Considerations for Hemodynamics

Abstract

Chapter 14 is about hemodynamics and the basics of the different pathophysiological states in children with congenital heart defects. Obstructive heart diseases are contrasted with dilated states, and the different effects of hemodynamic changes in different pathologic states are described. Basics about shunt diseases, valvular diseases, etc. and clinical and echocardiographic evaluation of these patients are presented. Chapter 14 lays the foundations for an understanding of the postoperative therapy described in Chaps. 15 and 16.

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1 Preliminary Considerations

The energy required for the cardiomyocytes to contract is provided in the healthy heart at rest by free fatty acids, glucose, and lactate in approximately equal proportions. With increased load and unchanged force-frequency coupling (Bowditch effect), increasingly more glucose and lactate are consumed proportionately until ultimately the limit of aerobic metabolism is reached, and, with little possibility of increasing O₂ extraction (>65% of the delivered O₂ is already extracted at rest), the heart itself produces lactate (= lactate reversal). In the process, mechanical systole (= phase of isovolumetric contraction plus the ejection phase) requires about three quarters of the energy, while diastole (= isovolumetric relaxation plus the ventricular filling phase) also requires about 25% of the energy. This energy is used essentially for structural maintenance, regeneration of membrane potentials, and atrial systole.

As energy supply is critical only in exceptional situations (severe hypoglycemia/metabolic defects), O₂ provision takes precedence. Coronary blood flow is therefore a crucial variable in this process: This can be enhanced by an increase in rate, coronary vasodilation, opening of precapillary sphincters with an enhanced blood supply requirement, and increased perfusion pressure. In this respect, two mechanisms must be considered critically from an economic perspective:

Firstly, an increase in rate can increase coronary O₂ delivery only up to a certain point – once this is exceeded, there is an O₂ delivery mismatch as a result of a reduction in the duration of diastole with a concurrent rate-dependent increase in contraction.

Secondly, adrenergic coronary vasodilation (predominantly mediated by β ₁ receptors) is also exhausted by increases in rate and contractility, and, as a result of additive adrenergic effects (α-receptor-mediated vasoconstriction), a disproportionate increase in O₂ consumption occurs as heart rate increases.

In addition to the previously described preload dependency of ventricular filling and the reduction in contractility on overdistension (see Starling curve), the afterload and the muscular condition of the myocardium itself naturally play important roles. Kinetically, the afterload describes how much energy must be used for isovolumetric contraction until the aortic valve opens in order to build up wall tension that overcomes systemic vascular resistance.

By increasing contractility and wall tension within limits, the healthy left ventricle can overcome a higher afterload – under exertion, systolic blood pressure rises (increased contractility) far more than diastolic. In resistance-induced concentric hypertrophy of the ventricle (the contractile elements of the individual muscle cells proliferate), a higher ventricular pressure can be generated with unchanged wall tension. According to Laplace’s law, wall tension is inversely proportional to wall thickness. Wall tension (κ) = ventricular pressure × ventricular radius/2 × wall thickness.

This compensation mechanism is acquired on the other hand at the expense of reduced diastolic compliance – the hypertrophic ventricle requires a comparatively longer diastole for optimal filling and is highly dependent on the active component of ventricular filling due to atrial systole (A wave of transmitral inflow or A wave of the atrial pressure curve). Therefore, compared with a ventricle with normal wall strength, cardiac output can barely be increased at all in a (hypertrophic) ventricle trained against resistance, even at lower heart rates or following loss of sinus rhythm.

In a dilated ventricle with a comparatively thin ventricular wall (= high wall tension), all compensation mechanisms are compromised in parallel. In order to be able to maintain cardiac output when contractility is impaired, the heart slips into the distension range on the Starling curve by a neuroendocrine, primarily RAAS-triggered, mechanism. As a result of sympathomimetic stimulation, the (overdistended) ventricle approaches the limit of its contractility reserves, while the heart rate also increases to maintain organ perfusion. An increase in afterload here can then rapidly overburden the heart. In this case, therapeutic afterload reduction must be accomplished carefully so as not to compromise coronary perfusion.

