Estimating mean circulatory filling pressure in clinical practice: a systematic review comparing three bedside methods in the critically ill

Abstract

The bedside hemodynamic assessment of the critically ill remains challenging since blood volume, arterial–venous interaction and compliance are not measured directly. Mean circulatory filling pressure (P_mcf) is the blood pressure throughout the vascular system at zero flow. Animal studies have shown P_mcf provides information on vascular compliance, volume responsiveness and enables the calculation of stressed volume. It is now possible to measure P_mcf at the bedside. We performed a systematic review of the current P_mcf measurement techniques and compared their clinical applicability, precision, accuracy and limitations. A comprehensive search strategy was performed in PubMed, Embase and the Cochrane databases. Studies measuring P_mcf in heart-beating patients at the bedside were included. Data were extracted from the articles into predefined forms. Quality assessment was based on the Newcastle–Ottawa Scale for cohort studies. A total of 17 prospective cohort studies were included. Three techniques were described: P_mcf hold, based on inspiratory hold-derived venous return curves, P_mcf arm, based on arterial and venous pressure equilibration in the arm as a model for the entire circulation, and P_mcf analogue, based on a Guytonian mathematical model of the circulation. The included studies show P_mcf to accurately follow intravascular fluid administration and vascular compliance following drug-induced hemodynamic changes. Bedside P_mcf measures allow for more direct assessment of circulating blood volume, venous return and compliance. However, studies are needed to determine normative P_mcf values and their expected changes to therapies if they are to be used to guide clinical practice.

Electronic supplementary material

The online version of this article (10.1186/s13613-018-0418-2) contains supplementary material, which is available to authorized users.

Background

It is difficult to determine the cause for hemodynamic instability in patients and to predict the best treatments. Currently, cardiovascular resuscitation options are triggered by arterial pressure and cardiac output (CO) measures, focusing on the oxygen delivery side of the circulation. However, the primary determinants of CO reside on the venous side. Veins are 30–50 times more compliant than arteries and contain approximately 75% of the total blood volume [1–5]. Mean circulatory filling pressure (P_mcf) provides vital information on this “forgotten venous side of the circulation” [6].

In 1894, P_mcf was defined as the equilibrium pressure throughout the circulation during circulatory arrest [7]. In the 1950s, Guyton and colleagues described a linear relationship between venous return (V_R) and right atrial pressure (P_ra), described as: V_R = (P_mcf − P_ra)/(RVR) [8, 9]. RVR is resistance to V_R and defines the slope of the V_R curve. This linearity has been confirmed in intact circulations in animal studies and is not affected by hypo- or hypervolemia [10–15]. V_R curves enable to determine the equilibrium point of the circulation, which is the intersection between the CO and V_R curve. Central venous pressure (CVP) is a surrogate of P_ra used in clinical practice. CVP at zero flow equals P_mcf (Fig. 1).

Fig. 1.

The venous return curve (a) combined with the cardiac output curve (b). The intersection of these two curves (c) is the working point of the circulation. The central venous pressure when venous return equals zero is the P_mcf (d). The slope of the V_R is determined by the resistance to venous return

Vascular volume requires a minimal volume before its distending pressure becomes positive. The amount of blood not causing pressure on the vessels is called unstressed volume (V_u) and reflects intravascular volume present with P_mcf of zero. Stressed volume (V_s) is the additional blood causing a distending pressure on the vascular walls and reflects the effective circulating volume. V_u and V_s together define the total blood volume. V_s is approximately 25% of the total blood volume [3–5]. V_s and vascular compliance (Csys) define P_mcf [16]. An increase in V_s increases P_mcf, and an increase in Csys decreases P_mcf. Fluid loading should increase P_mcf, but V_R only increases if the pressure gradient for V_R (i.e., P_mcf CVP) increases, RVR decreases, or both. Since in the steady state V_R = CO, knowing the determinants of V_R is relevant to understanding cardiovascular state.

Recently, methods have emerged to enable clinicians to estimate P_mcf at the bedside. Our objectives for this review were to describe the techniques and to highlight their clinical applicability, precision, accuracy and limitations in critically ill patients.

