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Gravity causes a vertical gradient in the distribution of pulmonary blood flow in the LDP for the same reason that it does in the upright position (see Chapter 17 ). Because the vertical hydrostatic gradient is less in the LDP than in the upright position, zone 1 blood flow (in the nondependent lung) is ordinarily less in the former than the latter position. Nevertheless, blood flow to the dependent lung is still significantly greater than that to the nondependent lung ( Fig. 49-9 ). Thus, when the right lung is nondependent, it should receive approximately 45% of total blood flow as opposed to the 55% that it receives in the upright and supine positions. When the left lung is nondependent, it should receive approximately 35% of total blood flow as opposed to the 45% that it receives in the upright and supine positions.[206] [207]
Figure 49-9
Schematic representation of the effects of gravity on
the distribution of pulmonary blood flow in the lateral decubitus position. The
vertical gradient in the lateral decubitus position is less than in the upright position.
Consequently, there is less zone 1 and more zone 2 and 3 blood flow in the lateral
decubitus position than in the upright position. Nevertheless, pulmonary blood flow
increases with lung dependency and is greater in the dependent lung than in the nondependent
lung. PA
, alveolar pressure; Ppa
, pulmonary artery pressure;
Ppv
, pulmonary venous pressure. (From Benumof JL: Anesthesia
for Thoracic Surgery. Philadelphia, WB Saunders, 1987.)
Because gravity also causes a vertical gradient in pleural pressure
(Ppl) in the LDP, ventilation is relatively increased in the dependent as compared
with the nondependent lung ( Fig. 49-10
).
In addition, in the LDP the dome of the lower part of the diaphragm is pushed higher
into the chest than the dome of the upper part of the diaphragm; therefore, the lower
portion is more sharply curved than the upper portion of the diaphragm. As a result,
the lower part of the diaphragm is able to contract more efficiently during spontaneous
respiration. Thus, in an awake patient in the LDP, the lower lung is normally better
ventilated than the upper lung, regardless of the side on which the patient is lying,
although there remains a tendency toward greater ventilation of the larger right
lung.[226]
As a result of greater perfusion to
the lower lung, the preferential ventilation to the lower lung is matched by increased
perfusion of this lung, so the distribution of the V̇/
ratios of the two
lungs is not greatly altered when an awake subject assumes the LDP. Because perfusion
increases to a greater extent than ventilation with lung dependency does, the V̇/
ratio decreases from the nondependent to the dependent lung (just as it does in upright
and supine lungs).
Comparison of an awake with an anesthetized patient in the LDP (see Chapter 28 ) reveals no difference in the distribution of pulmonary blood flow between the dependent and nondependent lungs. Thus, in an anesthetized patient, the dependent lung continues to receive relatively more perfusion than the nondependent lung does. Induction of general anesthesia, however, does cause significant changes in the distribution of ventilation between the two lungs.
Figure 49-10
Awake, closed-chest distribution of ventilation. A,
Pleural pressure (Ppl
) in an awake upright patient is most positive in
the dependent portion of the lung, and alveoli in this region are therefore most
compressed and have the least volume. Pleural pressure is least positive (most negative)
at the apex of the lung, and alveoli in this region are therefore least compressed
and have the largest volume. When these regional differences in alveolar volume
are translated to a regional transpulmonary pressure-alveolar volume curve, the small
dependent alveoli are on a steep (large-slope) portion of the curve, and the large
nondependent alveoli are on a flat (small-slope) portion of the curve. In this diagram,
regional slope equals regional compliance. Thus, for a given and equal change in
transpulmonary pressure, the dependent part of the lung receives a much larger share
of the tidal volume than the nondependent part of the lung does. B,
In the lateral decubitus position, gravity also causes pleural pressure gradients
and therefore similarly affects the distribution of ventilation. The dependent lung
lies on a relatively steep portion and the nondependent lung lies on a relatively
flat portion of the pressure-volume curve. Thus, in the lateral decubitus position,
the dependent lung receives most of the tidal ventilation. P, transpulmonary pressure;
V, alveolar volume. (From Benumof JL: Anesthesia for Thoracic Surgery.
Philadelphia, WB Saunders, 1987.)