This yields a range for the optimal, i.e., most economic, cardiac output for each heart in relation to the physiological requirements of the circulation that depends on contractility, filling pressures and volumes, wall thickness, and wall tension of the overloaded ventricle, as well as on the physiological demands that the body places on the system (= O₂ requirement).

On the venous side of the circulation, various mechanisms cause the return of blood from the capillary venous branch (capillary filling pressure about 15–25 mmHg) to the heart.

The well-contracting right ventricle generates a suction effect in the venous system in systole by displacing the AV valve level with the AV valves closed (valve level mechanism identifiable by the X wave of the atrial curve = lowest pressure level) and draws the returning blood to the heart. The negative intrathoracic pressure of inspiration in spontaneous breathing acts on venous blood flow in a similar fashion. The effect of the muscle pump (with preserved venous valves) is slight in immobile patients. However, atrial systole with an opened AV valve contributes up to 20–25% to ventricular filling (particularly in tachycardia) and thus to the forward transport of the venous blood into the ventricle.

All these mechanisms can only work if diastolic ventricular function is unimpaired:

In the case of diastolic dysfunction, on the one hand, energy-consuming (isovolumetric) ventricular relaxation can be impaired – in the short term, β ₁ stimulants can then improve lusitropy (= capacity of the cardiomyocytes to relax). On the other hand, a high wall tension on relaxation (ventricular hypertrophy) and an increased end-diastolic ventricular pressure due to a systolic function disorder can obstruct filling of the ventricle with venous blood.

The mechanism of interventricular interaction (interventricular dependency) comes into play to only a limited extent following cardiac surgery with opened pericardium, but it is certainly important following pericardial closure.

RV and LV share the space within the pericardium. The pericardium is distensible only to a very limited extent (exponential distension curve). Acute volume changes in one ventricle therefore also act directly on the other. If, for example, right heart dilation occurs in association with an acute increase in pulmonary afterload (e.g., PHT crisis), the septum is shifted to the left, and the LV is compressed (the position of the septum is dependent on the respective pressures in the right and left ventricle). If a ventricle is compressed from the “outside,” ventricular compliance decreases (ventricular filling disorder), thereby causing the end-diastolic volume (EDV) and stroke volume (SV) to decrease as well. As a result of the unloading of a dilated ventricle, therefore, the filling of the other ventricle is improved, which can lead to a “paradoxical” increase in cardiac output.

Echocardiographic measurement methods

Extensive experience is required for purely visual identification of the systolic function of the ventricles (eyeballing). If no regional wall movement disorders are detected here, the method of determining ventricular fractional shortening (FS) by measuring the diastolic and systolic internal diameter of the left ventricle in the parasternal long axis orthogonally in the area of the tendinous cords or in the parasternal short axis below the mitral valve level can be of use. (Paradoxical septal movements are also always found after arterial switch surgery in Bland-White-Garland syndrome or in patients with large atrial septal defects (ASD), so that FS is not 1:1 transposable, but nevertheless LV function can be estimated in this way.) At both levels, the right ventricle can also be assessed at the same time.

Formula 43

FS = [(Diam_ed − Diam_es)/Diam_ed] × 100%

Normal: ≥28–30%

Using the multislice summation method (biplanar) or other approximation procedures (including monoplanar), the ejection fraction (EF) can also be estimated from the ratio of the end-diastolic ventricular area to the end-systolic ventricular area if the endocardium is clearly identifiable.

