Cardiopulmonary monitoring

Non-invasive cardiopulmonary monitoring
Non-invasive cardiopulmonary monitoring includes data on heart rate, respiratory rate, blood pressure (non-invasive NIBP measurement), pulse oximetry and ECG.

Heart rate and respiratory rate (HR, RR)
Continuous monitoring with adjustable alarm values is best. Approximately, HR can be determined by counting the heartbeats over 6 seconds and multiplying the result by 10. When assessing RR, it is necessary to determine the value by counting over a 1-minute period, as the respiratory rate is slower and more variable than the heart rate and calculating over a shorter period of time would risk miscalculation. We always set upper and lower limits for the alarm, and the values depend on the age of the child and the underlying disease. In terms of RR, the most important thing is an alarm to catch apnoea, which is a superurgent condition in medicine. We usually set the threshold for apnoea at 15 seconds.

Non-invasive blood pressure measurement (NIBP)
In general, BP measurements should be a routine for children older than 3 years; it is clearly preferred to use a mercury manometer with auscultatory measurement. When measuring BP, simultaneous measurement of the child's height and weight should become the rule. If BP values exceed 90-95 during the measurement, BP is automatically measured on the lower limb as well. BP on the lower limbs tends to be 10-20 mmHg higher. In no case should the upper limb BP exceed the lower limb BP. In this case, we strongly suspect aortic coarctation.

In some children, echoes are audible when measuring BP, possibly down to 0 mmHg. In these cases, it is recommended to repeat the measurement with less pressure on the head of the stethoscope. If the echoes are still audible up to 0 mmHg, the diastolic BP value should be recorded at the first obvious weakening of the echoes. The width of the cuff of the tonometer should be approximately 40% of the circumference of the arm; a narrow cuff is the source of falsely high BP values, while a cuff that is too wide is the cause of falsely lower BP values (in this case, however, the significance of the error is small). The width of the cuff of the tonometer means the inner, i.e. rubber, part of the cuff.

The arm on which the BP is measured should be completely free (the child should be at least halfway undressed), the cuff should be placed in the middle of the arm between the olecranon and the acromion. If borderline BP values are detected, the measurement should be repeated.

Non-invasive BP monitoring:


 * oscilometric (most common)
 * sphygmomanometric (mercury tonometer) – determine Korotkov phenomena (the accuracy of the measurement affects the correct choice of cuff)
 * doppler principle

In children in the ICU, the mercury tonometer is disadvantageous in the youngest children, in uncooperative children and when frequent measurements are necessary.

Doppler technique is suitable for young children and conditions with impaired perfusion. A small Doppler probe is placed over the radial or brachial artery. The blood movement is well detected by sensitive ultrasound. A cuff placed on the upper arm is inflated until the Doppler signal has completely disappeared. It is then slowly deflated. Systolic pressure is read when the first Doppler signal appears, diastolic pressure is read when the length and quality of the signal decreases. Correlation with pressure measured directly intra-arterially is good, but the method is not suitable for continuous measurement.

The oscillometric method is easy to implement. When the cuff is inflated, the blood flow in the artery causes oscillations. If the pressure in the cuff begins to drop, the device registers the sBP, dBP and MAP. However, all techniques have limitations in conditions with a significant decrease in cardiac output, severe hypotension or systemic vasoconstriction, conditions with generalized edema, and extreme obesity. In addition to systolic BP and diastolic BP, the determination of mean arterial pressure (MAP) is very important. MAP represents the organ perfusion pressure and is useful to assess circulatory failure and to define hypotension. It is not the arithmetic mean of systolic sBP and diastolic dBP. MAP = (sBP + 2x dBP) / 3 Indirect methods of BP measurement have limited accuracy, therefore, intra-arterial monitoring is necessary in severe conditions such as shock, rhythm disturbances, and administration of vasoactive agents.

Perfusion pressure is of great significance in shock conditions. Arithmetically, it is the difference between MAP and CVP: $$PP = MAP - CVP$$

Pulse oxymetry
Pulse oximetry non-invasively measures the oxygen saturation of haemoglobin in the arterial part of the bloodstream (pulsatile flow).