Figure 49-11
Distribution of ventilation in a patient in the lateral
decubitus position when awake (A) and when anesthetized
(B). Induction of anesthesia has caused loss of lung
volume in both lungs, with the nondependent lung moving from a flat, noncompliant
portion to a steep, compliant portion of the pressure-volume curve and the dependent
lung moving from a steep, compliant part to a flat, noncompliant part of the pressure-volume
curve. Thus, an anesthetized patient in a lateral decubitus position has more of
the tidal ventilation in the nondependent lung (where perfusion is the least) and
less of the tidal ventilation in the dependent lung (where perfusion is the greatest).
P, transpulmonary pressure; V, alveolar volume. (From Benumof JL: Anesthesia
for Thoracic Surgery. Philadelphia, WB Saunders, 1987.)
In the LDP, more ventilation is switched from the dependent lung in an awake subject to the nondependent lung in an anesthetized patient ( Fig. 49-11 ).[227] [228] There are several interrelated reasons for this change in the relative distribution of ventilation between the nondependent and dependent lungs. First, induction of general anesthesia usually causes a decrease in FRC, and both lungs share in the loss of lung volume. Because each lung occupies a different initial position on the pulmonary pressure-volume curve while the subject is awake, a general anesthesia-induced reduction in the FRC of each lung causes each lung to move to a lower, but still different portion of the pressure-volume curve (see Fig. 49-11 ). The dependent lung moves from an initially steep part of the curve (with the subject awake) to a lower and flatter part of the curve (after anesthesia is induced), whereas the nondependent lung moves from an initially flat portion of the pressure-volume curve (with the subject awake) to a lower and steeper part of the curve (after anesthesia is induced). In fact, in the LDP the ratio of nondependent to dependent lung FRC is approximately 1.5 (average values are 1400 and 900 mL, respectively) in an adult patient.[229] [230] [231] [232] Similar findings may be expected in children.[230] Furthermore, in the LDP the compliance of the nondependent and dependent lung is 30 and 23 cm H2 O, respectively.[229] Thus, with induction of general anesthesia, the lower lung moves to a less favorable (flat, noncompliant) portion and the upper lung to a more favorable (steep, compliant) portion of the pressure-volume curve. Second, if an anesthetized patient in the LDP is also paralyzed and mechanically ventilated, the high, curved diaphragm of the lower lung no longer confers any advantage in ventilation (as it does in the awake state) because it is no longer actively contracting.[231] [232] Third, the mediastinum rests on the lower lung and physically impedes lower lung expansion, as well as selectively decreases lower lung FRC. Fourth, the weight of the abdominal contents pushing cephalad against the diaphragm is greatest in the dependent lung, which physically impedes lower lung expansion the most and disproportionately decreases lower lung FRC. Finally, suboptimal positioning that fails to provide room for lower lung expansion may considerably compress the dependent lung. Opening the nondependent hemithorax further disproportionately increases ventilation to the nondependent lung (see later).
In summary, an anesthetized patient, with or without paralysis,
in the LDP and with a closed chest has a nondependent lung that is well ventilated
but poorly perfused and a dependent lung that is well perfused but poorly ventilated,
which results in an increased degree of V̇/ mismatching. The application
of PEEP to both lungs restores more of the ventilation to the lower lung.[207]
Presumably, the lower lung returns to a steeper, more favorable part of the pressure-volume
curve, and the upper lung resumes its original position on a flat, unfavorable portion
of the curve.
When compared with the condition of an anesthetized, closed-chest
patient in the LDP, opening the chest wall and pleural space alone does not ordinarily
cause any significant alteration in partitioning of pulmonary blood flow between
the dependent and nondependent lungs; thus, the dependent lung continues to receive
relatively more perfusion than the nondependent lung does. Opening the chest wall
and pleural space, however, does have a significant impact on the distribution of
ventilation (which must now be delivered by positive pressure). The change in the
distribution of ventilation may result in further V̇/ mismatching ( Fig.