Formula 44

${\displaystyle \begin{array}{l}\mathrm{EF}\ \mathrm{in}\%=\left[\left({\mathrm{Vol}}_{\mathrm{ed}}-{\mathrm{Vol}}_{\mathrm{es}}\right)/{\mathrm{Vol}}_{\mathrm{ed}}\right]\times 100\\ {} or\\ {}\mathrm{EF}\mathrm{in}\%=\mathrm{SV}/\mathrm{EDV}\times 100\end{array}}$

Normal: >50% (<30% severely impaired)

Example:

• 1. Where EDV = 100 mL and SV = 60 mL → EF = 60%

• 2. Where EDV = 200 mL and SV = 60 mL → EF = 27%

Example 2 shows that a “weakly” dilated ventricle can still generate normal cardiac output despite a reduced ejection fraction (at rest). During exertion (increased VO₂) or stress (higher systemic vascular resistance [SVR]), however, systolic function sometimes is insufficient to cover the increased need (= sign of heart failure).

To measure this, the diameter of the lumen (= endocardium-to-endocardium) is determined. The dimensions mentioned are particularly important for ongoing monitoring in myocardial diseases of the left ventricle and in patients with biventricular anatomy and must be considered strictly in relation to the patient’s clinical situation.

An absolute estimation of stroke volume as would be required to determine cardiac output (CO = SV × HF) is more or less impossible by echocardiography. By way of approximation, the VTI (velocity-time integral), measured by pulsed-wave Doppler via the LVOT, for example, can be multiplied by the cross-sectional area instead. Most ultrasound devices provide algorithms for this purpose, as well as for the measurement of rates of pressure rise via valve regurgitations. When assessing FS and EF, it should be borne in mind that in patients with significant mitral or aortic insufficiency as a result of regurgitant blood flow, there is a high FS or EF as long as ventricular function is good. A borderline left ventricular FS/EF in these cases is already a sign of pump failure.

2 Managing Heart Defects

2.1 “Hypertrophy” Versus “Dilation”

As described earlier, concentric myocardial hypertrophy serves to overcome increased resistances with relatively little increase in wall tension at the expense of reduced diastolic compliance, which is dependent on a sufficiently long diastole and adequate filling pressure. In simplified terms, hypertrophied ventricles (“compliance disorder”) are preload-sensitive and afterload-insensitive and react early to tachycardia and loss of atrioventricular coordination with a decrease in cardiac output.

This means they react sensitively to an increase in preload with an increase in SV, whereas a reduction in afterload results in no increase in SV. This is so because such ventricles usually eject a large proportion of their EDV even under normal conditions (high EF).

Therefore, hypertrophied hearts respond well to an increase in preload by volume replacement, as well as to a reduction in heart rate with beta-blockers or central alpha-stimulants and to an increase in blood pressure, e.g., with noradrenaline. Increased blood pressure therefore tends to be beneficial for these hearts, since this prevents “hypercontractility” while maintaining an adequate perfusion pressure for the coronary arteries. With low diastolic blood pressure, there is a risk of intimal ischemia because of the long diffusion distance and the high end-diastolic pressure. A striking example is sudden cardiac death (SCD) in patients with hypertrophic cardiomyopathy under exertion.

Opposing the hypertrophied ventricle is a dilated, thin, and weak heart (e.g., dilative cardiomyopathy [DCM]). The weak contraction of these ventricles results in an increase in EDV and hence wall tension. As only a small percentage of the EDV can be ejected during systole (low EF), these hearts are working at an already high end-diastolic filling level. A further increase in preload can therefore provide little further contribution to an increase in SV (there is also a threat of acute decompensation on overdistension of sarcomeres). The main problems of these ventricles are the limited contractility and the “structural dilation” (as the contractile filaments no longer overlap optimally and therefore can produce less force). As a result of a reduction in afterload, there is an increase in stroke volume in this situation, as the ventricle can again eject more despite reduced contractility. Accordingly, EDV decreases again (caution: reduction in afterload beyond the limit of minimal organ perfusion pressures, particularly of diastolic coronary perfusion pressure).

In simplified terms, dilated ventricles (“contractility disorder” with low EF = SV/EDV) are afterload-sensitive and preload-insensitive. This means these hearts react sensitively to a reduction in afterload with an increase in SV, whereas an increase in preload results in a significant increase in SV only within a very limited range.