Materials and methods

Publication selection

This review was performed according to PRISMA guidelines [17] (Additional file 1) and methodology outlined in the Cochrane Handbook for systematic reviews [18]. No study protocol was published. A PubMed, Embase and Cochrane Library database search was performed with help of a clinical librarian with no restriction on publication date. The search was performed up to May 18, 2017. The search strategy combined the following concepts: (1) “mean systemic filling pressure” or “mean circulatory filling pressure” or “static filling pressure” and (2) “intensive care” or “critical care” or “perioperative” or “intraoperative” (Additional file 1). Titles, abstracts and full-texts were independently screened by two reviewers for relevance (MW and DPS), and discrepancies were resolved by a third reviewer (BFG). The references of the selected articles were examined for additional eligible articles. Studies were included when available in English and full-text, described prospective studies in which P_mcf estimation methods were examined in heart-beating ICU patients and contained a description of their clinical applicability, precision and accuracy or limitations.

Data extraction and analysis

Data were extracted into predefined forms. No additional analyses were performed. Critical appraisal was based on the Newcastle–Ottawa Scale for cohort studies [19] to assess the quality of non-randomized studies at study level. A modified version of the scale was used since only five out of nine questions were applicable, resulting in a possible highest score of five stars (Additional file 1).

Results

Study selection and characteristics

The initial search identified 369 articles, of which 300 were excluded after screening title and abstract. A total of 53 articles were excluded based on full-text. Two relevant articles were found by citation tracking. Consequently, 17 prospective cohort studies estimating P_mcf in heart-beating ICU patients were included (Additional file 1). Three different bedside measurement techniques were found. Eight studies estimated P_mcf applying inspiratory hold maneuvers (P_mcf hold), three studies during a circulatory stop-flow in the arm (P_mcf arm) and four studies using a mathematical algorithm (P_mcf analogue). Two studies compared multiple techniques.

Eleven studies were performed in postoperative cardiac surgery patients (Table 1). All patients were hemodynamically stable without alteration in vasopressor use or fluid therapy during the study protocol. All patients were sedated and mechanically ventilated. In one study, spontaneous breathing efforts were observed [20]. The number of included patients ranged from nine to 80. In all studies, CVP was measured via a catheter in the right internal jugular vein. CO measurement techniques differed between studies (Additional file 1).

Table 1.

