【Pediatric Anesthesia】Lung interaction in congenital heart disease surgery center

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Lung interaction in the Surgical Center for Congenital Heart Disease

Cardiopulmonary interactions vary significantly among patients, so a ventilation strategy or regimen may not be applicable to all patients

1

lung volume

Changes in lung volume have a huge impact on pulmonary circulatory resistance PVR, which is lowest in FRC, and hypoventilation and hyperventilation may lead to a significant increase in

Elevated PVR increases right ventricular afterload or wall tension, impairs RV function and leads to decreased

See figure:

2

Intrapleural pressure

Increased mean intrapleural pressure during positive pressure ventilation leads to a decrease in body and pulmonary ventricular preload, but the opposite effect on individual ventricular postloads

3

right ventricle

A decrease in RV preload during positive pressure ventilation may result in a decrease

In patients with normal RV compliance and no residual capacity or pressure load in the ventricles after surgery, changes in before and after load due to positive pressure ventilation hardly affect RV function

The potential harm of mechanical ventilation to the function of RV deserves attention

4

left ventricle

Changes in lung volume also affect LV preload

The pressure of the systemic circulating arteries is high and is not affected
by radial traction during lung expansion or contraction.

Thus changes in lung volume affect the preload of LV, but their afterload depends only on changes in intrapleural pressure and is not related to
changes in lung volume.

Unlike the RV, the main effect of positive pressure ventilation on LV is a reduction
in afterload.

According to LaPlace's law, the ventricular wall stress is proportional to the left ventricular pressure and the RADIUS of LV curvature
.

LV cross-wall pressure is the difference between
its intraluminal pressure and the surrounding intrathoracic pressure.

Assuming constant arterial pressure and LV diameter, an increase in intrapleural pressure during positive pressure ventilation will reduce the transmural gradient, thereby reducing LV wall stress
.

Therefore, positive pressure ventilation and PEEP are very beneficial in patients with left ventricular
failure.

Patients with LV insufficiency, end-diastolic volume, and increased pressure may impair respiratory mechanics
due to increased lung fluid, decreased lung compliance, and increased airway resistance.

Respiratory reserves are limited in newborns, and increased respiratory work can lead to early fatigue
.

A large part of the total oxygen consumption of newborns and infants with LV insufficiency is used for respiratory work, leading to poor feeding and stunting
.

Therefore, positive pressure ventilation reduces respiratory work and oxygen demand, which is beneficial
in patients with significant volume overload and ventricular dysfunction.

5

Lung injury

It is important to note that mechanical ventilation may cause severe lung damage, especially at orgasmic
volume.

Large and rapid changes in tidal volume may lead to shear stress in the alveolar septum and subsequent rupture
of alveolar capillaries.

Gas leakage under the same mechanism can also lead to microcirculation disruption, increase the total amount of lung water, which in turn increases airway resistance and reduces lung compliance
.

Lung disease is usually uneven, and the time constants of different lung areas are different, that is, the concept
of "fast" and "slow" alveoli.

When using volume-modulated ventilation strategies, the alveoli with high compliance will dilate preferentially over collapsed or slow time constant regions, resulting in local alveolar hyperdistention and trauma
.

This may be less pronounced under a pressure-controlled ventilation strategy, as alveoli with greater compliance or faster speeds will swell to a preset pressure, and subsequently, depending on the inspiratory time, lung areas with low time constants will gradually swell and reassert
.

Although a large tidal volume of 12 to 15 ml/kg is beneficial for maintaining lung volume at low PVR after surgery in patients with CHD, long-term use of the high-volume strategy may cause lung damage (i.
e.
, volume damage).


Pressure-controlled ventilation maintains a relatively constant tidal volume, but does not experience large fluctuations in peak inspiratory pressure or local alveolar overdiscaling
.

It is critical
to repeatedly evaluate ventilation patterns and adjust them according to hemodynamic responses.

Fortunately, most patients with CHD surgery do not have substantial lung disease and respiratory mechanical changes, such as secondary changes in pulmonary fluid, which are usually resolved after surgical repair and diuresis is resolved
after CPB.

6

Positive pressure at the end of exhalation

The use of PEEP in PATIENTS with CHD has been controversial
.

It was initially thought not to help improve gas exchange, and increasing airway pressure may adversely affect hemodynamics and lead to lung damage and air
leakage.

However, PEEP increases FRC, promotes pulmonary atelectasis, and redistributes pulmonary fluid from the alveolar septum around the hilars
with better compliance.

Both of these effects improve gas exchange and reduce PVR
.

However, too high levels of PEEP may increase RV afterload
.

PEPs of 3 to 5 cmH20 usually help maintain FRC and pulmonary fluid redistribution without affecting hemodynamics
.

Notes/Wu Yajun

Typography/Meat

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