As well as a reduction in afterload, dilated, weak ventricles obviously also benefit from increased contractility. Inodilators such as milrinone (Corotrop®) or the calcium sensitizer levosimendan (Simdax®) are best suited to this purpose. Higher-dosed betamimetics (e.g., adrenaline, dobutamine), which result in an increase in O₂ consumption and tachycardia, are better avoided in our opinion. On the basis of earlier arguments (see also Chap. 3), a (excessively) high HR (in the form also of a demand-tachycardia) is not beneficial and does not result in any improvement of the O₂ balance (target: age-commensurate normal HR).

The strengthening of systolic function also contributes to improved diastolic filling through the higher stroke volume and the decrease in EDV. The transmitral inflow profile is normalized and the interventricular interaction can improve as a result (see Fig. 14.1).

Fig. 14.1

Factors influencing arterial blood pressure

3 Classification of Cardiac Output and O₂ Balance

Cardiac output is of the greatest importance for the body’s O₂ supply (DO₂). Usually only cardiac output can be altered acutely, whereas Hb and SpO₂ are usually relatively “fixed” parameters (see also Chap. 1).

Formula 45

DO₂ = CO × Hb × SaO₂

The factors in oxygen supply (in the order of their importance for DO₂) are:

• Cardiac output

• Hb concentration

• SaO₂

Theoretically, it should be possible for cardiac output to continue to increase constantly through an increase in heart rate and ventricular filling (with appropriate contractility). However, there are limits:

• The Frank-Starling curve becomes increasingly flatter as filling increases (see Fig. 3.1).

• Ventricular volume is constrained by the pericardium.

• As HR increases, SV (at high rates) decreases again (shorter diastole).

• With excessive filling pressure (= preload), pressures at the venous capillary end also increase, which can result in edema.

The previous sections have described how cardiac output can be increased (see also above and Chap. 3) and how significant an inadequate oxygen supply (critical DO₂) is for the body.

The clinical parameters are checked to establish sufficient oxygen delivery (DO₂):

• Arterial blood pressure (perfusion pressure, afterload, heart function, see Fig. 14.1.)

• Central venous pressure (filling, preload, compliance disorder, heart function)

• Capillary filling (microcirculation)

• Urine output (perfusion pressure – kidney particularly pressure-sensitive)

• Temperature difference between body core and shell (microcirculation, afterload)

• Arterial blood gas analysis (pH, BE, lactate, SaO₂, pO₂, Hb)

• Ventilation parameters

• Venous blood gas analysis (SvO₂)

• Vigilance

As well as the abovementioned function and measurement parameters, causes of postoperative function disorders (residual defect, outlet stenoses, leaking valves or valve stenoses, etc.) should be identified by echocardiography so that a therapeutic decision is rapidly possible from the combination of previous history, clinical features, and supplementary technical examinations (see Table 14.1).

Table 14.1 Hemodynamic effects

In rare exceptional cases and only in biventricular serial circulation without relevant shunts, PICCO technology can provide vital prompts for therapeutic action by continuous pulse contour analysis combined with (repeated) static thermodilution: Benefits here can include detection and quantification of pulmonary edema, quantification of cardiac preload and cardiac output, measurement of volumetric preload parameters instead of filling pressures, and determination of afterload, contractility, or volume reactivity.

4 Postoperative Differential Diagnosis of Arterial Hypotension

One of the most common postoperative problems is arterial hypotension, which always raises the question: Is the hypotension due to

• Low cardiac output = reduced cardiac output (preload deficiency, anemia, poor contractility, high afterload, cardiac arrhythmia, pHT, etc.) or

• Reduced peripheral resistance (vasodilation; see Fig. 14.1)?

SvO₂ provides further help here:

• The combination of hypotension with low SvO₂ indicates a low cardiac output (preload deficiency, poor contractility, etc.).