Baseline characteristics for included studies

References	Method	N	Patient population (all adult ICU patients)	Age	Male	Timeframe Pmcf measurement
Maas et al. [21]	Pmcf hold	12	Postoperative cardiac surgery	64 (10)	10 (83%)	Not described
			10 CABG
			2 AVR
Keller et al. [23]	Pmcf hold	9	Postoperative cardiac surgery	Median 61	4 (44%)	Not described
			3 CABG	IQR 55–75
			6 AVR
Maas et al. [22]	Pmcf hold	10	Postoperative cardiac surgery	64 (11)	9 (90%)	Within 1 h after ICU admission
			2 AVR
			1 MVP + TVP
			7 CABG
Persichini et al. [27]	Pmcf hold	16	Septic shock	67 (16)	8 (50%)	Not described
Maas et al. [25]	Pmcf hold	16	Postoperative cardiac surgery	64 (11)	Not described	Within 1 h after ICU admission
			1 MVP
			15 CABG
Guerin et al. [28]	Pmcf hold	30	Shock	65 (12)	21 (70%)	Not described
De Wit et al. [24]	Pmcf hold	17	Postsurgical gastrointestinal	62 (9)	14 (82%)	Not mentioned
			16 esophageal resection
			1 pancreaticoduodenectomy
Helmerhorst et al. [26]	Pmcf hold	22	Postoperative cardiac surgery	63 (59–66)	17 (85%)	1 h after ICU admission
			22 CABG
Geerts et al. [43]	Pmcf arm	24	Postoperative cardiac surgery	64 (10)	19 (79%)	Within 2 h after ICU admission
			17 CABG
			7 CABG plus valve repair
Aya et al. [41]	Pmcf arm	20	Postoperative cardiac surgery	63 (11)	17 (85%)	Initial period at ICU (not further defined)
			13 CABG
			4 AVR
			4 MVR
Aya et al. [42]	Pmcf arm	80	Postoperative cardiac surgery	70	62 (78%)	Initial period at ICU (not further defined)
			36 CABG	Range 52–80
			27 AVR + CABG
			12 MVR + CABG
			5 Other
Parkin et al. [49]	Pmcf analogue	10	Multi-organ failing patients receiving CVVH for acute renal failure	65	7 (70%)	Not described
				Range 24–77
Cecconi et al. [48]	Pmcf analogue	39	22 Cardiac surgery	68 (12)	26 (67%)	Not described
			8 Shock
			6 Non-cardiac surgery
			3 Other
Gupta et al. [20]	Pmcf analogue	61	Postoperative cardiac surgery	63 (11)	46 (75%)	Within 6 h after ICU admission
			40 CABG
			8 CABG + valve replacement
			8 Valve replacement
			5 Bentall’s procedure
			7 DDD pacing
Aya et al. [51]	Pmcf analogue	26	Postoperative fluid challenge	68	16 (62%)	Initial period at ICU (not further defined)
			7 Cardiac surgery	Range 53–80
			19 Non-cardiac surgery
Maas et al. [16]	Pmcf hold	11	Postoperative cardiac surgery	64	9 (82%)	Within 2 h after ICU admission
	Pmcf arm	11	9 CABG	Range 50–80
	Pmcf analogue	11	2 AVR
Maas et al. [30]	Pmcf arm	15	Postoperative cardiac surgery	64 (11)	Not described	Within 1 h after ICU admission
	Pmcf hold	12	9 CABG
			5 Valve
			1 CABG + valve

Age is presented as mean with standard deviation (SD) or median with range or interquartile range (IQR). Number of males per study is presented as counts with percentage

CABG coronary artery bypass, MVR mitral valve replacement, MPV mitral valve prolapse, AVR aortic valve replacement, TVP tricuspid valve prolapse, CVVH continuous veno-venous hemodiafiltration

P_mcf hold

Technique description

P_mcf hold is based on the linear relation between CVP and V_R (P_mcf = (V_R − CVP)/RVR). CVP is raised by performing a series of end-inspiratory hold maneuvers. In 2009, the method was first studied in humans [21]. Inspiratory hold maneuvers at 5, 15, 25 and 35 cmH₂O incremental ventilatory plateau pressures (P_vent) were performed, and CO was measured in the last 3 s of the 12 s inspiratory hold. They validated that after 7–10 s a steady state consists when V_R = CO. By plotting the CVP and CO values, a V_R curve is constructed and the zero-flow pressure (P_mcf) extrapolated. Seven studies [16, 21–26] estimated P_mcf hold using these four plateau pressures. Two studies [27, 28] used two points (P_vent 5 and 30 cmH₂O) at 15-s inspiratory and expiratory hold plateau phase. Between the P_mcf hold measurements, either 1-min pauses were used to re-establish the initial hemodynamic steady state [16, 21, 22, 24, 28], or the consecutive inspiratory hold was performed when CO had returned to baseline [23, 26, 27].

Clinical applicability

The average baseline P_mcf hold values found in the eight included studies range from 19 to 33 mmHg with a wide standard deviation (Tables 2, 3). Five studies [21–23, 26, 28] demonstrated fluid administration caused an increase in P_mcf hold, confirming that in humans, as in animals before [14, 15], P_mcf hold follows hemodynamic changes (Table 2). One of these studies found passive leg raising (PLR) to significantly increase P_mcf hold values [28]. RVR was not significantly affected by different volumetric conditions nor by PLR. V_s was calculated from P_mcf as a measure for effective circulating volume [22]. In one study, P_mcf was used to assess the hemodynamic effects of arterial hyperoxia (FiO₂ = 90% for 15 min) in ICU patients [26]. During this hyperoxia, left ventricular afterload increased and contractility remained similar; however, CO did not decrease. Both P_mcf and RVR increased significantly (Table 3), explaining why V_R (thus CO) remained unaltered.

