Troubleshooting a dialysis line: when blue runs red

Abstract

We present the case of a critically ill woman whose dialysis line was noted to be circulating bright red blood. Located in the right internal jugular vein, the line had previously been working normally with the change occurring shortly after the patient was liberated from positive pressure mechanical ventilation. An arterial malposition was ruled out and subsequent investigations revealed the presence of a left-sided partial anomalous pulmonary venous connection (PAPVC) that had been previously undiagnosed. The identification of a left-sided PAPVC from blood gas measurements taken from a right internal jugular vein dialysis catheter in this case provides an informative opportunity to consider the intricate physiological relationship between the respiratory and cardiovascular systems in critically ill patients requiring invasive procedures and treatments.

Background

The recognition and intellectual approach to problems encountered in the intensive care setting often requires understanding of complex concepts in physiology. For example, the mixed venous oxygen saturation is a value commonly trended when evaluating and treating patients in shock, as it can provide information about cardiac output.^1–3 This is based on the equation: SvO₂=SaO₂–[(VO₂)/(Hbx1.36xCO)] where SvO₂=mixed venous oxygen saturation, VO₂=oxygen uptake, Hb=haemoglobin and CO=cardiac output. It is important to maintain an awareness of the factors that can lead to spurious values. Our case reveals an unusual condition in which the mixed venous oxygen saturation could provide misleading information in the assessment of a patient’s cardiac output. In addition, our diagnosis and its clinical presentation provide an opportunity to consider the physiological relationships between positive end-expiratory pressure and pulmonary vascular circulation.

Case presentation

A woman in her early 60s presented with shortness of breath and weakness that resulted in her having spent 5 days barely able to leave her couch. Her medical history included chronic obstructive pulmonary disease, hypertension, ischaemic cerebral infarction, vocal cord squamous cell carcinoma and a granular cell tumour of the epiglottis previously treated with transoral laser resection. She was found to be hypoglycaemic, hypoxaemic, hypotensive and severely anaemic without signs of active bleeding. She was subsequently found to have a plasma cell leukaemia, and experienced a complex subsequent course including septic shock, hypoxaemic respiratory failure requiring mechanical ventilation, right heart failure and renal failure requiring dialysis. Dialysis was initiated through a left femoral catheter that was later replaced with a Trialysis central venous catheter (CVC) placed in the right internal jugular vein. The location of the second catheter was confirmed radiographically and it was used uneventfully for continuous venovenous haemodialysis (CVVH). Around the same time, her condition was improving and she was liberated from the ventilator the next day. Several hours before this, a central venous blood gas (VBG) obtained through the third port (designated for venous blood access) demonstrated a partial pressure of oxygen (PO₂) of 68 mm Hg (normal range: venous 35–45 mm Hg; arterial 75–100 mm Hg). The central venous oxygen saturation (SvO₂) was 92% (normal range 60%–80%). Half an hour after she was taken off the ventilator and placed on high-flow nasal cannula, another VBG drawn from the Trialysis line demonstrated a partial pressure of oxygen (PO₂) of 133 mm Hg. CVVH was being performed at the same blood flow rate as prior. Soon after this VBG was done, the CVVH circuit had flow errors and dialysis was interrupted. When CVVH was restarted, the bedside nurse noted an unusually light-red colour of the blood coming from the proximal port and alerted the intensive care unit provider team out of concern for arterial translocation of the catheter.