• The combination of hypotension with normal or fairly high SvO₂ indicates a sufficient cardiac output with low afterload (vasodilation).

Typical constellations:

• BP low, HR high, CVP “low”, SvO₂ low to normal, volume-responsive (BP increases more strongly than CVP): hypovolemia

• BP low, HR high, CVP “high”, SvO₂ low, volume-nonresponsive (CVP increases more strongly than BP): tamponade, lower ventricular compliance, pump failure

• BP low, HR high, CVP “normal,” SvO₂ normal to high: vasodilation

As well as the clinical examination, echocardiography in particular can also be used for rapid differentiation. As depicted in Fig. 14.1, blood pressure is related to cardiac output.

No flow, no pressure (macrocirculation) – no pressure, no flow (organ circulation).

If organ functions are normal and cardiac output (oxygen delivery/VO₂) and perfusion pressure are therefore adequate, the numerical value of the blood pressure is of no interest (“Don’t care about the numbers”).

While flow and hence the supply of nutrients and oxygen to the organs are the most important parameters overall, a certain minimum perfusion pressure must also be maintained at the organ level (best example: in the absence of adequate diastolic blood pressure, myocardial ischemia, pump dysfunction, etc. occur!).

In critical situations, therefore, pressure comes before flow (“centralization” in favor of organs such as the brain and heart that are vital to survival); in stabilized situations, flow comes before pressure (best example: severe heart failure in which afterload reduction results in an increase in cardiac output). (In cardiac arrest, it is obvious that flow must be generated first in this situation before there can be pressure.)

4.1 Tamponade (“Wet” and “Dry” Due to Myocardial Edema)

When the pericardium is filled with fluid (usually blood – more rarely chyle or serious fluids), diastolic filling of the heart can very rapidly be impaired with the resultant development of symptoms of acute heart failure. Retrosternal and intrapericardial drains are therefore usually put in place at the end of surgery. Similarly, and with similar symptoms (tachycardia, LOCS, increase in CVP, etc.), postoperative swelling of the myocardium is manifested after prolonged ischemia or insufficient myocardial protection in the form of a ventricular filling disorder. An acute diastolic function disorder develops in which the ventricle is able to receive the delivered blood only at high preload pressures – in the case of this filling disorder, which can be differentiated clinically and by echocardiography, it is usually possible to provide sufficient physical assistance by opening the chest (left open postoperatively or reopened) and to try to close it when filling pressures fall and there is sufficient urine output (caution: increased risk of infection).

5 Considerations for Shunt Defects

In deviations from the normal (serial) anatomy of the blood circulation, the blood chooses the path of least resistance.

As the right ventricle exhibits considerably greater compliance than the left and pulmonary vascular resistance (PVR) is markedly lower than systemic vascular resistance (SVR), an L/R shunt normally occurs in defects at the vascular level (anomalous pulmonary venous return, AV fistula, or aortopulmonary window) and at the cardiac level (VSD/ASD). If there is an additional (pathological) change, this rule may not apply:

• RV compliance reduced and PVR still high (postnatally with physiological pulmonary hypertension and a still “trained” ventricle). In this case, ASD can lead to an R/L shunt.

• If the resistance of an RV outlet stenosis (e.g., pulmonary stenosis) is higher than the peripheral systemic resistance, this results in an R/L shunt (Fallot with hypoxemia).

• Similarly with PVR (hypoxia/pneumonia/acidosis/Eisenmenger syndrome) – in this case also a shunt reversal can occur.

• SVR ↓↓ (as in sepsis): Here also the blood can be conducted more to the systemic circulation and less to the pulmonary circulation.

The ratio describing the relationship between pulmonary and systemic perfusion is known as the Qp/Qs ratio. In serial circulation, the ratio is about 1:1; in L/R shunts, it reflects the degree of hyperperfusion of the lung relative to systemic perfusion and therefore contributes to the degree of heart failure of the LV and the risk of development of an increase in PVR.