Table 2.

Mean circulatory filling pressure during different volumetric state

Study	Method	N	Patient population	Baseline position	Baseline Pmcf	Hypervolemia (induced by fluid administration)	p value*	Amount of fluid administered to induce hypervolemia	Hypovolemia (induced by HUT)	p value†
Maas et al. [21]	Pmcf hold	12	Cardiac surgery	Supine	18.7 (4.5)	29.1 (5.2)	0.001	500 mL colloid in 15–20 min	14.5 (3.0)	0.005
Keller et al. [23]	Pmcf hold	9	Cardiac surgery	Semirecumbent	19.7	26.9	< 0.05	500 mL colloid	–	–
					IQR 17.0–22.6	IQR 18.4–31.0
Maas et al. [22]	Pmcf hold	10	Cardiac surgery	Not described	18.7 (4.0)	26.4 (3.2)	< 0.001	500 mL colloid	–	–
Guerin et al. [28]	Pmcf hold	30	Shock	Semirecumbent	Responder: 25 (13)	32 (17)	< 0.01	500 mL saline in 10 min
					Non-responders: 24 (10)	28 (12)	< 0.01
Geerts et al. [43]	Pmcf arm	24	Cardiac surgery	Supine	Responders: 16.2 (6.3)	22.0 (7.6)	< 0.001	500 mL colloid	–	–
					Non-responders: 24.3 (8.2)	29.9 (9.1)	< 0.001
Aya et al. [41]	Pmcf arm	20	Cardiac surgery	Supine	22.4 (7.7)	–	–	–	–	–
Aya et al. [42]	Pmcf arm	80	Cardiac surgery	Supine	23.0	–	–	–	–	–
					Range: 17.3–29.8
Parkin et al. [49]	Pmcf analogue	10	CVVH	Not described	Target state = 15.9	–	–	CVVHD	–	–
Cecconi et al. [48]	Pmcf analogue	39	Heterogenous	Not described	Responders: 17.8 (5.1)	20.9 (5.1)	< 0.001	Mean 252 (8.9) mL	–	–
					Non-responders: 17.9 (5.1)	21.0 (4.9)	< 0.001	52.5% crystalloid
								37.6% colloid
								8.8% FFP & RBC
Gupta et al. [20]	Pmcf analogue	61	Cardiac surgery	Supine	Responders: 17 (3.7)	19 (4.3)	0.02	Mean 264 (16) mL	–	–
					Non-responders: 17 (3.6)	19 (4.1)	0.03	50% saline. Other 50%: mix of FFP, platelets, albumin, packed RBC, return of pump blood
Aya et al. [51]	Pmcf analogue	26	Heterogenous	Not described	Responders: 13.7 IQR: 10.9–16.9			250 mL crystalloid
					Non-responders: 16.7 IQR: 10.5–18.9
Maas et al. [16]	Pmcf hold	11	Cardiac surgery	Supine	19.7 (3.9)	28.3 (3.6)	< 0.001	500 mL colloid	16.2 (3.0)	0.001
	Pmcf arm				18.4 (3.7)	27.1 (4.0)	< 0.001		15.4 (3.1)	0.001
	Pmcf analogue				14.7 (2.7)	19.2 (1.1)	< 0.001		10.9 (2.0)	< 0.001
Maas et al. [30]	Pmcf arm	15	Cardiac surgery	Supine	21.0 (6.8)	27.7 (7.4)	< 0.001	500 mL colloid (10 steps of 50 mL)	–	–
	Pmcf hold#

Data presented as mean with SD or median with interquartile range (IQR). P_mcf in mmHg. Hypovolemic state induced by head up tilt (HUT) to 30°. Responders = fluid responsiveness was defined by a 10% increase in CO

* p value, difference between baseline and hypervolemia induced by fluid administration

^†p value, difference between baseline and hypovolemic state

Table 3.