49-12
).[233]
Figure 49-12
Schematic depiction of a patient in the lateral decubitus
position in which the closed-chest anesthetized condition is compared with the open-chest
anesthetized and paralyzed condition. Opening the chest increases nondependent lung
compliance and reinforces or maintains the larger part of tidal ventilation going
to the nondependent lung. Paralysis also reinforces or maintains the larger part
of tidal ventilation going to the nondependent lung because the pressure of the abdominal
contents (PAB
) pressing against the upper part of the diaphragm is minimal
(smaller arrow), and it is therefore easier for positive-pressure
ventilation to displace this lesser resisting dome of the diaphragm. P, transpulmonary
pressure; V, alveolar volume. (From Benumof JL: Anesthesia for Thoracic
Surgery. Philadelphia, WB Saunders, 1987.)
If the upper lung is no longer restricted by a chest wall and the total effective compliance of that lung is equal to that of the lung parenchyma alone, it will be relatively free to expand and will consequently be overventilated (and remain underperfused). Conversely, the dependent lung may continue to be relatively noncompliant and poorly ventilated and overperfused.[206] Surgical retraction and compression of the exposed upper lung can provide a partial, though nonphysiologic solution to this problem in that if expansion of the exposed lung is mechanically or externally restricted, ventilation will be diverted to the dependent, better-perfused lung.[208]
In an open-chest anesthetized patient in the LDP, induction of paralysis alone does not cause any significant alteration in the partitioning of pulmonary blood flow between the dependent and nondependent lungs. Thus, the dependent lung continues to receive relatively more perfusion than the nondependent lung does. There are, however, strong theoretical and experimental considerations indicating that paralysis might cause significant changes in the distribution of ventilation between the two lungs under these conditions.
In the supine position and the LDP, the weight of the abdominal contents pressing against the diaphragm is greatest on the dependent part of the diaphragm (posterior lung and lower lung, respectively) and least on the nondependent part of the diaphragm (anterior lung and upper lung, respectively) (see Fig. 49-12 ). In an awake, spontaneously breathing patient, the normally present active tension in the diaphragm overcomes the weight of the abdominal contents, and the diaphragm moves the most (largest excursion) in the dependent portion and least in the nondependent portion. This circumstance is healthy because this is another factor that maintains the greatest amount of ventilation where perfusion is the greatest (dependent lung) and the least amount of ventilation where perfusion is the least (nondependent lung). During paralysis and positive-pressure breathing, the passive and flaccid diaphragm is preferentially displaced in the nondependent area, where the resistance to passive diaphragmatic movement by the abdominal contents is least; conversely, the diaphragm is minimally displaced in the dependent portion, where the resistance to passive diaphragmatic movement by the abdominal contents is greatest. [234] This circumstance is unhealthy because the greatest amount of ventilation may occur where perfusion is the least (nondependent lung) and the least amount of ventilation may occur where perfusion is the greatest (dependent lung).[234]
In summary ( Fig.
49-13
), the preceding section has developed the concept that an anesthetized,
paralyzed patient in the LDP with an open chest may have a considerable V̇/
mismatch consisting of greater ventilation but less perfusion to the nondependent
lung and less ventilation but more perfusion to the dependent lung. The blood flow
distribution is mainly and simply determined by
Figure 49-13
Schematic summary of ventilation-perfusion relationships
in an anesthetized patient in the lateral decubitus position who has an open chest
and is paralyzed and suboptimally positioned. The nondependent lung is well ventilated
(as indicated by the large dashed lines) but poorly
perfused (small perfusion vessel); the dependent lung is poorly ventilated (small
dashed lines) but well perfused (large perfusion vessel). In addition,
an atelectatic shunt compartment (indicated on the left side of the lower lung) may
also develop in the dependent lung because of the circumferential compression of
this lung. (See the text for a detailed explanation.) PAB
, pressure of
the abdominal contents. (Modified from Benumof JL: Anesthesia for Thoracic
Surgery. Philadelphia, WB Saunders, 1987.)
A physiologic solution to the adverse effects of anesthesia and surgery in the LDP on the distribution of ventilation and perfusion during two-lung ventilation would be selective application of PEEP to the dependent lung (through a DLT).[236] Selective PEEP to the lower lung should increase the ventilation to this lung by moving it up to a steeper, more favorable portion of the lung pressure-volume curve. Indeed, this has been done with reasonably good success.[236] [237] A series of 22 mechanically ventilated patients (both lungs) undergoing thoracotomy in the LDP were divided into two groups.[237] Group I patients had
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