Differential diagnosis

The light-red colour we usually observe with arterial blood is due to the presence of oxyhaemoglobin (fully oxygenated haemoglobin), which absorbs light at 660 nm wavelength, as opposed to deoxyhaemoglobin (haemoglobin without the bound oxygen), which absorbs light at 940 nm which conveys a dark red colour.⁴ Small increases in the venous oxyhaemoglobin concentrations can be seen with several physiological states such as high pH, low 2,3-diphosphoglycerate levels or hypothermia, which all shift the oxygen dissociation curve to the left.⁵ These dysregulations are usually not significant enough to be clinically relevant from the perspective of oxyhaemoglobin levels. Conversely, high venous blood oxygen saturations can be seen in states of cytopathic hypoxia, such as sepsis⁶ or cyanide poisoning,⁷ in which tissues fail to extract oxygen from the haemoglobin molecules. Carbon monoxide poisoning can also make the venous blood appear bright red even when there is a low PO₂ in the blood. The carbon monoxide molecule attaches to one of the four oxygen binding sites of the haemoglobin tetramer increasing the oxygen affinity of the other three.⁷ An air bubble or froth left in the sample poses another potential source of spuriously elevated PO₂ in a blood gas measurement, but this is unlikely to result in more than 5 mm Hg increase, and would not explain the colour change observed in the dialysis circuit.⁸

When light-red blood with elevated PO₂ is being drawn from a CVC, and none of the above conditions are present, the potential explanations are limited. The primary concern should be the malposition of the catheter into a systemic artery. The incidence of this occurring has been previously reported as high as 11% when choosing an internal jugular vein as the site of catheter insertion, although with the emergence of ultrasound guidance, such complications have become less common.⁹ Most inadvertent arterial punctures can be detected at the time of the introducer needle placement prior to line insertion based on the colour or pulsatile backflow of the blood. These features can be more difficult to appreciate in the presence of hypotension and hypoxaemia. In such conditions, the central line can be misplaced in the artery and the mistake further overlooked on plain chest X-rays, particularly if congenital malformations or overlapping vessels are present.¹⁰

A less common cause for obtaining blood with elevated oxygen saturations and partial pressures from a CVC is the presence of congenital cardiac or venous abnormalities resulting in redirection of oxygenated blood to the right atrium (RA) or its tributaries. Among these, partial anomalous pulmonary venous connections (PAPVCs) have been described in a series of case reports in which the CVC was inserted through the left internal jugular vein. In each case, the concern for the malposition arose from the postprocedural chest X-ray that revealed failure of the CVC to cross the midline to the expected location of the cavoatrial junction. Alzghoul et al reviewed 10 such cases, in addition to their own, and put forward a systematic approach to follow when managing a similar malposition of a CVC. The steps include: (1) checking the insertion site with an ultrasound to confirm venous versus arterial placement; (2) connecting the catheter to a pressure transducer which should show a venous waveform, unless the catheter is wedged, in which case the waveform could be misleading; (3) comparing the PO₂ of the blood drawn from the CVC with that from a sample taken from an arterial site; and (4) evaluating the location of the CVC via further imaging such as fluoroscopy or CT.¹¹

Outcome and follow-up

For our patient, repeat blood gas measured through the proximal port reconfirmed an elevated PO₂ of 124 concerning for arterial blood. CVVH was stopped out of concern for arterial translocation of the catheter. CT scan of the chest and neck without contrast showed the central catheter in the right jugular vein without indication of arterial malpositioning (video 1) and a pressure transducer also confirmed a venous wave form. Nursing staff made the observation that while blood drawn rapidly demonstrated a PO₂ of 155 mm Hg, a slow draw resulted in a value of 55 mm Hg, suggesting the presence of a left-to-right shunt with minimal contribution to venous blood at low flow rates. Suspecting the presence of a PAPVC, prior imaging was reviewed carefully. A CT angiogram from 2 years prior with fortuitous contrast timing permitting clear differentiation between pulmonary arteries and pulmonary veins confirmed a left PAPVC draining into the left innominate (or brachiocephalic) vein (figure 1A,B, video 1). The line was subsequently used for CVVH without complications. We suspect that the previous flow errors were unrelated to the presence of the PAPVC although it is noted that the intake port for the line was aligned with the inflow from the left innominate vein.

Video 1.

Download video file ^{(39.9MB, mp4)}

DOI: 10.1136/bcr-2018-225972.video01

Review of CT scans demonstrating dialysis line position and the presence of a partial anomalous pulmonary venous connection.

Figure 1.

Coronal (A) and cross-sectional (B) CT scan images capturing the partial anomalous pulmonary artery connection (light-grey arrow) entering the innominate vein (bright white arrow).

