood vessels therein—and thereby increasing vascular resistance
Whereas in humans older volatile anesthetics were thought to inhibit Hypoxic Pulmonary Vasoconstriction more than intravenously based anesthesia, modern volatile anesthetics, including sevoflurane and desflurane, have little effect
T or F
T
The amount of gas that can diffuse across a membrane in a given period is the diffusing capacity. Describe the equation
Diffusing capacity = (SA × ∆P × Sol) / h × √MW
SA - surface area of the membrane exposed to
gas
ΔP - gradient of partial pressure between administered gas versus blood tension
Sol - solubility of the gas in the membrane
h - thickness of the membrane
MW - is the molecular weight of the gas
Brief description of anesthetic changes in lung physiology
Anesthesia causes loss of muscle tone with a subsequent change in the balance between outward forces (i.e., respiratory muscles) and inward forces (i.e., elastic tissue in the lung) leading to a fall in FRC.
This causes or is paralleled by an increase in the elastic behavior of the lung (reduced compliance) and an increase in respiratory resistance. The decrease in FRC affects the patency of lung tissue with the formation of atelectasis (made worse with the use of high concentrations of inspired oxygen) and airway closure. This alters the distribution of ventilation and matching of ventilation and blood flow and impedes oxygenation of blood and removal of CO2.
Resting lung volume (i.e., FRC) is reduced by almost … L by moving from upright to supine position; induction of anesthesia further decreases the FRC by approximately … L.
This reduces the FRC from approximately 3.5 to … L, a value close to RV.
1
0.5
2
A sigh maneuver, or a large VT (tidal volume), has been suggested for reversing atelectasis; however, atelectasis is not uniformly reduced by a VT increase or sigh up to PAW of 20 cm H2O. Instead, a PAW (alveolar ventilation pressure) of … cm H2O is required for initial opening, and … cm H2O for more complete reversal
30
40
Inflation with a PAW (alveolar ventilation pressure) of 40 cm H2O for … seconds appears to successfully open almost all anesthesia-induced atelectasis
7 to 8
How to minimize reocurrance of atelectasis after a recruitment maneuver?
Although recruitment of anesthesia-induced atelectasis is completely possible with either PEEP or a VC maneuver, continuous application of some level of PEEP is required to prevent rapid recurrence of the atelectasis.
However, nitrogen (N2)—an insoluble gas that is not absorbed into the blood—can “splint” the alveolus if the alveolus is already opened. As a result, in anesthetized patients, a VC maneuver followed by ventilation with a gas mixture containing 60% N2 (40% O2) reduced the propensity for reaccumulation of atelectasis, with only 20% reappearing 40 minutes after recruitmen
Because loss of muscle tone in the diaphragm or chest wall appears to increase the risk of atelectasis, techniques that preserve muscle tone may have advantages. Intravenous
… does not impair muscle tone and is the only individual anesthetic that does not cause atelectasis
ketamine