P_mcf and pharmacodynamics

References	Method	n	Situation A	Situation B	p value*	Situation C	p value#
Persichini et al. [27]		16	NE 0.30	NE 0.19
			Range 0.10–1.40	Range 0.08–1.15
	Pmcf hold (in mmHg)		33 (12)	26 (10)	0.003
Maas et al. [25]		16	Baseline 1	NE increase of 0.04 (0.02)		Baseline 2
			NE 0.04 (0.03)			NE 0.04 (0.03)
	Pmcf hold (in mmHg)		21.4 (6.1)	27.6 (7.4)	< 0.001	22.0 (5.3)
de Wit et al. [24]		17	Propofol low	Propofol medium		Propofol high
			Cb 3.0 (0.90) μg/mL	Cb 4.5 (1.0) μg/mL		Cb 6.5 (1.2) μg/mL
	Pmcf hold (in mmHg)		27.9 (5.4)	24.6 (4.9)	0.01	21.4 (4.2)	< 0.001
Helmerhorst et al. [26]		22	FiO2 21–30%	FiO2 90%
	Pmcf hold (in mmHg)		20.8 (3.5)	23.1 (4.0)	< 0.001

NE norepinephrine dose in μg/kg/min presented as mean with range or mean with standard deviation. P_mcf values are presented as mean with standard deviation. Cb target blood concentration of propofol in μg/mL. P_mcf hold values presented in mmHg. FiO₂ fractional oxygen concentration

* p value, p value for situation A compared to B

^#p value, p value for situation A compared to C

Studies have used P_mcf hold to describe hemodynamic changes caused by propofol [24] and norepinephrine [25, 27] (Table 3). In septic shock patients, decreasing the dose of norepinephrine decreased both P_mcf and RVR [27]. Further, after increasing norepinephrine CO decreased in ten patients and CO increased in six patients [25]. In all patients, P_mcf and RVR increased, though the “balance” between the two values determined whether CO increased. One study showed an increase in propofol caused a decrease in V_s without a change in CO [24]. These studies show P_mcf behaves within the framework of hemodynamic reasoning and lends itself to being used as a less invasive method to assess drug-induced physiology. Since P_mcf exists at the intersection of arterial and venous flow, it enables to calculate the true arterial and venous resistance by calculating the critical closing pressure (P_cc). P_cc is the mean arterial pressure (MAP) to zero CO-intercept. Arterial resistance is calculated as (MAP − P_cc)/CO [22].

Precision and accuracy

The technique precision has not yet been assessed in humans. However, in an animal study the averaged coefficient of variation for repeated measurements of P_mcf hold was 6% [29]. Comparing the techniques’ accuracy, no significant differences between P_mcf hold and P_mcf arm existed, whereas P_mcf analogue values were significantly lower [16, 30].

Limitations

The use of P_mcf hold is restricted to mechanically ventilated and sedated patients with a central venous catheter. The procedure of the inspiratory hold maneuvers is not yet automated and requires a direct link between monitor and ventilator, or advanced monitor analytics to detect the inspiratory holds and to perform the instantaneous CO calculations. Furthermore, it is not suitable during cardiac arrhythmia. This method is not suitable to measure rapid changes in hemodynamic status since it takes a couple of minutes to perform the multiple end-inspiratory (and end-expiratory) holds. Potentially, this technique is operator-dependent because a proper inspiratory plateau pressure is needed. CVP can be altered due to incorrect catheter placement. An absolute CO value is not necessary for P_mcf hold as the technique extrapolates to zero CO. If the trend measurements are accurate, the RVR slope might change, but the intersection P_mcf point remains constant. The latter holds only true for the P_mcf itself, the RVR is dependent of the slope of the curve. In clinical practice, a physician would use P_mcf together with RVR; therefore, for clinical use of the P_mcf an accurate CO value is needed.