In all noncyanotic heart defects Qp/Qs is 1 or greater, i.e., there is no mixing of deoxygenated blood in the systemic ventricle (or the atrium), and only “saturated” blood is ejected into the systemic circulation.

Conversely, in cyanotic defects, an R/L shunt occurs with the mixing of nonoxygenated blood with the systemic perfusion: As a result, organ perfusion may be compromised; this tends to be due more to reduced oxygen delivery rather than to low cardiac output.

In the univentricular heart (a mixing chamber) with a constant SvO₂ and SpvO₂ (= pulmonary venous saturation), SaO₂ varies according to the Qp/Qs ratio (SpO₂ ≈ SaO₂).

Qp/Qs = 1 with SpO₂ about 80%, as long as SpvO₂ ≈ 100% is the case:

Formula 46

Qp/Qs = (SaO₂ − SvO₂)/(SpvO₂ − SpaO₂)

Calculation: (80–60) / (100–80) = 1

(Since SpvO₂ can only be determined by cardiac catheterization, the only clinical option remains an approach by means of SpO₂/SaO₂ and pulmonary function.) An ideal pulmonary venous saturation (SpvO₂) of 100% is assumed in the calculation:

Qp/Qs = 2 with SaO₂ 90%, “good” SvO₂ (70%): (90–70)/(100–90) = 2. (This condition does not persist for long since, with predominant pulmonary circulation, systemic cardiac output is insufficient in the long term for a “good” SvO₂.)

• Instead, with SpO₂ of 90%, a SvO₂ of 40% or less is seen:

• Qp/Qs = 5: (90–40)/(100–90) = 5 → threat of massive heart failure from pulmonary flooding.

• With low systemic arterial saturation (60% and good lung), cardiac output may be fine, but pulmonary flow is impaired. Example SvO₂ = 40

• Qp/Qs = 0.5 with a calculation: (60–40)/(100–60) = 0.5

If there is a decrease of SpvO₂ (“pulmonary disorder”) or SvO₂ (fall in cardiac output, Hb, SaO_2, or increase of VO₂), the resultant SaO₂ naturally also falls accordingly, and the actual values would have to be used in the formula:

Qp/Qs = 3 with SpO₂ 80% SvO₂ 50%, but SpvO₂ only 90%

It is clear from this example that the ratio of pulmonary to systemic perfusion in univentricular circulation can be deduced not just from the transcutaneously measured saturation.

6 Considerations for Pressure Gradients

Pressure gradients develop at stenoses, vascular connections, or septal defects. The pressure gradient at a stenosis or a defect depends on various factors:

• 1. The extent of the constriction or the size of the defect

• 2. The amount of blood flow via the stenosis

• 3. The pressure or resistance conditions before and after the stenosis or the defect

These conditions can be described by means of a VSD. If the VSD is large (nonpressure-separating), the pressures of both ventricles equal out (LVP = RVP). With a relatively small VSD (pressure-separating), there is a pressure gradient. If left ventricular systolic pressure decreases (reduced SVR, reduced contractility), the pressure gradient also is reduced without any change occurring in the defect. When RV pressure is high (e.g., neonatally or in association with pulmonary hypertension), the gradient also is lower if LV pressure is unchanged.

7 Classification of Heart Defects

Table 14.2 contains a simplified classification of heart defects, while Chap. 13 discusses them in greater detail.

Table 14.2 Simplified classification of heart defects

If we look at non-cyanotic heart defects with an L/R shunt taking VSD as an example, the blood recirculating in the pulmonary circulation generates a pressure and volume overload of the pulmonary vascular bed and a volume overload of the left heart. The higher the pulmonary hypercirculation, the less volume is available for the systemic circulation (Qs). This not only impairs peripheral organ perfusion (resulting in activation of the renin-angiotensinaldosterone system = RAAS ↑), but also limits LV reserves.