Astrup – blood gas testing
The aim of the blood gas examination is to obtain data to assess the oxygenation function of the lungs, the adequacy of alveolar ventilation and, together with other biochemical parameters, to detect the possible existence of an ABR disorder and to determine the degree of its compensation.

Capnometry, capnography
The measurement of the CO2 concentration (capnometry) and the graphical representation of this value (capnography) in exhaled air is based on the measurement of the absorption of infrared light.

Under normal circumstances, the gradient between arterial tension paCO2 and end-expiratory CO2 tension (end-tidal CO2 = etCO2) is 2-5 torr (0.25-0.66 kPa) and reflects the size of the ventilatory dead space and the ratio of the size of the tidal volume to the dead space. An increase in anatomical or alveolar dead space under pathological conditions in which pulmonary perfusion is reduced leads to an increase in the gradient between paCO2 and etCO2. In practice, this change is usually manifested by a decrease in etCO2.

Clinical causes of the increase in the gradient between paCO2 and etCO2:


 * enlargement of the anatomical dead space;
 * enlargement of the alveolar dead space;


 * hypotention;
 * low cardiac output;
 * high PIP and/or PEEP;
 * pulmonary embolism;
 * bronchospasm

PF index
In patients with severe forms of pulmonary dysfunction, the PF index = hypoxemic index = Horowitz index is often used to assess the oxygenation function of the lungs. Its determination requires blood gas testing with knowledge of FiO2. The actual value is strongly dependent on the FiO2 parameter used and the level of airway pressures at the time of the blood gas examination. In patients with hypercapnia, the effect of changes in partial pressure of CO2 may also need to be taken into account, as a significant rise in alveolar partial pressure of CO2 results in a decrease in pAO2 and subsequently in paO2. PFi = paO2 : FiO2


 * FiO2 is given as a decimal number
 * paO2 is given in torr

Normal values are > 500, values < 300 represent acute lung injury, and values < 200 are one of the criteria defining ARDS. PFi < 200 corresponds to a lung shunt value > 20%.

Alveolo-arterial oxygen gradient A-aDO2
The alveolar-arterial oxygen gradient A-aDO2, sometimes referred to as the alveolar-arterial difference, is a parameter used to assess the degree of impaired oxygenation of the lungs. It primarily indicates the quality of alveocapillary diffusion.

A–aDO2 = pAO2 &minus; paO2 A–aDO2 = (760 x FiO2) &minus; {(paO2 + paCO2) + 47}


 * FiO2 is given as a decimal number
 * paO2 and paCO2 are given in torr
 * 760 = barometric pressure at sea level
 * 47 = partial pressure of water vapour in the inhaled air

This formula can be used if data on inhaled O2 concentration and arterial gas values are available. Values >350 are indicative of respiratory insufficiency, values >550 are one of the criteria for ECMO (extracorporeal membrane oxygenation).

Oxygenation index
The oxygenation index OI is widely used in paediatrics, unlike the PFi it also reflects pressure changes. OI = (FiO2 x Pmaw) : paO2


 * FiO2 is given in percentage!
 * Pmaw is given in cmH2O.
 * paO2 is given in torr.

Normal values are < 5.

Dead space ventilation
For an indicative assessment of the relationship between the size of the functional dead space and the size of the tidal volume (Vd/Vt), the difference between the arterial tension of CO2 and the tension of CO2 in the exhaled mixture at the end of expiration (etCO2) is used. Under normal circumstances, this difference is minimal (2-5 torr), but under pathological circumstances it increases significantly.

Vd/Vt = (paCO2 &minus; etCO2) : paCO2 If an increase in airway pressure (e.g. after PEEP adjustment) results in an increase in this parameter without a concomitant beneficial effect of the increase in pressure on oxygenation, this may be considered a sign of an exceedance of the optimal airway pressure. Similarly, changes in the magnitude of the Vd/Vt ratio may occur when cardiac output or pulmonary pressures decrease.