Potentially, the inspiratory hold maneuver overestimates P_mcf by the blood translocation from the pulmonary into the systemic circulation [31–33]. However, the potential volume shifts relative to Csys suggest that this effect is minimal [10, 34]. During inspiratory hold maneuvers, arterial pressure decreases. If sustained, baroreflex-induced increased sympathetic tone may cause P_mcf to increase [35, 36]. Indeed one study performed in pigs found the P_mcf hold overestimating compared to a method using right atrial balloon occlusion in euvolemic conditions, in bleeding and hypervolemia; however, the values found between the two methods were similar [34]. Two clinical studies [16, 30] have shown P_mcf hold and P_mcf arm values not being significantly different, debating the former result found in pigs. Future studies in humans are needed. Moreover, all patients undergoing inspiratory holds are on neuro-humoral suppressive agents, probably dampening the baroreflex and other autonomic influences [37–39].

P_mcf arm

Technique description

As P_mcf is defined as the steady-state blood pressure during no-flow conditions, instantaneously P_mcf should mainly be similar for different vascular compartments even though each compartment may have different V_u and V_s [2, 40]. Four studies [16, 41–43] used the arm to estimate P_mcf. For arm occlusion, a rapid cuff inflator (inflates in 0.3 s) [16, 43] or a pneumatic tourniquet (inflates in 1.4 s) [41, 42] was inflated around the upper arm to 50 mmHg above systolic blood pressure. Arterial and venous pressures were measured via a radial artery catheter and a peripheral venous cannula in the forearm. When these two pressures equalize, P_mcf arm values are achieved. An initial study determined that a 25–30 s stop-flow time was adequate to achieve this equilibration [16]. Following this, in two studies P_mcf arm was measured as the average radial arterial pressure at 30 s after stop-flow [16, 43]. One study found the smallest difference between venous and arterial pressure after 60 s of stop-flow [41]. This discrepancy could be explained by different inflation time, i.e., induction of stop-flow.

Clinical applicability

The average baseline P_mcf arm values found in the included studies range from 16 to 24 mmHg (Table 2). P_mcf arm can be performed in spontaneously breathing subjects and requires only one measure. In two studies, P_mcf arm was assessed as a predictor of fluid loading responsiveness (FLR) [16, 43]. One study showed that a low P_mcf arm (< 22 mmHg) predicts FLR with 71% sensitivity and 88% specificity, where responders were defined when CO increased > 10% after 500 mL colloid administration [43]. Another study showed changes in circulating volume (500 mL colloid) are tracked well by changes in P_mcf arm [16]. Finally, one study indicated a minimum of 4 mL/kg fluid challenge was needed to define FLR [42].

Precision and accuracy

Repeated measurements of P_mcf arm showed no significant differences [41]. The coefficient of variation for a single measurement was 5%, which reduced to 3% after four measurements. Bland–Altman analysis showed a bias of − 0.1 ± 1.68 mmHg for the first two measurements. The least significant change [44] for a single measurement was 14% (i.e., ± 3 mmHg for a P_mcf arm of 22 mmHg). One study observed a negligible bias of two P_mcf arm determinations at baseline position and after fluid expansion [16]. Two studies [16, 30] found no significant differences in P_mcf arm to P_mcf hold measures.

Limitations

Theoretically, a limitation of the technique is the influence of an auto regulatory hypoxia-induced response causing arterial vasodilation. The time of measuring P_mcf after arm occlusion should be enough for arterial and venous pressures to equilibrate, but before hypoxia-induced vasodilation causes an underestimation of P_mcf [45]. One study observed plateau pressures after 20–30 s and saw a further decrement after 35–40 s which indicates hypoxia-induced vasodilation [16]. Potentially, arm occlusion causes a small accumulation of blood volume because the venous outflow stops before the arterial inflow stops [16]. Though, this potential overestimation is negligible since the inflow is small compared to the total distal arm volume as long as cuff inflation is rapid. To note, P_mcf arm is only reliable when a stable plateau pressure is achieved [2].

In contrast to P_mcf hold, P_mcf arm measures can be made in non-sedated patients with cardiac arrhythmias. However, the possible influence of the rapid cuff inflator on reflex mechanisms needs to be studied. In septic patients, central and peripheral vasomotor tone might be altered differently [46]. Shortly after cardiac surgery differences between aortic and radial pressure can occur [47], still, the original validation studies were on postoperative cardiac surgery patients.