Discussion

Our case provides an informative opportunity for discussion around troubleshooting a CVC when oxygen saturations are unexpectedly high. Our observations may facilitate recognition of a PAPVC as a cause, but the clinical relevance extends to more common practice as central venous saturations are often monitored in intensive care settings as a correlate of cardiac function.^{2 3} To our knowledge, all prior descriptions of left-sided anomalous pulmonary veins detected after placement of CVCs have involved left-sided CVCs tracking into anomalous pulmonary veins detected radiographically. Our case is distinct and informative as it exemplifies how a left-sided PAPVC can be detected from a right-sided line that does not directly enter the aberrant structure. In this clinical scenario, by applying well-established cardiopulmonary physiology, we were able to understand some relevant clinical consequences of an asymptomatic PAPVC in the setting of hypoxaemic respiratory failure and shock. Knox et al have previously suggested that an unappreciated PAPVC could be an exacerbating factor in the mechanism of cardiorespiratory collapse.¹²

PAPVCs are rare congenital abnormalities, previously thought to be more prevalent on the right side and associated with other heart malformations as observed in autopsy or surgical series of patients. More recent studies using CT, however, showed higher prevalence of left-sided PAPVCs, usually incidentally detected in asymptomatic patients.^{13 14}

Prior literature suggests that a PAPVC is predictably problematic when more than 50% of the pulmonary venous blood drains into the RA or its tributaries by creating a significant left-to-right shunt and leading to pulmonary vasculature remodelling, pulmonary arterial hypertension (PAH), and eventually right heart failure. Most patients with this pathology have more than just one PAPVC, an associated congenital heart malformation or another contributing disease process.^{15 16} However, cases of isolated left-sided PAPVC leading to PAH have been reported.^{17 18} In our case, an echocardiogram done several months before the current presentation showed normal right heart pressures, size and function.

In our case, positive pressure ventilation seemed to reduce blood flow through the PAPVC. We noted that the PO₂ obtained after the patient was taken off mechanical ventilation (133 mm Hg) was significantly higher than the one done several hours prior to extubation (68 mm Hg). Once we ruled out malposition into an arterial vessel and confirmed the presence of a left-sided PAPVC, we hypothesised that the differences in the blood gas composition was the result of physiological changes in the venous blood return to the heart caused by switching between mechanical ventilation and spontaneous breathing (figure 2A–C).

Figure 2.

Cardiovascular pressure changes under different ventilation conditions in the presence of a PAPVC. (A) Respiratory distress generates profound swings in negative intrathoracic pressure increasing the left ventricle’s afterload and, consequently, the Ppv. Since the PAPVC is under the same Ppv, blood flow through it increases leading to RA and RV overload. (B) Under normal conditions, the chronic left-to-right blood shunt through the PAPVC is not significant enough to cause right heart problems, but the pulmonary arterial blood will have a higher PO₂ than normal. (C) When the patient is switched to positive pressure ventilation, the Ppv and Ppapvc decrease, while the RAP goes up. Blood flow through the PAPVC is minimised, as a result. LA, left atrium; LV, left ventricle; LVEDP, left ventricle end diastolic pressure; PAPVC, partial anomalous pulmonary vein connection; Ppapvc, partial anomalous pulmonary vein connection pressure; Ppv, pulmonary venous vasculature pressure; Psv, systemic venous vasculature pressure; PVR, pulmonary venous resistance; RA, right atrium; RAP, right atrial pressure; RV, right ventricle.