In normal subjects, the Vd/Vt value is in the range of 0.2-0.3. A rise in Vd/Vt is associated with the development of both hypoxemia and hypercapnia. Hypercapnia usually occurs when Vd/Vt is greater than 0.5.

Gastric tonometry
The principle of the method is the regional measurement of the partial pressure of CO2 (PtCO2) of the gastric mucosa. Using this method, we can detect very early perfusion disturbances of the splanchnic region, which will be manifested by a very early rise in mucosal PtCO2.

Invasive BP measurement
Measuring arterial blood pressure is an essential part of monitoring any acute condition. The mean arterial pressure MAP depends on cardiac CO output and systemic SVR resistance: MAP = CO x SVR For children, it is necessary to use the indexed values of these parameters, i.e. the values relative to the body surface. Then the equation will look like this: MAP = CI x SVRI The equation itself shows the limits in the measurement of arterial blood pressure. Blood pressure does not inform about blood flow. It can therefore be normal even with increasing peripheral resistance and simultaneously decreasing cardiac output, and therefore with reduced organ blood flow. Thus, we consider MAP as only a crude indicator of organ perfusion, especially since many organs have the ability to autoregulate, i.e., their perfusion is kept constant over a wide range of perfusion pressures through changes in vascular resistance.

Arterial BP is measured directly or indirectly. Indirect methods are simple and non-invasive. Direct methods are more accurate. The differences between indirect and direct blood pressure measurement are especially obvious in shock, hypertension, hypothermia, and obesity.

Advantages of direct BP measurement:


 * continuous monitoring;
 * consistent measurement accuracy;
 * rapid recognition of circulatory disorders;
 * direct monitoring of the hemodynamic effects of heart rhythm disturbances;
 * indirect assessment of myocardial contractility from the rate of rise of the arterial pressure curve;
 * estimation of pulse volume from the systolic part of the pressure curve;
 * access to the artery to take blood samples: the Astrup and other laboratory.

Indication:


 * hemodynamically unstable patient: shock states, hypertensive crisis, hypotension;
 * intracranial hypertension;
 * the need to administer vasoactive substances: catecholamines, sodium nitroprusside;
 * ventilationally unstable patient (need for repeated and frequent blood gas testing);
 * the need for repeated blood draws;
 * regular blood sampling;
 * angiographic examination;
 * hemofiltration/hemoperfusion.

Central venous pressure, CVP
A central venous catheter is a catheter whose distal end lies in a hollow vein. Normal values of central venous pressure CVP are 2-12 cm H2O (ideal 3-10 cm H2O).

Transfer relations:


 * 1 cm H2O = 0,74 mmHg
 * 1 mmHg = 1,36 cm H2O
 * 1 kPa = 7,5 mmHg = 10,2 cm H2O

Reduced CVP values are found in hypovolemia.

Increased CVP values in hypervolemia, right heart insufficiency, pulmonary embolism, superior vena cava obstruction, cardiac tamponade.

Catheters for long-term insertion are coated with an antibacterial surface. Currently, all catheters are radiocontrast. To eliminate risks, the latest catheters are fitted with a one-way valve to prevent air embolism.

When choosing an access to the superior vena cava, the following factors in particular should be taken into account: the experience of the physician with a particular method, the accessibility of the veins suitable for puncture, the risks of each approach for a particular patient and the expected time of catheter insertion.

For long-term cannulation, we prefer a central approach (v. jugularis interna, v. subclavia) because catheters inserted in this way have a lower risk of infectious and thrombotic complications than catheters inserted from the periphery (swimming catheters). Never introduce catheters through an infected puncture site.

Central venous cannulation is a very common procedure in intensive care. The availability of quality sets has expanded its use and safety. In this context, it is appropriate to emphasize the need for correct indication and professional humility of the physician in the decision-making process, including strict adherence to the methodology for each approach.