P_mcf analogue

Technique description

Based on a Guytonian model of the systemic circulation (CO = V_R = (P_mcf − CVP)/RVR), an analogue of P_mcf can be derived using a mathematical model: P_mcf analogue = axCVP + bxMAP + cxCO [5, 20, 48, 49]. In this formula, a and b are dimensionless constants (a + b = 1). Assuming a veno-arterial compliance ratio of 24:1, a = 0.96 and b = 0.04; c resembles arteriovenous resistance and is based on a formula including age, height and weight [5, 48–50].

Clinical applicability

The average baseline P_mcf analogue values found in the included studies range from 14 to 18 mmHg (Table 2). One study compared fluid replacement based on target P_mcf analogue compared to conventional treatment in continuous veno-venous hemodiafiltration [49]. Fluid replacement based on target P_mcf analogue led to significantly less fluid administration with stable cardiovascular variables (CVP, MAP, CO) and no complications. So, P_mcf analogue measurement adequately follows intravascular volume status in patients. P_mcf analogue measurements are automatic making it an attractive alternative to P_mcf hold and P_mcf arm.

More recently, the P_mcf analogue dynamics, measured with the Navigator™ device (Applied Physiology, Pty Ltd, Australia), were observed [20, 48, 51]. Patients were defined as responders with an increase in stroke volume or CO > 10% after 250 mL fluid administration. P_mcf analogue increased after fluid administration; however, baseline P_mcf analogue did not differ between responders and non-responders [20, 45, 48] (Table 2). This is contrary to results of another study [43] using P_mcf arm, possibly due to different fluid volume (250 vs. 500 mL) [42]. Although the driving pressure for V_R (P_mcf CVP) was different between responders and non-responders, it showed low sensitivity (79%) and specificity (56%) to predict FLR [20, 48].

Precision and accuracy

Precision has not been assessed for P_mcf analogue (Table 4). Comparing measurement techniques revealed a lower P_mcf analogue value compared to P_mcf hold [16]. However, a significant regression of P_mcf analogue and P_mcf hold was observed enabling to adjust the P_mcf analogue value using calibration factor [5].

Table 4.

Comparison of bedside P_mcf measurement techniques

	Pmcf hold	Pmcf arm	Pmcf analogue
	CO = (Pmcf CVP)/RVR	Pa = Pv	Pmcf = axCVP + bxMAP + cxCO
Applicability to a broad patient population	−	±	±
	Restricted to fully sedated and mechanically ventilated patients	In theory applicable in all patients (sedated or awake) with an radial artery catheter	In theory applicable in all patients (sedated or awake)
	Restricted to patients without a contraindication for inspiratory holds (such as COPD with bullae)		Continuous and accurate CO, MAP and CVP measurements needed
	Continuous and accurate CO and CVP measurements needed		Not suitable in cardiac arrhythmia
	Not suitable in cardiac arrythmia
Accuracy	+	+	−
	Values interchangeable with Pmcf arm	Values interchangeable with Pmcf hold	Values significantly lower than derived with Pmcf hold
	When sedated baroreflex probably of little influence	Dependent on time of measurement: > Pa and Pv equilibration. < hypoxia-induced vasodilatation	Pmcf analogue can be transformed to Pmcf hold values (constant error)
	Mechanical ventilation may overestimate Pmcf value	Possible influence rapid cuff inflator on reflex mechanism altering Pmcf value in non-sedated patients. This is not studied	Mathematical coupling and the equation is based on assumptions that may not be generalizable to all patient populations in ICU
Precision	?	+	?
	Not studied	No significant differences during repeated measurements. LSC for a single measurement is 14%	Not studied
Outcome operator independent	−	±	+
	Inspiratory holds	Timing of measurement	CVP transducer position and CO measurement technique
	CVP transducer position and CO measurement technique
	Extrapolation of curve
Responding time	−	+	+
	> 4 min	30–60 s	Fast, no exact times mentioned
Costs	−	+	+
	Theoretically no extra devices needed than standard present in ICU	Rapid Cuff Inflator (Hokanson E20, Bellevue, Washington, USA) = 3000 euro	Navigator™ (Applied Physiology, Pty Ltd, Sydney, Australia)
			Price unknown
Risk of complications	+	±	−
	No complications reported in published studies. In theory:	No complications reported in published studies. In theory:	No complications reported in published studies. In theory:
	Barotrauma from inspiratory holds	In sedated patients attention should be paid deflating the rapid cuff before hypoxemia-induced damage can occur	Complications associated with central venous catheters and CO measurement
	Severe hemodynamic instability induced by inspiratory holds	In awake patients local pain could be caused by inflating the rapid cuff inflator
	Complications associated with central venous catheters and CO measurement