Positive intrathoracic pressure affects blood return to both the right and left atria. In normal states, the systemic venous blood return to the RA is the main determinant of cardiac output and is dependent on the pressure gradient between the RA and the systemic venous system.¹⁹ Guyton along with other pioneers of pulmonary-cardiac physiology showed that there is an inverse proportional relationship between the right atrial pressure (RAP) and venous blood return.^{19 20} While positive intrathoracic pressure is known to increase RAP, studies have shown that parallel increases in Psv preserve the pressure gradient, venous return and cardiac output.²¹ However, if the system is sensitive to the effects of positive airway pressures due to pulmonary hypertension and/or right ventricular dysfunction, the RAP will experience a disproportionate increase. As a result, the drop in venous blood return to the RA in the setting of positive intrathoracic pressure leads to a decrease in the overall blood flow through the pulmonary arteries, pulmonary veins and subsequently the PAPVC. This phenomenon is supported by the significantly lower central venous PO₂ seen when our patient was on mechanical ventilation as compared with when she was spontaneously breathing. We suspect that the blood flow was even lower through the PAPVC than the other pulmonary veins for two reasons. Positive intrathoracic pressure decreases the left ventricle (LV) afterload²² optimising the left side filling volumes and potential pressures, although this has been previously under debate and difficult to demonstrate.²⁰ Regardless, in the setting of mechanical ventilation, right ventricle (RV) dysfunction and a normal LV function as seen in our patient, the left atrial pressure can become lower than the RAP and the pulmonary venous blood may preferentially flow to the left atrium in that setting.²³ Another potential contributing mechanism for our observation is that the PAPVC was located in the left upper lobe with high likelihood of experiencing Zone 1 type physiology, in which, according to West,⁵ the effect of gravity in someone who is not lying flat leads to a pulmonary arterial pressure lower than the alveolar pressure with vessel collapse and minimal blood flow. Based on these physiological principles and our clinical observations, we suspect that the blood flow through the PAPVC was minimised and the RA preferentially filled with systemic venous blood in the setting of positive pressure ventilation.

Once the transition to spontaneous negative intrathoracic pressure ventilation was made, the RAP decreased, cardiac output augmented and pulmonary venous vasculature pressure (Ppv) likely increased. As a result, the pressure gradient between the Ppv (including PAPVC) and the RAP was restored to its previous state in which RAP<Ppv allowing for pulmonary venous blood return to the RA through the PAPVC. While the chronic left-to-right shunt might not have caused an apparent clinical problem, it is conceivable that in the setting of respiratory distress, negative intrathoracic pressure swings may have increased the LV afterload and likely the Ppv. Blood flow through the PAPVC in that setting may have increased and contributed to RV overload (figure 2A).

Restoring venous PO₂ to normal values when blood was withdrawn very slowly through a syringe was an observation that warrants some consideration. It supports the fact that under relatively normal conditions (in this case, in the absence of respiratory distress), the left-to-right shunt through the left side PAPVC is minimal. We suspect that when applying negative pressure to the catheter, the widening of the pressure difference between the PAPVC and catheter site results in increased blood flow from the PAPVC. This phenomenon was minimised during positive pressure ventilation, however, as blood flow is decreased through the PAPVC due to the physiological changes described above.

Surgical correction of isolated partial PAPVCs is recommended when signs of RV dysfunction, volume overload or increased pulmonary pressures are present, as these can progress over time and result in impairment that can be prevented through surgical repair.¹⁵ Patients selected for repair by experienced surgeons have been described as having low morbidity and mortality associated with the required surgical procedures.^{15 16} The management of asymptomatic isolated partial PAPVCs is more controversial and, generally, conservative close monitoring of the right heart function is recommended.

In our case, it was important to identify the previously asymptomatic PAPVC as it had several clinically significant consequences including reassurance that the line was not malpositioned into an arterial bed as well as recognition of the inability to extrapolate cardiac output trends from the central venous blood oxygen saturation. Due to her multiple health problems, our patient was not likely to benefit from surgical repair and died later of complications unrelated to the PAPVC.

Learning points.

• Abnormally elevated central venous oxygen saturations or partial pressures from an otherwise correctly placed central venous catheter should raise suspicion for the presence of partial anomalous pulmonary venous connection (PAPVC).

• The presence of a PAPVC can lead to misleading central venous oxygen saturations, which is important to know when values are trended as part of a patient’s intensive care unit management.

• A left-sided PAPVC is usually asymptomatic, but in the setting of respiratory distress, the increased blood flow through it can lead to right ventricle overload and even failure.

• Blood flow through a PAPVC may decline during positive pressure ventilation as a consequence of increased right atrial pressure and decreased pulmonary venous vasculature pressure.