We need to check the position of each CVK and adjust if necessary to avoid severe complications. The optimal position is immediately in front of the orifice of the superior vena cava into the right atrium. There are no longer any venous valves in this area. Two procedures are appropriate: chest X-ray and ECG check with a catheter tip lead. On the chest radiograph, the carina tracheae is an important landmark for the position of the tip of the central venous catheter. The carina always lies cranial to the pericardium. For safety reasons, the end of the catheter must lie just above the carina. ECG diagnosis is simple, without great expense, and should therefore be preferred to the less prompt and more expensive radiographic imaging. The position of the catheter tip in the right atrium is indicated by a clearly elevated P wave in the ECG image on the monitor. The catheter is then advanced until the normal P wave reappears on the monitor.

Saturation of hemoglobin in the central venous system - SvO2
In the critically ill patient, it is essential to determine whether the oxygen supply to the tissues is adequate in relation to the tissue oxygen demand. SvO2 monitoring allows a fairly accurate determination of the ability to determine whether tissue oxygen demand is in balance with oxygen delivery. SvO2 represents the average percentage of oxygen bound in mixed venous blood.

The delivery of oxygen to the tissues is a fundamental task of the cardiovascular system and directly depends on cardiac output, arterial blood O2 saturation SaO2 and haemoglobin concentration. Adequate oxygen delivery to the tissues is ensured by a SaO2 > 92% and an optimal hemoglobin value (depending on the age of the child). Cardiac output is an equally important component of maintaining good tissue oxygenation. In critical conditions, the effort is always to maximize cardiac output by manipulating preload, afterload, contractility and heart rate.

The physiological SvO2 value represents a range of 60-80% and means that the tissue oxygen demand is covered by a sufficient supply. In case of significant deviations, the values of SaO2, Hb, CO/CI and O2 consumption should always be reassessed.

Changes in SvO2 values that require reassessment of the patient's condition:


 * change of plus/minus 10 % persisting for at least 5 minutes
 * decrease < 60 % or increase > 80 %
 * trend showing a gradual but steady decline

Drop in SvO2 indicates that the patient needs to use up O2 stores to meet their needs. This occurs when the O2 supply decreases despite an equal or increased O2 requirement, or when the O2 requirement increases despite an equal or decreased O2 supply.

An increased SvO2 value is observed in the case of increased O2 supply despite equal or decreased demand, or decreased O2 demand despite equal or increased supply.

Reduced value of SvO2:


 * reduced supply of O2:
 * reduced cardiac output,
 * high PEEP,
 * cardiogenic shock,
 * hypovolemia,
 * hypotension,
 * arrhythmias.
 * reduced SaO2:
 * hypoxia,
 * respiratory failure,
 * dyspnoea,
 * decrease in hemoglobin (anemia, bleeding).
 * increased need for O2:
 * hyperthermia,
 * pain,
 * increased physical activity,
 * cramps,
 * increased respiratory work.

Conditions with increased value of SvO2:


 * increased supply of O2:
 * increased cardiac output (inotropics, sepsis),
 * increase in SaO2 (high FiO2, hyperoxia),
 * increased Hb (transfusions).
 * reduced need for O2:
 * hypothermia,
 * anesthesia.
 * other causes of the increase in SvO2:
 * VVV heart with L-R shunt,
 * tissue necrosis,
 * nitroprusside toxicity,
 * septic shock.

Oxygen delivery (DO2)
Oxygen delivery (oxygen delivery, DO2) is directly proportional to cardiac output and arterial blood oxygen content (arterial oxygen content, CaO2). For paediatrics, we always choose indexed values, i.e. values relative to body surface area. DO2 (index) = CI x CaO2 x 10

CI = HR x SV

CaO2 = (Hb x 1,34 x SaO2) + (0,003 x PaO2)

CvO2 = (Hb x 1,34 x SvO2) + (0,003 x PvO2)

a–v DO2 = CaO2 &minus; CvO2

DO2 = oxygen delivery, represents oxygen delivered to tissues per minute, reference values DO2 = 550–650 ml/min/m2

SV = stroke volume

HR = heart rate

CI = cardiac index → cardiac output per unit body surface area

CaO2 = oxygen content in arterial blood, reference values CaO2 = 17–20 ml

CvO2 = oxygen content in mixed venous blood, reference values CvO2 = 12–15 ml

SaO2 = O2 saturation in arterial blood, is referred to as SaO2/100

SvO2 = saturace smíšené žilní krve, je uváděna jako SvO2/100

PaO2 = partial pressure of oxygen in arterial blood, is given in torr

PvO2 = partial pressure of oxygen in mixed venous blood, is given in torr

a–v DO2 = arteriovenous oxygen content difference (oxygen content difference), reference values a–v DO2 = 3–5 ml/dl

Hb = hemoglobin, given in quantity of g/dl !