In the presence of hypercirculation and poor left ventricular function, left atrial pressure, and hence PCWP, also increases in parallel and generates an increase in post-capillary pulmonary resistance with a tendency to pulmonary edema. This in turn results in a compensatory increase in precapillary pulmonary vascular resistance to protect against pulmonary edema.

Right ventricular afterload is thereby increased with the possible consequence of RV failure (hepatic congestion and peripheral edema) and a further reduction in organ perfusion (RAAS ↑↑). The increase in pulmonary vascular resistance becomes established in the second 6 months of life, so that in the event of a severe (particularly post-tricuspid) L/R shunt, surgery should be performed by that time.

In cyanotic defects, nonoxygenated blood is mixed with the blood in the systemic ventricle. This is possible through various shunts and hence different types of circulatory anatomy, which need to be treated differently (particularly in the preoperative phase).

In the case of cyanotic heart defects with reduced pulmonary perfusion, the right ventricle is obstructed, e.g., at the inlet (tricuspid valve) or outlet (pulmonary valve). An ASD or VSD is essential in order to conduct the caval blood to the systemic side (R/L shunt). The same applies to univentricular hearts. In these patients, Qp is < Qs, and massive hypoxemia develops as soon as pulmonary perfusion is limited by closure of the ductus arteriosus (PDA-dependent pulmonary perfusion).

These patients are at risk from hypoxemic acidosis, although the systemic perfusion is initially wholly sufficient; obviously, acidosis and reduced O₂ delivery (DO₂↓) also affect ventricular function secondarily. The focus of preoperative therapy thus lies on keeping the PDA open, administering O₂ (as long as a pulmonary component plays a role in hypoxia), and providing sufficient O₂ transporters (Hb > 12–14 g/dL). Naturally the channeling of blood to the systemic ventricle must not be obstructed in these heart defects either (e.g., restrictive ASD), since otherwise this results in inflow congestion and an excessively low cardiac output.

A good balance between systemic perfusion and pulmonary perfusion is achieved in the patient with PDA-dependent circulation (assuming constant pulmonary ventilation and pulmonary perfusion) with arterial saturation of about 80% (Qp/Qs about 1:1).

This contrasts with left heart obstructions with reduced systemic perfusion. In this case, structures of the left heart or aorta are malformed so that the right ventricle is entirely or largely responsible for systemic perfusion via the open PDA (PDA-dependent systemic perfusion in HLHS, critical aortic stenosis, high grade CoA, or hypoplastic aortic arch).

Oxygenation of the blood in this case is initially unproblematic, but the systemic supply is compromised by reduced perfusion on PDA closure or decreasing pulmonary vascular resistance and the associated pulmonary hypercirculation. As well as maintaining PDA and ensuring the outflow of the oxygenated blood from the left to the right atrium (a highly restrictive atrial septum here results in pulmonary congestion), the primary measures here are those that improve systemic perfusion (careful afterload reduction), that do not reduce pulmonary vascular resistance (no O₂, no ventilation if not clinically necessary, and no hyperventilation), and that improve the heart function of the right ventricle, which now carries the systemic circulation.

With other univentricular heart defects in which there is no inflow obstruction of the pulmonary or systemic circulation, the postnatally high PVR balances the pulmonary to systemic perfusion ratio. However, with decreasing resistance in the pulmonary vascular bed, this leads here to pulmonary hypercirculation with the aforesaid consequences for pulmonary and systemic perfusion. Restricting the blood flow to the pulmonary vascular bed is of primary importance here in the initial therapy.

In any understanding of the preoperative approaches to treatment (including TGA, the circulatory conditions of which are addressed in detail later), knowledge of the possibilities of influencing the pulmonary/systemic vascular bed, heart function, and pulmonary function are vital.

8 Influencing Pulmonary Artery Resistance

For details on how to influence pulmonary artery resistance, refer to Table 14.3 and Chap. 9.

Table 14.3 Influences on pulmonary arterial and systemic vascular resistance