CO cardiac output, CVP central venous pressure, RVR resistance to venous return, MAP mean arterial pressure, P_a arterial pressure, P_v venous pressure (the latter two measured in the arm)

Limitations

The mathematical model is based on CVP, MAP and CO measurements. As CVP values vary during ventilation, usually end-expiratory CVP-recordings can be used. Furthermore, CVP values depend on the position of the transducer. Accurate CO values are needed for this method. The limitation of P_mcf analogue is that the algorithm is based on a mathematical model with mathematical coupling between CO and P_mcf and fixed Csys and resistance parameters [5], therefore presumably not applicable for all patient populations or clinical conditions. We are unable to assess the availability of the Navigator™ for routine care.

Discussion

We found three bedside techniques to measure P_mcf: P_mcf hold, P_mcf arm and P_mcf analogue. They were used to follow volumetric state and to study drug-induced hemodynamic changes in patients.

The interpretation of V_R curves and P_mcf in clinical practice is subject to debate [52–59]. The values found in heart-beating ICU patients are higher (14–33 mmHg) than in deceased ICU patients (12.8 ± 5.6 mmHg, mean ± sd), probably because of alteration of vasomotor tone after dying [53]. Furthermore, ICU patients often receive vasopressors which increase P_mcf and the study populations differed making it not one-to-one comparable. It is also speculated that the pressure described by Guyton is not measurable in heart-beating patients and the extrapolated pressure of the curve represents a different physiological parameter. Nevertheless, in two studies P_mcf arm was interchangeable with P_mcf hold [16–30]. Furthermore, although P_mcf values may differ, the CVP values do as well, which may account for a similar driving pressure for V_R. The reviewed studies illustrate the possible clinical benefits of using the bedside derived P_mcf values.

This review is limited since we were unable to pool the data because of the variety in used conditions and interventions. The 16 included studies were performed by only a few research groups with a limited amount of included patients. In most of the studies, each patient served as their own control since it is not clear what would be an appropriate outside control group.

Still, all studies testing the accuracy of P_mcf to follow intravascular changes and pharmacodynamics found significant results. Therefore, it is unlikely that a larger number of patients will show different outcomes. It is possible only positive studies were published, indicating publication bias. P_mcf values differ between the studies and have a wide range within studies (Table 2). Normal values for different patient populations need to be defined before P_mcf can be implemented into standard (ICU) care. The increase in P_mcf values after fluid administration depends on vascular redistribution, vasomotor tone and fluid loss into the interstitial space. Studies focusing on clinical decision-making based on P_mcf, driving pressure for V_R, V_s or Csys have not yet been performed. Study designs need to be created to see if using these measures improves outcomes. Also, no precision studies examining P_mcf hold or P_mcf analogue exist yet.

Conclusions

Presently, three bedside P_mcf measurement techniques are available. All require invasive hemodynamic monitoring. Though P_mcf measures allow for more direct assessment of circulating blood volume, V_R and Csys, studies are needed to determine cutoff values to allow P_mcf to trigger therapeutic interventions and to determine its value in clinical practice.

Abbreviations

CO: cardiac output; Csys: vascular compliance, CVP: central venous pressure; FiO₂: fractional oxygen concentration; FLR: fluid loading responsiveness; ICU: intensive care unit; MAP: mean arterial pressure; RVR: resistance for venous return.

List of symbols

P_cc: critical closing pressure; P_mcf: mean circulatory filling pressure; P_ra: right atrial pressure; V_R: venous return; V_s: stressed volume; V_u: unstressed volume.