Oxygen consumption (oxygen uptake, VO2)
The measure of O2 consumption is VO2 (oxygen consumption, oxygen uptake), reference values VO2 (index) = 120–200 ml/min/m2

VO2 (index) = CI x (CaO2 &minus; CvO2) x 10 The basic task of the cardiopulmonary unit is to ensure the balance between VO2 and DO2. The balance is determined by :


 * oxygen content in mixed venous blood CvO2
 * O2 extraction (O2ER), i.e. the ratio between the amount of oxygen consumed and delivered VO2/DO2, expressed as a percentage.

Normal extraction values are around 25%, but with significantly increased tissue demand, O2 extraction can rise to 50%. In shock conditions, we try to keep O2 extraction below 30%. O2ER = VO2 / DO2

Both CvO2 and O2ER depend on the values of mixed venous blood SvO2 saturation and cardiac CO output. CO/CI depends on the values of heart rate, heart volume, preload, afterload and contractility. Increasing heart rate, improving myocardial contractility and relaxation in diastole, and optimizing preload and afterload increase CO/CI. Oxygen carrying capacity can be improved by optimizing hematocrit. By improving all these parameters, DO2 can be increased. In some specific situations (fever, sepsis, trauma, thyrotoxicosis), metabolic needs may exceed even normal DO2.

PiCCO System
The PiCCO (Pulse Contour Cardiac Output) system is a less invasive method than the Swan-Ganz pulmonary artery catheter - it requires the insertion of a central venous catheter and a thermodilution arterial catheter (inserted via the a.axillaris or a.radialis or, more commonly, a.femoralis) to determine cardiac output, without the need for pulmonary artery catheterization. With this system, in addition to cardiac output, preload volume parameters can be determined and pulmonary oedema quantified.

Cardiac output is measured intermittently by transpulmonary thermodilution and continuously by heart rate curve analysis. During the three bolus thermodilution measurements, the shape of the heart rate curve is analysed and calibrated; cardiac output is then continuously monitored by comparing these "calibrated" curves with several consecutive heart rate curves. Since a regular heart rhythm is required, the system fails in the presence of arrhythmias (e.g. atrial fibrillation).

In case of sudden fluctuations in hemodynamics, it is necessary to re-calibrate using thermodilution (standard calibration is performed after at least 6 hours).

The PiCCO system uses thermodilution curve analysis and knowledge of individual volumes (end-diastolic volumes of both chambers and atria) from thermodilution measurements between the application site and the detection of the tracer (solution of known temperature). Further, from the volumes determined by thermodilution techniques between the site of application and detection, the "extravascular lung water" (EVLW) can be calculated to quantify pulmonary edema. This is the difference between total lung fluid content (pulmonary thermal volume, PTV) and intravascular lung fluid (pulmonary blood volume, PBV).

LiDCCO System
A variant of this system is a system using lithium chloride dilution (LiDCO) instead of thermodilution. Calibration is performed by detecting the presence of LiCl in peripheral arterial blood (a.radialis) after its bolus administration into the venous part of the vasculature. Cardiac output is continuously monitored by subsequent comparison of heart rate curves.

In addition to routine methods such as CVP or aTK measurements, modern thermodilution methods and the possibility of arterial pressure pulse curve analysis (e.g. the PICCO method) allow more detailed hemodynamic parameters to be determined. For paediatric purposes, the most important are the indexed values of the individual parameters, which are related to the body surface and thus allow comparison between the values of different patients.

Parameters defining preload
In addition to CVP (the pressure parameter defining right ventricular preload), which is the most commonly used marker of preload, a number of other parameters can be monitored as part of more detailed haemodynamic measurements:


 * global enddiastolic volume (GEDV) indicates the volume of blood contained in all 4 cavities of the heart at the end of diastole
 * intrathoracic blood volume (ITBV) indicates the volume of blood contained in all 4 cavities of the heart at the end of diastole + the volume of blood in the pulmonary vessels

ITBV and GEDV show greater sensitivity and specificity to determine cardiac preload than standard CVP and PAWP filling pressures, but also than right ventricular end-diastolic volume calculated by echocardiography. Another advantage of ITBV and GEDV is that they do not interfere with artificial pulmonary ventilation. In children, as mentioned above, the indexed values, i.e. GEDVI and ITBVI, should be used.

n patients on UPV we can use another hemodynamic parameter - stroke volume variation (SVV - dynamic parameter). SVV reflects changes in cardiac preload in relation to UPV cycles. A rise in SVV may predict the need for volume expansion

Parametry defining afterload
In practice, the systemic and pulmonary vascular resistance are evaluated (based on Ohm's law) as a determinant of afterload. Knowing the values of CO, we can calculate the value of systemic vascular resistance (SVR): SVR = (MAP &minus; CVP) x 80 / CO PP = MAP &minus; CVP SVR = (MAP &minus; CVP) x 80 / CO = PP x 80 / CO


 * PP = perfusion pressure; difference between mean arterial pressure and central venous pressure

The indexed SVR value related to body surface area is SVRI : SVRI = (MAP &minus; CVP) x 80 / CI = PP x 80 / CI

Based on these relationships, it is therefore possible to increase cardiac output by reducing vascular resistance, but it also follows that good blood pressure does not necessarily mean good cardiac output - vascular resistance can rise at the same time as cardiac output falls!

By analogy, for pulmonary vascular resistance, : PVR = (MPAP &minus; PAOP) x 80 / CO respectively, PVRI = (MPAP &minus; PAOP) x 80 / CI

MPAP is the mean pulmonary artery pressure and PAOP is pulmonary artery opening pressure. .

Extravascular pulmonary water
Extravascular lung water (EVLW) and its index EVLWI indicate the volume of free water in the lungs and allow bedside quantification of the severity of pulmonary edema. In addition to pulmonary edema, it correlates with the severity of ARDS or the duration of UPV. It is a better indicator of pulmonary oedema than chest X-ray.

Contractility
Contractility is the intrinsic inotropic activity of the myocardium independent of preload and afterload. It is influenced by ionized calcium, compliance and myocardial delivery of energy substrates.

An indicator of contractility is the ability to exert pressure per unit time, in practice it is used :

LVSW = 0,0136 x SV x (MAP &minus; PAOP) RVSW = 0,0136 x SV x (MPAP &minus; CVP)
 * values of left and right ventricular stroke work: LVSW and RVSW (left/right ventricular stroke work)


 * global ejection fraction (GEF) and cardiac function index (CFI) derived from parameters measured by the PiCCO system;
 * the level of myocardial contractility can also be estimated from the steepness of the rise in the pulse curve during direct measurement of arterial pressure.

Cardiac Output
Within the limits of more detailed haemodynamics we are able to determine the stroke volume (SV). Based on this value, we can calculate cardiac output (CO), which is the product of stroke volume and heart rate: CO = HR × SV By converting to body surface area we get cardiac index = CI.

Calculation of CO using Fick's formula: CO = {VO2 / (CaO2 &minus; CvO2)} × 10

BP measurements in pulmonary artery wedging, PAWP
The PAWP value is measured with a Swan-Ganz catheter. It is the resultant of pulmonary resistance and left heart function. Its values are close to the left atrial pressure. It is used to determine the exact CI. It has a rare application in paediatrics.


 * reference values: 6–16 cm H2O (ideally 7–15 cm H2O)

Indications for Swan-Ganz catheter insertion:


 * unclear intravascular volume
 * PEEP > 12 cm H2O
 * heart failure
 * need for intensive inotropic myocardial support

Source

 * HAVRÁNEK, Jiří: Kardiopulmonální monitoring